18-Bit, 2.5 LSB INL, 570 kSPS SAR ADC
AD7679
Rev. 0
Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its use.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective companies.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.326.8703
2003 Analog Devices, Inc. All rights reserved.
FEATURES
18-bit resolution with no missing codes
No pipeline delay (SAR architecture)
Differential input range: V
REF
(V
REF
up to 5 V)
Throughput: 570 kSPS
INL: 2.5 LSB max (9.5 ppm of full scale)
Dynamic range : 103 dB typ (V
REF
= 5 V)
S/(N+D): 100 dB typ @ 2 kHz (V
REF
= 5 V)
Parallel (18-,16-, or 8-bit bus) and serial 5 V/3 V interface
SPI
/QSPI
TM
/MICROWIRE
TM
/DSP compatible
On-board reference buffer
Single 5 V supply operation
Power dissipation: 76 mW @ 500 kSPS
150 W @ 1 kSPS
48-lead LQFP or 48-lead LFCSP package
Pin-to-pin compatible upgrade of AD7674/AD7676/AD7678
APPLICATIONS
CT scanners
High dynamic data acquisition
Geophone and hydrophone sensors
-
replacement (low power, multichannel)
Instrumentation
Spectrum analysis
Medical instruments
GENERAL DESCRIPTION
The AD7679 is an 18-bit, 570 kSPS, charge redistribution SAR,
fully differential analog-to-digital converter that operates on a
single 5 V power supply. The part contains a high speed 18-bit
sampling ADC, an internal conversion clock, an internal
reference buffer, error correction circuits, and both serial and
parallel system interface ports.
The part is available in a 48-lead LQFP or 48-lead LFCSP with
operation specified from 40C to +85C.
FUNCTIONAL BLOCK DIAGRAM
SWITCHED
CAP DAC
18
CONTROL LOGIC AND
CALIBRATION CIRCUITRY
CLOCK
AD7679
D[17:0]
BUSY
RD
CS
MODE0
OGND
OVDD
DGND
DVDD
AVDD
AGND
REF REFGND
IN+
IN
PD
RESET
SERIAL
PORT
PARALLEL
INTERFACE
CNVST
PDBUF
REFBUFIN
MODE1
030850001
Figure 1. Functional Block Diagram
Table 1. PulSAR Selection
Type/kSPS 100250
500570
800
1000
Pseudo-
Differential
AD7651
AD7660
/
AD7661
AD7650
/
AD7652
AD7664
/
AD7666
AD7653
AD7667
True Bipolar
AD7663
AD7665
AD7671
True
Differential
AD7675
AD7676
AD7677
18-Bit
AD7678
AD7679
AD7674
Multichannel/
Simultaneous
AD7654
AD7655
PRODUCT HIGHLIGHTS
1. High Resolution, Fast Throughput.
The AD7679 is a 570 kSPS, charge redistribution, 18-bit
SAR ADC (no latency).
2. Excellent
Accuracy.
The AD7679 has a maximum integral nonlinearity of
2.5 LSB with no missing 18-bit codes.
3. Serial or Parallel Interface.
Versatile parallel (18-, 16-, or 8-bit bus) or 3-wire serial
interface arrangement compatible with both 3 V and
5 V logic.
AD7679
Rev. 0 | Page 2 of 28
TABLE OF CONTENTS
Specifications..................................................................................... 3
Timing Specifications....................................................................... 5
Absolute Maximum Ratings............................................................ 7
Pin Configuration and Functional Descriptions.......................... 8
Definition of Specifications........................................................... 11
Typical Performance Characteristics ........................................... 12
Circuit Information ........................................................................ 15
Converter Operation.................................................................. 15
Typical Connection Diagram ................................................... 17
Power Dissipation versus Throughput .................................... 19
Conversion Control ................................................................... 19
Digital Interface.......................................................................... 20
Parallel Interface......................................................................... 20
Serial Interface............................................................................ 20
Master Serial Interface............................................................... 21
Slave Serial Interface .................................................................. 22
Microprocessor Interfacing ...................................................... 24
Application Hints ........................................................................... 25
Layout .......................................................................................... 25
Evaluating the AD7679's Performance.................................... 25
Outline Dimensions ....................................................................... 26
Ordering Guide............................................................................... 26
REVISION HISTORY
Revision 0: Initial Version
AD7679
Rev. 0 | Page 3 of 28
SPECIFICATIONS
Table 2. 40C to +85C, V
REF
= 4.096 V, AVDD = DVDD= 5 V, OVDD = 2.7 V to 5.25 V, unless otherwise noted.
Parameter Conditions
Min
Typ
Max
Unit
RESOLUTION
18
Bits
ANALOG INPUT
Voltage Range
V
IN+
V
IN
V
REF
+V
REF
V
Operating Input Voltage
V
IN+
, V
IN
to AGND
0.1
AVDD+0.1
V
Analog Input CMRR
f
IN
= 100 kHz
68
dB
Input Current
570 kSPS Throughput
25
A
Input Impedance
1
THROUGHPUT SPEED
Complete Cycle
1.75
s
Throughput Rate
0
570
kSPS
DC ACCURACY
Integral Linearity Error
2.5
+2.5
LSB
2
Differential Linearity Error
1
+1.75
LSB
No Missing Codes
18
Bits
Transition Noise
V
REF
= 5 V
0.7
LSB
Zero Error, T
MIN
to T
MAX
3
40
+40
LSB
Zero Error Temperature Drift
0.5
ppm/C
Gain Error, T
MIN
to T
MAX
3
0.048
See Note 3
+0.048
% of FSR
Gain Error Temperature Drift
1.6
ppm/C
Power Supply Sensitivity
AVDD = 5 V 5%
4
LSB
AC ACCURACY
f
IN
= 2 kHz, V
REF
= 5 V
101
dB
4
V
REF
= 4.096 V
97.5
99
dB
f
IN
= 10 kHz, V
REF
= 4.096 V
98
dB
Signal-to-Noise
f
IN
= 100 kHz, V
REF
= 4.096 V
97
dB
Dynamic Range
V
IN+
= V
IN
= V
REF
/2 = 2.5 V
103
dB
f
IN
= 2 kHz
120
dB
f
IN
= 10 kHz
118
dB
Spurious-Free Dynamic Range
f
IN
= 100 kHz
105
dB
f
IN
= 2 kHz
115
dB
f
IN
= 10 kHz
113
dB
Total Harmonic Distortion
f
IN
= 100 kHz
98
dB
f
IN
= 2 kHz, V
REF
= 4.096 V
98
dB
Signal-to-(Noise + Distortion)
f
IN
= 2 kHz, 60 dB Input
40
dB
3 dB Input Bandwidth
26
MHz
SAMPLING DYNAMICS
Aperture Delay
2
ns
Aperture Jitter
5
ps rms
Transient Response
Full-Scale Step
250
ns
Overvoltage Recovery
250
ns
REFERENCE
External Reference Voltage Range
REF
3
4.096
AVDD + 0.1
V
REF Voltage with Reference Buffer
REFBUFIN = 2.5 V
4.05
4.096
4.15
V
Reference Buffer Input Voltage Range
REFBUFIN
1.8
2.5
2.6
V
REFBUFIN Input Current
1
+1
A
REF Current Drain
570 kSPS Throughput
235
A
AD7679
Parameter Conditions
Min
Typ
Max
Unit
DIGITAL INPUTS
Logic Levels
V
IL
0.3
+0.8
V
V
IH
2.0
DVDD + 0.3
V
I
IL
1
+1
A
I
IH
1
+1
A
DIGITAL OUTPUTS
Data Format
5
Pipeline Delay
6
V
OL
I
SINK
= 1.6 mA
0.4
V
V
OH
I
SOURCE
= 500 A
OVDD 0.6
V
POWER SUPPLIES
Specified Performance
AVDD
4.75
5
5.25
V
DVDD
4.75
5
5.25
V
OVDD
2.7
DVDD + 0.3
7
V
Operating Current
500 kSPS Throughput
AVDD PDBUF
High
10.8
mA
DVDD
8
4.5
mA
OVDD
8
50
A
PDBUF High @ 500 kSPS
76
90
mW
PDBUF High @ 1 kSPS
150
W
POWER DISSIPATION
8
PDBUF Low @ 500 kSPS
89
103
mW
TEMPERATURE RANGE
9
Specified Performance
T
MIN
to T
MAX
40
+85
C
1
See
section.
Analog Inputs
2
LSB means Least Significant Bit. With the 4.096 V input range, 1 LSB is 31.25 V.
3
See
section. The nominal gain error is not centered at zero and is +0.273% of FSR. This specification is the deviation from this nominal
value. These specifications do not include the error contribution from the external reference, but do include the error contribution from the reference buffer if used.
Definition of Specifications
4
All specifications in dB are referred to a full-scale input, FS. Tested with an input signal at 0.5 dB below full scale unless otherwise specified.
5
Parallel or Serial 18-Bit.
6
Conversion results are available immediately after completed conversion.
7
The max should be the minimum of 5.25 V and DVDD + 0.3 V.
8
Tested in Parallel Reading mode.
9
Contact factory for extended temperature range.
Rev. 0 | Page 4 of 28
AD7679
TIMING SPECIFICATIONS
Table 3. 40C to +85C, AVDD = DVDD = 5 V, OVDD = 2.7 V to 5.25 V, unless otherwise noted.
Parameter Symbol
Min
Typ
Max
Unit
Refer to Figure 32 and Figure 33
Convert Pulsewidth
t
1
10
ns
Time between Conversions
t
2
1.75
s
CNVST LOW to BUSY HIGH Delay
t
3
35
ns
BUSY HIGH All Modes Except Master Serial Read after Convert
t
4
1.5
s
Aperture Delay
t
5
2
ns
End of Conversion to BUSY LOW Delay
t
6
10
ns
Conversion Time
t
7
1.5
s
Acquisition Time
t
8
250
ns
RESET Pulsewidth
t
9
10
ns
Refer to Figure 34, Figure 35, and Figure 36 (Parallel Interface Modes)
CNVST LOW to Data Valid Delay
t
10
1.5
s
Data Valid to BUSY LOW Delay
t
11
20
ns
Bus Access Request to Data Valid
t
12
45
ns
Bus Relinquish Time
t
13
5
15
ns
Refer to Figure 38 and Figure 39 (Master Serial Interface Modes)
1
CS LOW to SYNC Valid Delay
t
14
10
ns
CS LOW to Internal SCLK Valid Delay
t
15
10
ns
CS LOW to SDOUT Delay
t
16
10
ns
CNVST LOW to SYNC Delay
t
17
525
ns
SYNC Asserted to SCLK First Edge Delay
2
t
18
3
ns
Internal SCLK Period
2
t
19
25
40
ns
Internal SCLK HIGH
2
t
20
12
ns
Internal SCLK LOW
2
t
21
7
ns
SDOUT Valid Setup Time
2
t
22
4
ns
SDOUT Valid Hold Time
2
t
23
2
ns
SCLK Last Edge to SYNC Delay
2
t
24
3
ns
CS HIGH to SYNC HI-Z
t
25
10
ns
CS HIGH to Internal SCLK HI-Z
t
26
10
ns
CS HIGH to SDOUT HI-Z
t
27
10
ns
BUSY HIGH in Master Serial Read after Convert
2
t
28
See Table 4
CNVST LOW to SYNC Asserted Delay
t
29
1.5
s
SYNC Deasserted to BUSY LOW Delay
t
30
25
ns
Refer to Figure 40 and Figure 41 (Slave Serial Interface Modes)
External SCLK Setup Time
t
31
5
ns
External SCLK Active Edge to SDOUT Delay
t
32
3
18
ns
SDIN Setup Time
t
33
5
ns
SDIN Hold Time
t
34
5
ns
External SCLK Period
t
35
25
ns
External SCLK HIGH
t
36
10
ns
External SCLK LOW
t
37
10
ns
1
In serial interface modes, the SYNC, SCLK, and SDOUT timings are defined with a maximum load C
L
of 10 pF; otherwise, the load is 60 pF maximum.
2
In Serial Master Read during Convert mode. See
for Serial Master Read after Convert mode.
Table 4
Rev. 0 | Page 5 of 28
AD7679
Table 4. Serial Clock Timings in Master Read after Convert
DIVSCLK[1]
0 0 1 1
DIVSCLK[0]
Symbol
0 1 0 1 Unit
SYNC to SCLK First Edge Delay Minimum
t
18
3
17
17
17
ns
Internal SCLK Period Minimum
t
19
25 60 120
240
ns
Internal SCLK Period Maximum
t
19
40 80 160
320
ns
Internal SCLK HIGH Minimum
t
20
12 22 50 100
ns
Internal SCLK LOW Minimum
t
21
7
21
49
99
ns
SDOUT Valid Setup Time Minimum
t
22
4
18
18
18
ns
SDOUT Valid Hold Time Minimum
t
23
2 4 30
89
ns
SCLK Last Edge to SYNC Delay Minimum
t
24
3
60
140
300 ns
Busy High Width Maximum
t
28
2.25 3
4.5
7.5 s
Rev. 0 | Page 6 of 28
AD7679
ABSOLUTE MAXIMUM RATINGS
Table 5. AD7679 Absolute Maximum Ratings
1
Parameter Rating
Analog Inputs
IN+
2
, IN
2
, REF, REFBUFIN, REFGND
to AGND
AVDD + 0.3 V to
AGND 0.3 V
Ground Voltage Differences
AGND, DGND, OGND
0.3 V
Supply Voltages
AVDD, DVDD, OVDD
0.3 V to +7 V
AVDD to DVDD, AVDD to OVDD
7 V
DVDD to OVDD
0.3 V to +7 V
Digital Inputs
0.3 V to DVDD + 0.3 V
Internal Power Dissipation
3
700
mW
Internal Power Dissipation
4
2.5
W
Junction Temperature
150C
Storage Temperature Range
65C to +150C
Lead Temperature Range
(Soldering 10 sec)
300C
1
Stresses above those listed under Absolute Maximum Ratings may cause
permanent damage to the device. This is a stress rating only; functional
operation of the device at these or any other conditions above those
indicated in the operational section of this specification is not implied.
Exposure to absolute maximum rating conditions for extended periods may
affect device reliability.
2
See An
section.
alog Inputs
3
Specification is for device in free air: 48-Lead LQFP:
JA
= 91C/W,
JC
= 30C/W.
4
Specification is for device in free air: 48-Lead LFCSP:
JA
= 26C/W.
TO OUTPUT
PIN
C
L
60pF
1
500
A
I
OH
1.6mA
I
OL
1.4V
IN SERIAL INTERFACE MODES,THE SYNC, SCLK, AND
SDOUT TIMINGS ARE DEFINED WITH A MAXIMUM LOAD
C
L
OF 10pF; OTHERWISE,THE LOAD IS 60pF MAXIMUM.
1
030850002
Figure 2. Load Circuit for Digital Interface Timing
SDOUT, SYNC, SCLK Outputs, C
L
= 10 pF
0.8V
2V
2V
0.8V
t
DELAY
2V
0.8V
t
DELAY
030850003
Figure 3. Voltage Reference Levels for Timing
Rev. 0 | Page 7 of 28
AD7679
Rev. 0 | Page 8 of 28
PIN CONFIGURATION AND FUNCTIONAL DESCRIPTIONS
36
35
34
33
32
31
30
29
28
27
26
25
13 14 15 16 17 18 19 20 21 22 23 24
1
2
3
4
5
6
7
8
9
10
11
12
48 47 46 45 44
39 38 37
43 42 41 40
PIN 1
IDENTIFIER
TOP VIEW
(Not to Scale)
AGND
CNVST
PD
RESET
CS
RD
DGND
AGND
AVDD
MODE0
MODE1
D0/OB/2C
NC
NC
NC = NO CONNECT
D1/A0
D2/A1
D3
D4/DIVSCLK[0]
BUSY
D17
D16
D15
AD7679
D5/DIVSCLK[1]
D14
P
DBUF
AV
DD
RE
FBUFIN
NC
AGND
IN+
NC
NC
NC
IN
RE
FGND
RE
F
D6
/E
X
T
/
I
NT
D
7
/IN
VSYN
C
D8/INVSCLK
D9
/RDC/S
D
IN
OGND
OV
DD
DV
DD
DGND
D
10/SD
OU
T
D
11/SC
LK
D
12/SYN
C
D1
3
/
RDE
RR
OR
030850004
Figure 4. 48-Lead LQFP(ST-48) and 48-Lead LFCSP (CP-48)
Table 6. Pin Function Descriptions
Pin No.
Mnemonic
Type
1
Description
1, 44
AGND
P
Analog Power Ground Pin.
2, 47
AVDD
P
Input Analog Power Pins. Nominally 5 V.
3
MODE0
DI
Data Output Interface Mode Selection.
4
MODE1
DI
Data Output Interface Mode Selection:
Interface MODE
MODE1
MODE0
Description
0 0
0
18-Bit
Interface
1 0
1
16-Bit
Interface
2 1
0
Byte
Interface
3 1
1
Serial
Interface
5
D0/OB/2C
DI/O
When MODE = 0 (18-bit interface mode), this pin is Bit 0 of the parallel port data output bus and the
data coding is straight binary. In all other modes, this pin allows choice of straight binary/binary twos
complement. When OB/2C is HIGH, the digital output is straight binary; when LOW, the MSB is
inverted, resulting in a twos complement output from its internal shift register.
6, 7, 40
42, 45
NC
No
Connect.
8 D1/A0 DI/O
When MODE = 0 (18-bit interface mode), this pin is Bit 1 of the parallel port data output bus. In all
other modes, this input pin controls the form in which data is output, as shown in Table 7.
9 D2/A1 DI/O
When MODE = 0 or 1 (18-bit or 16-bit interface mode), this pin is Bit 2 of the parallel port data output
bus. In all other modes, this input pin controls the form in which data is output, as shown in Table 7.
10 D3
DO
In all modes except MODE = 3, this output is used as Bit 3 of the parallel port data output bus. This pin
is always an output, regardless of the interface mode.
11, 12
D[4:5]or
DIVSCLK[0:1]
DI/O
In all modes except MODE = 3, these pins are Bit 4 and Bit 5 of the parallel port data output bus.
When MODE = 3 (serial mode), EXT/INT is LOW, and RDC/SDIN is LOW (serial master read after
convert), these inputs, part of the serial port, are used to slow down, if desired, the internal serial clock
that clocks the data output. In other serial modes, these pins are not used.
13 D6
or EXT/INT
DI/O
In all modes except MODE = 3, this output is used as Bit 6 of the parallel port data output bus.
When MODE = 3 (serial mode), this input, part of the serial port, is used as a digital select input for
choosing the internal data clock or an external data clock. With EXT/INT tied LOW, the internal clock is
selected on the SCLK output. With EXT/INT set to a logic HIGH, output data is synchronized to an
external clock signal connected to the SCLK input.
AD7679
Pin No.
Mnemonic
Type
1
Description
14 D7
or INVSYNC
DI/O
In all modes except MODE = 3, this output is used as Bit 7 of the parallel port data output bus.
When MODE = 3 (serial mode), this input, part of the serial port, is used to select the active state of the
SYNC signal. When LOW, SYNC is active HIGH. When HIGH, SYNC is active LOW.
15 D8
or INVSCLK
DI/O
In all modes except MODE = 3, this output is used as Bit 8 of the parallel port data output bus.
When MODE = 3 (serial mode), this input, part of the serial port, is used to invert the SCLK signal. It is
active in both master and slave mode.
16 D9
or RDC/SDIN
DI/O
In all modes except MODE = 3, this output is used as Bit 9 of the parallel port data output bus.
When MODE = 3 (serial mode), this input, part of the serial port, is used as either an external data
input or a read mode selection input depending on the state of EXT/INT. When EXT/ INT is HIGH,
RDC/SDIN could be used as a data input to daisy-chain the conversion results from two or more ADCs
onto a single SDOUT line. The digital data level on SDIN is output on SDOUT with a delay of 18 SCLK
periods after the initiation of the read sequence. When EXT/INT is LOW, RDC/SDIN is used to select the
read mode. When RDC/SDIN is HIGH, the data is output on SDOUT during conversion. When
RDC/SDIN is LOW, the data can be output on SDOUT only when the conversion is complete.
17
OGND
P
Input/Output Interface Digital Power Ground.
18 OVDD
P Output Interface Digital Power. Nominally at the same supply as the host interface (5 V or 3 V). Should
not exceed DVDD by more than 0.3 V.
19
DVDD
P
Digital Power. Nominally at 5 V.
20
DGND
P
Digital Power Ground.
21 D10
or SDOUT
DO
In all modes except MODE = 3, this output is used as Bit 10 of the parallel port data output bus.
When MODE = 3 (serial mode), this output, part of the serial port, is used as a serial data output
synchronized to SCLK. Conversion results are stored in an on-chip register. The AD7679 provides the
conversion result, MSB first, from its internal shift register. The data format is determined by the logic
level of OB/2C. In serial mode when EXT/INT is LOW, SDOUT is valid on both edges of SCLK. In serial
mode when EXT/INT is HIGH and INVSCLK is LOW, SDOUT is updated on the SCLK rising edge and is
valid on the next falling edge; if INVSCLK is HIGH, SDOUT is updated on the SCLK falling edge and is
valid on the next rising edge.
22 D11
or SCLK
DI/O
In all modes except MODE = 3, this output is used as Bit 11 of the parallel port data output bus.
When MODE = 3 (serial mode), this pin, part of the serial port, is used as a serial data clock input or
output, dependent upon the logic state of the EXT/INT pin. The active edge where the data SDOUT is
updated depends upon the logic state of the INVSCLK pin.
23 D12
or SYNC
DO
In all modes except MODE = 3, this output is used as Bit 12 of the parallel port data output bus.
When MODE = 3 (serial mode), this output, part of the serial port, is used as a digital output frame
synchronization for use with the internal data clock (EXT/INT = Logic LOW). When a read sequence is
initiated and INVSYNC is LOW, SYNC is driven HIGH and remains HIGH while the SDOUT output is
valid. When a read sequence is initiated and INVSYNC is HIGH, SYNC is driven LOW and remains LOW
while SDOUT output is valid.
24 D13
or RDERROR
DO
In all modes except MODE = 3, this output is used as Bit 13 of the parallel port data output bus.
In MODE = 3 (serial mode) and when EXT/ INT is HIGH, this output, part of the serial port, is used as an
incomplete read error flag. In slave mode, when a data read is started and not complete when the
following conversion is complete, the current data is lost and RDERROR is pulsed high.
2528 D[14:17]
DO Bit 14 to Bit 17 of the Parallel Port Data Output Bus. These pins are always outputs regardless of the
interface mode.
29 BUSY
DO
Busy Output. Transitions HIGH when a conversion is started. Remains HIGH until the conversion is
complete and the data is latched into the on-chip shift register. The falling edge of BUSY could be
used as a data ready clock signal.
30
DGND
P
Must Be Tied to Digital Ground.
31
RD
DI
Read Data. When CS and RD are both LOW, the interface parallel or serial output bus is enabled.
32
CS
DI
Chip Select. When CS and RD are both LOW, the interface parallel or serial output bus is enabled. CS is
also used to gate the external clock.
33 RESET DI
Reset Input. When set to a logic HIGH, reset the AD7679. Current conversion, if any, is aborted. If not
used, this pin could be tied to DGND.
34 PD
DI
Power-Down Input. When set to a logic HIGH, power consumption is reduced and conversions are
inhibited after the current one is completed.
Rev. 0 | Page 9 of 28
AD7679
Pin No.
Mnemonic
Type
1
Description
35
CNVST
DI
Start Conversion. If CNVST is held HIGH when the acquisition phase (t
8
) is complete, the next falling
edge on CNVST puts the internal sample/hold into the hold state and initiates a conversion. If CNVST
is held LOW when the acquisition phase is complete, the internal sample/hold is put into the hold
state and a conversion is started immediately.
36
AGND
P
Must Be Tied to Analog Ground.
37 REF
AI Reference Input Voltage and Internal Reference Buffer Output. Apply an external reference on this pin
if the internal reference buffer is not used. Should be decoupled effectively with or without the
internal buffer.
38
REFGND
AI
Reference Input Analog Ground.
39
IN
AI
Differential Negative Analog Input.
43
IN+
AI
Differential Positive Analog Input.
46 REFBUFIN
AI Reference Buffer Input Voltage. The internal reference buffer has a fixed gain. It outputs 4.096 V
typically when 2.5 V is applied on this pin.
48 PDBUF DI
Allows Choice of Buffering Reference. When LOW, buffer is selected. When HIGH, buffer is
switched off.
1
AI = Analog Input; AO = Analog Output; DI = Digital Input; DI/O = Bidirectional Digital; DO = Digital Output; P = Power.
Table 7. Data Bus Interface Definitions
MODE MODE1 MODE0 D0/OB/2C D1/A0 D2/A1 D[3] D[4:9] D[10:11] D[12:15] D[16:17] Description
0
0
0
R[0]
R[1] R[2] R[3]
R[4:9]
R[10:11] R[12:15] R[16:17] 18-Bit
Parallel
1 0 1 OB/2C
A0:0 R[2] R[3]
R[4:9]
R[10:11]
R[12:15] R[16:17] 16-Bit
High
Word
1 0 1 OB/2C
A0:1
R[0]
R[1]
All Zeros
16-Bit Low Word
2 1 0 OB/2C
A0:0
A1:0
All Hi-Z
R[10:11]
R[12:15]
R[16:17]
8-Bit HIGH Byte
2 1 0 OB/2C
A0:0
A1:1
All Hi-Z
R[2:3]
R[4:7]
R[8:9]
8-Bit MID Byte
2 1 0 OB/2C
A0:1
A1:0
All Hi-Z
R[0:1]
All Zeros
8-Bit LOW Byte
2 1 0 OB/2C
A0:1
A1:1
All Hi-Z
All Zeros
R[0:1]
8-Bit LOW Byte
3 1 1 OB/2C
All Hi-Z
Serial Interface
Serial Interface
R[0:17] is the 18-bit ADC value stored in its output register.
Rev. 0 | Page 10 of 28
AD7679
Rev. 0 | Page 11 of 28
DEFINITION OF SPECIFICATIONS
Integral Nonlinearity Error (INL)
Linearity error refers to the deviation of each individual code
from a line drawn from negative full scale through positive full
scale. The point used as negative full scale occurs LSB before
the first code transition. Positive full scale is defined as a level
1 LSB beyond the last code transition. The deviation is
measured from the middle of each code to the true straight line.
Differential Nonlinearity Error (DNL)
In an ideal ADC, code transitions are 1 LSB apart. Differential
nonlinearity is the maximum deviation from this ideal value. It
is often specified in terms of resolution for which no missing
codes are guaranteed.
Gain Error
The first transition (from 000...00 to 000...01) should occur for
an analog voltage LSB above the nominal full scale
(4.095991 V for the 4.096 V range). The last transition (from
111...10 to 111...11) should occur for an analog voltage
1 LSB below the nominal full scale (4.095977 V for the
4.096 V range). The gain error is the deviation of the
difference between the actual level of the last transition and the
actual level of the first transition from the difference between
the ideal levels.
Zero Error
The zero error is the difference between the ideal midscale
input voltage (0 V) from the actual voltage producing the
midscale output code.
Spurious-Free Dynamic Range (SFDR)
SFDR is the difference, in decibels (dB), between the rms
amplitude of the input signal and the peak spurious signal.
Effective Number of Bits (ENOB)
ENOB is a measurement of the resolution with a sine wave
input, and is expressed in bits. It is related to S/(N+D) by the
following formula:
ENOB = (S/[N+D]dB 1.76)/6.02
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of the first five harmonic
components to the rms value of a full-scale input signal, and is
expressed in decibels.
Dynamic Range
Dynamic range is the ratio of the rms value of the full scale to
the rms noise measured with the inputs shorted together. The
value for dynamic range is expressed in decibels.
Signal-to-Noise Ratio (SNR)
SNR is the ratio of the rms value of the actual input signal to the
rms sum of all other spectral components below the Nyquist
frequency, excluding harmonics and dc. The value for SNR is
expressed in decibels.
Signal-to-(Noise + Distortion) Ratio (S/[N+D])
S/(N+D) is the ratio of the rms value of the actual input signal
to the rms sum of all other spectral components below the
Nyquist frequency, including harmonics but excluding dc. The
value for S/(N+D) is expressed in decibels.
Aperture Delay
Aperture delay is a measure of the acquisition performance and
is measured from the falling edge of the CNVST input to when
the input signal is held for a conversion.
Transient Response
Transient response is the time required for the AD7679 to
achieve its rated accuracy after a full-scale step function is
applied to its input.
AD7679
Rev. 0 | Page 12 of 28
TYPICAL PERFORMANCE CHARACTERISTICS
CODE
2.5
0
65536
131072
196608
262144
INL-LSB (18-Bit)
1.5
1.0
0
2.5
0.5
0.5
03085-0-005
1.0
2.0
2.0
1.5
Figure 5. Integral Nonlinearity vs. Code
CODE IN HEX
70000
COUNTS
50000
30000
0
20000
10000
40000
1FEBE
1FEBD
1FEC0 1FEC1 1FEC2 1FEC3 1FEC4 1FEC5 1FEC6
V
REF
= 5V
60000
0
0
0
0
03085-0-006
73
7584
58510 59001
4616
264
Figure 6. Histogram of 131,072 Conversions of a
DC Input at the Code Transition
POSITIVE INL (LSB)
100
0
NUMBE
R OF UNITS
60
20
0
40
80
0.5
1.0
1.5
2.0
03085-0-007
2.5
Figure 7. Typical Positive INL Distribution (424 Units)
CODE
2.0
0
65536
131072
196608
262144
DNL-LS
B
(1
8
-
Bi
t)
1.5
1.0
0
1.0
0.5
0.5
03085-0-008
Figure 8. Differential Nonlinearity vs. Code
CODE IN HEX
90000
COUNTS
60000
40000
20000
0
30000
10000
50000
1FEBF
1FEBE
1FEC0 1FEC1 1FEC2 1FEC3 1FEC4 1FEC5 1FEC6
V
REF
= 5V
70000
80000
0
59
799
2
0
22496
23080
3838
03085-0-009
Figure 9. Histogram of 131,072 Conversions of a
DC Input at the Code Center
NEGATIVE INL (LSB)
120
2.5
NUMBE
R OF UNITS
80
60
20
0
2.0
1.5
1.0
0.5
03085-0-010
100
40
0
Figure 10. Typical Negative INL Distribution (424 Units)
AD7679
Rev. 0 | Page 13 of 28
POSITIVE DNL (LSB)
120
0
NUMBE
R OF UNITS
100
60
20
0
40
80
0.5
1.0
1.5
03085-0-011
2.0
Figure 11. Typical Positive DNL Distribution (424 Units)
NEGATIVE DNL (LSB)
180
1.00
NUMBE
R OF UNITS
160
100
20
0
60
140
0.75
0.50
0.25
03085-0-014
0
120
40
80
Figure 12. Typical Negative DNL Distribution (424 Units)
FREQUENCY (kHz)
0
0
30
60
240
270
AMP
L
ITUDE
(dB of Full S
c
a
l
e
)
40
60
100
180
80
120
03085-0-012
140
20
160
90
120
150
180
210
f
S
= 570kSPS
f
IN
= 10kHz
V
REF
= 4.096V
SNR = 98.5dB
THD = 115.8dB
SFDR = 117.4dB
S/(N+D) = 98.4dB
Figure 13. FFT (10 kHz Tone)
FREQUENCY (kHz)
105
1
S
NR AND S
/
[N+D] (dB)
100
90
80
75
85
10
100
1000
03085-0-015
95
ENOB
S/(N+D)
SNR
16.5
17.0
15.5
16.0
14.5
15.0
14.0
ENOB (
B
it
s)
Figure 14. SNR, S/(N+D), and ENOB vs. Frequency
FREQUENCY (kHz)
60
1
THD, HARMONICS
(dB)
70
100
130
120
80
10
100
1000
03085-0-016
90
110
THD
THIRD
HARMONIC
SFDR
140
60
20
40
0
S
F
DR (dB)
120
80
100
SECOND
HARMONIC
Figure 15. THD, SFDR, and Harmonics vs. Frequency
INPUT LEVEL (dB)
60
S
NR RE
FE
RRE
D TO FULL S
CALE
(dB)
98
96
100
03085-0-017
SNR
50
0
10
20
30
40
102
V
REF
= 4.096V
S/(N+D)
Figure 16. SNR and S/(N+D) vs. Input Level
AD7679
Rev. 0 | Page 14 of 28
55
S
NR,
S
/
[N+D] (dB) 98
96
03085-0-018
99
97
35
125
85
65
5
15
100
SNR
ENOB
14.5
15.0
15.5
16.5
16.0
S/(N+D)
25
45
105
TEMPERATURE (
C)
Figure 17. SNR, S/(N+D), and ENOB vs. Temperature
TEMPERATURE (
C)
55
THD, HARMONICS
(dB)
120
140
03085-0-019
110
130
35
125
85
65
5
15
100
25
45
105
THIRD
HARMONIC
SECOND
HARMONIC
THD
Figure 18. THD and Harmonics vs. Temperature
SAMPLING RATE (SPS)
100000
OP
E
RATING CURRE
NTS
(
A)
0.001
1M
10000
1000
100
10
1
0.1
0.01
100k
10k
1k
100
1
AVDD
10
DVDD
OVDD
PDBUF HIGH
03085-0-020
Figure 19. Operating Current vs. Sampling Rate
TEMPERATURE (
C)
1000
55
P
O
WE
R-DOWN OP
E
RATING CURRE
NTS
(nA)
700
400
200
0
300
100
500
125
DVDD
35
15
5
25
45
65
85
105
AVDD
OVDD
03085-0-021
500
800
900
Figure 20. Power-Down Operating Currents vs. Temperature
TEMPERATURE (
C)
55
ZE
RO E
RROR,P
O
S
I
TIV
E
AND
N
E
GA
TIVE FU
LL SC
A
L
E (
L
SB
)
15
25
03085-0-022
20
35
125
85
65
5
15
25
25
45
105
5
5
10
0
10
15
20
POSITIVE
FULL SCALE
NEGATIVE
FULL SCALE
ZERO ERROR
Figure 21. Zero Error Positive and Negative Full Scale vs. Temperature
C
L
(pF)
0
t
12
DE
LAY
(ns
)
10
0
03085-0-024
200
150
50
50
100
30
20
40
OVDD = 2.7V @ 85C
OVDD = 5V @ 85C
OVDD = 5V @ 25C
OVDD = 2.7V @ 25C
Figure 22. Typical Delay vs. Load Capacitance C
L
AD7679
Rev. 0 | Page 15 of 28
CIRCUIT INFORMATION
IN+
REF
REFGND
IN
MSB
4C
2C
C
C
LSB
SW+
SWITCHES
CONTROL
262,144C 131,072C
MSB
4C
2C
C
C
LSB
SW
BUSY
OUTPUT
CODE
CNVST
CONTROL
LOGIC
COMP
262,144C 131,072C
030850025
Figure 23. ADC Simplified Schematic
The AD7679 is a very fast, low power, single-supply, precise
18-bit analog-to-digital converter (ADC) using successive
approximation architecture.
CONVERTER OPERATION
The AD7679 is a successive approximation ADC based on a
charge redistribution DAC. Figure 23 shows the simplified
schematic of the ADC. The capacitive DAC consists of two
identical arrays of 18 binary weighted capacitors that are
connected to the two comparator inputs.
The AD7679's linearity and dynamic range are similar or better
than many - ADCs. With the advantages of its successive
architecture, which ease multiplexing and reduce power with
throughput, it can be advantageous in applications that
normally use - ADCs.
During the acquisition phase, terminals of the array tied to the
comparator's input are connected to AGND via SW+ and SW.
All independent switches are connected to the analog inputs.
Thus, the capacitor arrays are used as sampling capacitors and
acquire the analog signal on IN+ and IN inputs. When the
acquisition phase is complete and the CNVST input goes low, a
conversion phase is initiated. When the conversion phase
begins, SW+ and SW are opened first. The two capacitor arrays
are then disconnected from the inputs and connected to the
REFGND input. Therefore, the differential voltage between the
IN+ and IN inputs captured at the end of the acquisition phase
is applied to the comparator inputs, causing the comparator to
become unbalanced. By switching each element of the capacitor
array between REFGND and REF, the comparator input varies
by binary weighted voltage steps (V
REF
/2, V
REF
/4...V
REF
/262144).
The control logic toggles these switches, starting with the MSB
first, to bring the comparator back into a balanced condition.
After completing this process, the control logic generates the
ADC output code and brings the BUSY output low.
The AD7679 provides the user with an on-chip track/hold,
successive approximation ADC that does not exhibit any
pipeline or latency, making it ideal for multiple multiplexed
channel applications.
The AD7679 can be operated from a single 5 V supply and can
be interfaced to either 5 V or 3 V digital logic. It is housed in a
48-lead LQFP, or a tiny 48-lead LFCSP that offers space savings
and allows for flexible configurations as either a serial or
parallel interface. The AD7679 is pin-to-pin compatible with
the AD7674, AD7676, and AD7678.
AD7679
Transfer Functions
Except in 18-bit interface mode, the AD7679 offers straight
binary and twos complement output coding when using OB/2C.
See Figure 24 and Table 8 for the ideal transfer characteristic.
000...000
000...001
000...010
111...101
111...110
111...111
ANALOG INPUT
+FS 1.5 LSB
+FS 1 LSB
FS + 1 LSB
FS
FS + 0.5 LSB
ADC CODE
(S
tra
i
ght Bina
ry
)
03085-0-026
Figure 24. ADC Ideal Transfer Function
Table 8. Output Codes and Ideal Input Voltages
Description
Analog Input
V
REF
= 4.096 V
Straight
Binary
(Hex)
Twos
Complement
(Hex)
FSR 1 LSB
4.095962 V
3FFFF
1
1FFFF
1
FSR 2 LSB
4.095924 V
3FFFE
1FFFE
Midscale +
1 LSB
31.25 V
20001
00001
Midscale
0 V
20000
00000
Midscale
1 LSB
31.25 V
1FFFF
3FFFF
FSR + 1 LSB
-4.095962 V
00001
20001
FSR -4.096
V
00000
2
20000
2
1
This is also the code for overrange analog input (V
IN+
V
IN
above V
REF
V
REFGND
).
2
This is also the code for underrange analog input (V
IN+
V
IN
below V
REF
+ V
REFGND
).
AVDD
AGND
DGND
DVDD
OVDD
OGND
CNVST
BUSY
SDOUT
SCLK
RD
CS
RESET
PD
2.5V REF
NOTE 1
REFBUFIN
20
D
CLOCK
AD7679
C/P/DSP
SERIAL PORT
DIGITAL SUPPLY
(3.3V OR 5V)
ANALOG
SUPPLY
(5V)
DVDD
OB/2C
NOTE 6
PDBUF
DVDD
50k
100nF
1M
IN+
ANALOG INPUT+
C
C
2.7nF
U1
NOTE 3
NOTE 4
AD8021
+
30
NOTE 2
NOTE 5
ADR421
10
F
100nF
+
10
F
100nF
+
100nF
+
10
F
IN
ANALOG INPUT
C
C
2.7nF
U2
NOTE 3
NOTE 4
AD8021
+
30
50
100nF
NOTES
1. SEE VOLTAGE REFERENCE INPUT SECTION.
2. OPTIONAL CIRCUITRY FOR HARDWARE GAIN CALIBRATION.
3.THE AD8021 IS RECOMMENDED. SEE DRIVER AMPLIFIER CHOICE SECTION.
4. SEE ANALOG INPUTS SECTION.
5. OPTION, SEE POWER SUPPLY SECTION.
6. OPTIONAL LOW JITTER CNVST, SEE CONVERSION CONTROL SECTION.
47
F
MODE1
MODE0
NOTE 1
C
REF
REF
REFGND
03085-0-027
Figure 25. Typical Connection Diagram (Internal Reference Buffer, Serial Interface)
Rev. 0 | Page 16 of 28
AD7679
TYPICAL CONNECTION DIAGRAM
Figure 25 shows a typical connection diagram for the AD7679.
Different circuitry shown on this diagram is optional and is
discussed later in this data sheet.
Analog Inputs
Figure 26 shows a simplified analog input section of the
AD7679. The diodes shown in Figure 26 provide ESD
protection for the inputs. Care must be taken to ensure that the
analog input signal never exceeds the absolute ratings on these
inputs. This will cause these diodes to become forward biased
and start conducting current. These diodes can handle a
forward-biased current of 120 mA max. This condition could
eventually occur when the input buffer's U1 or U2 supplies are
different from AVDD. In such a case, an input buffer with a
short-circuit current limitation can be used to protect the part.
IN+
IN
AGND
AVDD
R+ = 102
C
S
C
S
R = 102
03085-0-028
Figure 26. Simplified Analog Input
This analog input structure is a true differential structure. By
using these differential inputs, signals common to both inputs
are rejected as shown in Figure 27, which represents typical
CMRR over frequency.
FREQUECY (kHz)
80
CMRR (dB)
75
50
100
1000
10000
1
10
70
65
60
55
03085-0-029
Figure 27. Analog Input CMRR vs. Frequency
During the acquisition phase for ac signals, the AD7679 behaves
like a 1-pole RC filter consisting of the equivalent resistance,
R+, R, and C
S
. Resistors R+ and R are typically 102 and are
lumped components made up of a serial resistor and the on
resistance of the switches. C
S
is typically 60 pF and mainly
consists of the ADC sampling capacitor. This 1-pole filter with a
3 dB cutoff frequency of 26 MHz typ reduces any undesirable
aliasing effect and limits the noise coming from the inputs.
Because the input impedance of the AD7679 is very high, the
part can be driven directly by a low impedance source without
gain error. This allows the user to put an external 1-pole RC
filter between the amplifier output and the ADC analog inputs,
as shown in Figure 25, to even further improve the noise
filtering done by the AD7679 analog input circuit. However, the
source impedance has to be kept low because it affects the ac
performance, especially the total harmonic distortion (THD).
The maximum source impedance depends on the amount of
THD that can be tolerated. The THD degrades as a function of
source impedance and the maximum input frequency, as shown
in Figure 28.
INPUT RESISTANCE (
)
95
THD (dB)
120
45
75
105
15
100
105
110
115
20kHz
10kHz
2kHz
03085-0-030
Figure 28. THD vs. Analog Input Frequency and Source Resistance
Driver Amplifier Choice
Although the AD7679 is easy to drive, the driver amplifier
needs to meet the following requirements:
The driver amplifier and the AD7679 analog input circuit
have to be able to settle for a full-scale step of the capacitor
array at an 18-bit level (0.0004%). In the amplifier's data
sheet, settling at 0.1% or 0.01% is more commonly
specified. This could differ significantly from the settling
time at an 18-bit level and, therefore, should be verified
prior to driver selection. The tiny op amp AD8021, which
combines ultralow noise and high gain-bandwidth, meets
this settling time requirement.
The noise generated by the driver amplifier needs to be
kept as low as possible in order to preserve the SNR and
transition noise performance of the AD7679. The noise
coming from the driver is filtered by the AD7679 analog
input circuit 1-pole low-pass filter made by R+, R, and C
S
.
Rev. 0 | Page 17 of 28
AD7679
Rev. 0 | Page 18 of 28
U2
8.25k
2.5V
AD8021
590
AD7679
IN+
IN REF
U1
ANALOG INPUT
(UNIPOLAR
0V TO 4.096V)
10pF
AD8021
590
10pF
10
F
100nF
1.82k
2.7nF
30
2.7nF
30
REFBUFIN
03085-0-031
The SNR degradation due to the amplifier is
=
+
2
)
(
625
25
log
20
N
3dB
LOSS
Ne
f
SNR
where:
f
3dB
is the 3 dB input bandwidth in MHz of the AD7679
(26 MHz) or the cutoff frequency of the input filter, if used.
N is the noise factor of the amplifiers (1 if in buffer
configuration).
e
N
is the equivalent input noise voltage of each op amp in
nV/Hz.
Figure 29. Single-Ended-to-Differential Driver Circuit
(Internal Reference Buffer Used)
For instance, for a driver with an equivalent input noise of
2 nV/Hz (e.g., AD8021) configured as a buffer, thus with a
noise gain of +1, the SNR degrades by only 0.34 dB with
the filter in Figure 25, and by 1.8 dB without it.
Voltage Reference
The AD7679 allows the use of an external voltage reference
either with or without the internal reference buffer.
Using the internal reference buffer is recommended when
sharing a common reference voltage between multiple ADCs is
desired.
The driver needs to have a THD performance suitable to
that of the AD7679.
The
AD8021
meets these requirements and is usually
appropriate for almost all applications. The AD8021 needs a
10 pF external compensation capacitor, which should have good
linearity as an NPO ceramic or mica type.
However, the advantages of using the external reference voltage
directly are
The SNR and dynamic range improvement (about 1.7 dB)
resulting from the use of a reference voltage very close to
the supply (5 V) instead of a typical 4.096 V reference when
the internal buffer is used.
The
AD8022
could be used if a dual version is needed and gain
of 1 is present. The
AD829
is an alternative in applications
where high frequency (above 100 kHz) performance is not
required. In gain of 1 applications, it requires an 82 pF
compensation capacitor. The
AD8610
is another option when
low bias current is needed in low frequency applications.
The power saving when the internal reference buffer is
powered down (PDBUF high).
To use the internal reference buffer, PDBUF should be LOW. A
2.5 V reference voltage applied on the REFBUFIN input will
result in a 4.096 V reference on the REF pin.
Single-to-Differential Driver
For applications using unipolar analog signals, a single-ended-
to-differential driver will allow for a differential input into the
part. The schematic is shown in Figure 29. When provided an
input signal of 0 to V
REF
, this configuration will produce a
differential V
REF
with midscale at V
REF
/2.
In both cases, the voltage reference input REF has a dynamic
input impedance and therefore requires an efficient decoupling
between REF and REFGND inputs. The decoupling consists of a
low ESR 47 F tantalum capacitor connected to the REF and
REFGND inputs with minimum parasitic inductance.
If the application can tolerate more noise, the AD8138,
differential driver can be used.
Care should also be taken with the reference temperature
coefficient of the voltage reference, which directly affects the
full-scale accuracy if this parameter matters. For instance, a
4 ppm/C temperature coefficient of the reference changes the
full scale by 1 LSB/C.
AD7679
Rev. 0 | Page 19 of 28
SAMPLING RATE (SPS)
1000000
POW
E
R
D
I
SSA
PA
TION
(
W)
1M
100000
10000
1000
100
10
1
0.1
100k
10k
1k
100
1
10
PDBUF HIGH
03085-0-033
Power Supply
The AD7679 uses three sets of power supply pins: an analog 5 V
supply (AVDD), a digital 5 V core supply (DVDD), and a digital
output interface supply (OVDD). The OVDD supply defines the
output logic level and allows direct interface with any logic
working between 2.7 V and DVDD + 0.3 V. To reduce the
number of supplies needed, the digital core (DVDD) can be
supplied through a simple RC filter from the analog supply, as
shown in Figure 25. The AD7679 is independent of power
supply sequencing once OVDD does not exceed DVDD by
more than 0.3 V, and is therefore free from supply voltage
induced latch-up. Additionally, it is very insensitive to power
supply variations over a wide frequency range (see Figure 30).
FREQUECY (kHz)
65
P
S
RR (dB)
40
100
1000
10000
1
10
60
55
50
45
03085-0-032
Figure 31. Power Dissipation vs. Sample Rate
CONVERSION CONTROL
Figure 32 shows the detailed timing diagrams of the conversion
process. The AD7679 is controlled by the CNVST signal, which
initiates conversion. Once initiated, it cannot be restarted or
aborted, even by PD, until the conversion is complete. The
CNVST signal operates independently of CS and RD.
CNVST
t
1
t
2
MODE ACQUIRE
CONVERT
ACQUIRE
CONVERT
t
7
t
8
BUSY
t
4
t
3
t
5
t
6
03085-0-034
Figure 30. PSRR vs. Frequency
POWER DISSIPATION VERSUS THROUGHPUT
The AD7679 automatically reduces its power consumption at
the end of each conversion phase. During the acquisition phase,
the operating currents are very low, which allows for a
significant power savings when the conversion rate is reduced,
as shown in Figure 31. This feature makes the AD7679 ideal for
very low power battery applications.
Figure 32. Basic Conversion Timing
Although CNVST is a digital signal, it should be designed with
special care with fast, clean edges and levels with minimum
overshoot and undershoot or ringing.
It should be noted that the digital interface remains active even
during the acquisition phase. To reduce the operating digital
supply currents even further, the digital inputs need to be
driven close to the power rails (DVDD and DGND), and
OVDD should not exceed DVDD by more than 0.3 V.
For applications where SNR is critical, the CNVST signal should
have very low jitter. This may be achieved by using a dedicated
oscillator for CNVST generation, or to clock it with a high
frequency low jitter clock, as shown in Figure 25.
For other applications, conversions can be automatically
initiated. If CNVST is held low when BUSY is low, the AD7679
controls the acquisition phase and automatically initiates a new
conversion. By keeping CNVST low, the AD7679 keeps the
conversion process running by itself. It should be noted that the
analog input has to be settled when BUSY goes low. Also, at
power-up, CNVST should be brought low once to initiate the
conversion process. In this mode, the AD7679 could sometimes
run slightly faster than the guaranteed limits of 570 kSPS.
AD7679
Rev. 0 | Page 20 of 28
DIGITAL INTERFACE
The AD7679 has a versatile digital interface; it can be interfaced
with the host system by using either a serial or parallel interface.
The serial interface is multiplexed on the parallel data bus. The
AD7679 digital interface also accommodates both 3 V and 5 V
logic by simply connecting the AD7679's OVDD supply pin to
the host system interface digital supply. Finally, by using the
OB/2C input pin in any mode but 18-bit interface mode, both
twos complement and straight binary coding can be used.
The two signals, CS and RD, control the interface. When at least
one of these signals is high, the interface outputs are in high
impedance. Usually, CS allows the selection of each AD7679 in
multicircuit applications, and is held low in a single AD7679
design. RD is generally used to enable the conversion result on
the data bus.
t
9
RESET
DATA
BUS
BUSY
CNVST
t
8
03085-0-035
Figure 33. RESET Timing
CNVST
BUSY
DATA
BUS
CS = RD = 0
PREVIOUS CONVERSION DATA
NEW DATA
t
1
t
10
t
4
t
3
t
11
03085-0-036
Figure 34. Master Parallel Data Timing for Reading (Continuous Read)
PARALLEL INTERFACE
The AD7679 is configured to use the parallel interface with an
18-bit, a 16-bit, or an 8-bit bus width, according to Table 7. The
data can be read either after each conversion, which is during
the next acquisition phase, or during the following conversion,
as shown in Figure 35 and Figure 36, respectively. When the
data is read during the conversion, however, it is recommended
that it is read only during the first half of the conversion phase.
This avoids any potential feedthrough between voltage
transients on the digital interface and the most critical analog
conversion circuitry. Refer to Table 7 for a detailed description
of the different options available.
DATA
BUS
t
12
t
13
BUSY
CS
RD
CURRENT
CONVERSION
03085-0-037
Figure 35. Slave Parallel Data Timing for Reading (Read after Convert)
CS = 0
CNVST,
RD
t
1
PREVIOUS
CONVERSION
DATA
BUS
t
12
t
13
BUSY
t
4
t
3
03085-0-038
Figure 36. Slave Parallel Data Timing for Reading (Read during Convert)
CS
RD
A0, A1
PINS D[15:8]
PINS D[7:0]
HI-Z
HI-Z
HIGH BYTE
LOW BYTE
LOW BYTE
HIGH BYTE
HI-Z
HI-Z
t
12
t
12
t
13
03085-0-039
Figure 37. 8-Bit and 16-Bit Parallel Interface
SERIAL INTERFACE
The AD7679 is configured to use the serial interface when
MODE0 and MODE1 are held high. The AD7679 outputs 18
bits of data, MSB first, on the SDOUT pin. This data is
synchronized with the 18 clock pulses provided on the SCLK
pin. The output data is valid on both the rising and falling edge
of the data clock.
AD7679
MASTER SERIAL INTERFACE
In Read during Conversion mode, the serial clock and data
toggle at appropriate instants, minimizing potential feedthrough
between digital activity and critical conversion decisions.
Internal Clock
The AD7679 is configured to generate and provide the serial
data clock SCLK when the EXT/INT pin is held low. The
AD7679 also generates a SYNC signal to indicate to the host
when the serial data is valid. The serial clock SCLK and the
SYNC signal can be inverted if desired. Depending on the
RDC/SDIN input, the data can be read after each conversion or
during the following conversion. Figure 38 and Figure 39 show
the detailed timing diagrams of these two modes.
In Read after Conversion mode, it should be noted that unlike
in other modes, the BUSY signal returns low after the 18 data
bits are pulsed out and not at the end of the conversion phase,
which results in a longer BUSY width.
To accommodate slow digital hosts, the serial clock can be
slowed down by using DIVSCLK.
Usually, because the AD7679 is used with a fast throughput, the
mode master read during conversion is the most recommended
serial mode.
t
3
BUSY
CS, RD
CNVST
SYNC
SCLK
SDOUT
1
2
3
16
17
18
D17
D16
D2
D1
D0
X
EXT/INT = 0
RDC/SDIN = 0
INVSCLK = INVSYNC = 0
t
14
t
20
t
15
t
16
t
22
t
23
t
29
t
28
t
18
t
19
t
21
t
30
t
25
t
24
t
26
t
27
03085-0-040
Figure 38. Master Serial Data Timing for Reading (Read after Convert)
Rev. 0 | Page 21 of 28
AD7679
RDC/SDIN = 1
INVSCLK = INVSYNC = 0
D17
D16
D2
D1
D0
X
1
2
3
16
17
18
BUSY
SYNC
SCLK
SDOUT
CS, RD
CNVST
t
3
t
1
t
17
t
14
t
15
t
19
t
20
t
21
t
16
t
22
t
23
t
24
t
27
t
26
t
25
t
18
EXT/INT = 0
03085-0-041
Figure 39. Master Serial Data Timing for Reading (Read Previous Conversion during Convert)
SLAVE SERIAL INTERFACE
External Clock
The AD7679 is configured to accept an externally supplied
serial data clock on the SCLK pin when the EXT/INT pin is
held high. In this mode, several methods can be used to read the
data. The external serial clock is gated by CS. When CS and RD
are both low, the data can be read after each conversion or
during the following conversion. The external clock can be
either a continuous or a discontinuous clock. A discontinuous
clock can be either normally high or normally low when
inactive. Figure 40 and Figure 41 show the detailed timing
diagrams of these methods.
While the AD7679 is performing a bit decision, it is important
that voltage transients not occur on digital input/output pins or
degradation of the conversion result could occur. This is
particularly important during the second half of the conversion
phase because the AD7679 provides error correction circuitry
that can correct for an improper bit decision made during the
first half of the conversion phase. For this reason, it is
recommended that when an external clock is being provided, it
is a discontinuous clock that toggles only when BUSY is low or,
more importantly, that it does not transition during the latter
half of BUSY high.
External Discontinuous Clock Data Read after
Conversion
This mode is the most recommended of the serial slave modes.
Figure 40 shows the detailed timing diagrams of this method.
After a conversion is complete, indicated by BUSY returning
low, the result of this conversion can be read while both CS and
RD are low. Data is shifted out MSB first with 18 clock pulses,
and is valid on the rising and falling edge of the clock.
Among the advantages of this method, the conversion
performance is not degraded because there are no voltage
transients on the digital interface during the conversion process.
Also, data can be read at speeds up to 40 MHz, accommodating
both slow digital host interface and the fastest serial reading.
Finally, in this mode only, the AD7679 provides a daisy-chain
feature using the RDC/SDIN input pin to cascade multiple
converters together. This feature is useful for reducing
component count and wiring connections when desired (for
instance, in isolated multiconverter applications).
An example of the concatenation of two devices is shown in
Figure 42. Simultaneous sampling is possible by using a
common CNVST signal. It should be noted that the RDC/SDIN
input is latched on the edge of SCLK opposite the one used to
shift out data on SDOUT. Thus, the MSB of the upstream
converter follows the LSB of the downstream converter on the
next SCLK cycle.
Rev. 0 | Page 22 of 28
AD7679
Rev. 0 | Page 23 of 28
SCLK
SDOUT
D17
D16
D1
D0
D15
X17
X16
X15
X1
X0
Y17
Y16
BUSY
SDIN
INVSCLK = 0
X17
X16
X
1
2
3
17
18
19
20
EXT/INT = 1
RD = 0
t
35
t
36
t
37
t
31
t
32
t
34
t
16
t
33
CS
03085-0-042
Figure 40. Slave Serial Data Timing for Reading (Read after Convert)
SDOUT
SCLK
D1
D0
X
D17
D16
D15
1
2
3
17
18
BUSY
INVSCLK = 0
EXT/INT = 1
RD = 0
t
35
t
36
t
37
t
31
t
32
t
16
t
3
CS
CNVST
03085-0-043
Figure 41. Slave Serial Data Timing for Reading (Read Previous Conversion during Convert)
AD7679
BUSY
BUSY
AD7679
#2 (UPSTREAM)
AD7679
#1 (DOWNSTREAM)
RDC/SDIN
SDOUT
CNVST
CS
SCLK
RDC/SDIN
SDOUT
CNVST
CS
SCLK
DATA
OUT
SCLK IN
CS IN
CNVST IN
BUSY
OUT
03085-0-044
Figure 42. Two AD7679s in a Daisy-Chain Configuration
External Clock Data Read during Conversion
Figure 41 shows the detailed timing diagrams of this method.
During a conversion, while both CS and RD are low, the result
of the previous conversion can be read. The data is shifted out
MSB first with 18 clock pulses, and is valid on both the rising
and falling edge of the clock. The 18 bits have to be read before
the current conversion is complete. If that is not done,
RDERROR is pulsed high and can be used to interrupt the host
interface to prevent incomplete data reading. There is no daisy-
chain feature in this mode, and the RDC/SDIN input should
always be tied either high or low.
To reduce performance degradation due to digital activity, a fast
discontinuous clock is recommended to ensure that all bits are
read during the first half of the conversion phase. It is also
possible to begin to read the data after conversion and continue
to read the last bits even after a new conversion has been
initiated.
MICROPROCESSOR INTERFACING
The AD7679 is ideally suited for traditional dc measurement
applications supporting a microprocessor, and for ac signal
processing applications interfacing to a digital signal processor.
The AD7679 is designed to interface either with a parallel 8-bit
or 16-bit wide interface, or with a general-purpose serial port or
I/O ports on a microcontroller. A variety of external buffers can
be used with the AD7679 to prevent digital noise from coupling
into the ADC. The following section illustrates the use of the
AD7679 with an SPI equipped DSP, the ADSP-219x.
SPI Interface (ADSP-219x)
Figure 43 shows an interface diagram between the AD7679 and
the SPI equipped ADSP-219x. To accommodate the slower
speed of the DSP, the AD7679 acts as a slave device, and data
must be read after conversion. This mode also allows the daisy-
chain feature. The convert command could be initiated in
response to an internal timer interrupt. The 18-bit output data
are read with 3-byte SPI access. The reading process could be
initiated in response to the end-of-conversion signal (BUSY
going low) using an interrupt line of the DSP. The serial
interface (SPI) on the ADSP-219x is configured for master
mode (MSTR) = 1, Clock Polarity Bit (CPOL) = 0, Clock Phase
Bit (CPHA) = 1, and SPI interrupt enable (TIMOD) = 00, by
writing to the SPI Control register (SPICLTx). It should be
noted that to meet all timing requirements, the SPI clock should
be limited to 17 Mbits/s, which allow it to read an ADC result in
about 1.1 s. When a higher sampling rate is desired, use of one
of the parallel interface modes is recommended.
AD7679*
ADSP-219x*
SER/PAR
PFx
MISOx
SCKx
PFx or TFSx
BUSY
SDOUT
SCLK
CNVST
EXT/INT
CS
RD
INVSCLK
DVDD
* ADDITIONAL PINS OMITTED FOR CLARITY
SPIxSEL (PFx)
03085-0-045
Figure 43. Interfacing the AD7679 to an SPI Interface
Rev. 0 | Page 24 of 28
AD7679
Rev. 0 | Page 25 of 28
APPLICATION HINTS
LAYOUT
The AD7679 has very good immunity to noise on the power
supplies. However, care should still be taken with regard to
grounding layout.
The printed circuit board that houses the AD7679 should be
designed so that the analog and digital sections are separated
and confined to certain areas of the board. This facilitates the
use of ground planes that can be easily separated. Digital and
analog ground planes should be joined in only one place,
preferably underneath the AD7679, or at least as close to the
AD7679 as possible. If the AD7679 is in a system where
multiple devices require analog-to-digital ground connections,
the connection should still be made at one point only, a star
ground point that should be established as close to the AD7679
as possible.
The user should avoid running digital lines under the device, as
these will couple noise onto the die. The analog ground plane
should be allowed to run under the AD7679 to avoid noise
coupling. Fast switching signals like CNVST or clocks should be
shielded with digital ground to avoid radiating noise to other
sections of the board, and should never run near analog signal
paths. Crossover of digital and analog signals should be avoided.
Traces on different but close layers of the board should run at
right angles to each other. This will reduce the effect of
feedthrough through the board. The power supply lines to the
AD7679 should use as large a trace as possible to provide low
impedance paths and reduce the effect of glitches on the power
supply lines. Good decoupling is also important to lower the
supply's impedance presented to the AD7679 and to reduce the
magnitude of the supply spikes. Decoupling ceramic capacitors,
typically 100 nF, should be placed close to and ideally right up
against each power supply pin (AVDD, DVDD, and OVDD)
and their corresponding ground pins. Additionally, low ESR 10
F capacitors should be located near the ADC to further reduce
low frequency ripple.
The DVDD supply of the AD7679 can be a separate supply or
can come from the analog supply, AVDD, or the digital interface
supply, OVDD. When the system digital supply is noisy or when
fast switching digital signals are present, and if no separate
supply is available, the user should connect the DVDD digital
supply to the analog supply AVDD through an RC filter, (see
Figure 25), and connect the system supply to the interface
digital supply OVDD and the remaining digital circuitry. When
DVDD is powered from the system supply, it is useful to insert a
bead to further reduce high frequency spikes.
The AD7679 has four different ground pins: REFGND, AGND,
DGND, and OGND. REFGND senses the reference voltage and
should be a low impedance return to the reference because it
carries pulsed currents. AGND is the ground to which most
internal ADC analog signals are referenced. This ground must
be connected with the least resistance to the analog ground
plane. DGND must be tied to the analog or digital ground plane
depending on the configuration. OGND is connected to the
digital system ground.
The layout of the decoupling of the reference voltage is
important. The decoupling capacitor should be close to the
ADC and should be connected with short and large traces to
minimize parasitic inductances.
EVALUATING THE AD7679'S PERFORMANCE
A recommended layout for the AD7679 is outlined in the
documentation of the
EVAL-AD7679CB
evaluation board for
the AD7679. The evaluation board package includes a fully
assembled and tested evaluation board, documentation, and
software for controlling the board from a PC via the
EVAL-
CONTROL BRD2
.
AD7679
Rev. 0 | Page 26 of 28
OUTLINE DIMENSIONS
TOP VIEW
(PINS DOWN )
1
12
13
25
24
36
37
48
0.27
0.22
0.17
0.50
BSC
7.00
BSC SQ
SEATING
PLANE
1.60
MAX
0.75
0.60
0.45
VIEW A
9.00 BSC
SQ
PIN 1
0.20
0.09
1.45
1.40
1.35
0.10 MAX
COPLANARITY
VIEW A
ROTATED 90 CCW
SEATING
PLANE
7
3.5
0
10
6
2
0.15
0.05
COMPLIANT TO JEDEC STANDARDS MS-026BBC
Figure 44. 48-Lead Low Profile Quad Flatpack [LQFP] (ST-48)
(Dimensions shown in millimeters)
PIN 1
INDICATOR
TOP
VIEW
6.75
BSC SQ
7.00
BSC SQ
1
48
12
13
37
36
24
25
BOTTOM
VIEW
5.25
5.10
4.95
0.50
0.40
0.30
0.30
0.23
0.18
0.50 BSC
12 MAX
0.20
REF
0.80 MAX
0.65 NOM
1.00
0.90
0.80
5.50
REF
0.05 MAX
0.02 NOM
0.60 MAX
0.60 MAX
PIN 1
INDICATOR
COPLANARITY
0.08
SQ
SEATING
PLANE
COMPLIANT TO JEDEC STANDARDS MO-220-VKKD-2
PADDLE CONNECTED TO AGND.
THIS CONNECTION IS NOT
REQUIRED TO MEET THE
ELECTRICAL PERFORMANCE
Figure 45. 48-Lead Leadframe Chip Scale Package [LFCSP] (CP-48)
(Dimensions shown in millimeters)
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the
human body and test equipment and can discharge without detection. Although this product features proprietary
ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges.
Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
ORDERING GUIDE
Model
Temperature Range
Package Description
Package Option
AD7679AST
40C to +85C
Quad Flatpack (LQFP)
ST-48
AD7679ASTRL
40C to +85C
Quad Flatpack (LQFP)
ST-48
AD7679ACP
40C to +85C
Lead Frame Chip Scale (LFCSP)
CP-48
AD7679ACPRL
40C to +85C
Lead Frame Chip Scale (LFCSP)
CP-48
EVAL-AD7679CB
1
Evaluation
Board
EVAL-CONTROL BRD2
2
Controller
Board
1
This board can be used as a standalone evaluation board or in conjunction with the EVAL-CONTROL BRD2 for evaluation/demonstration purposes.
2
This board allows a PC to control and communicate with all Analog Devices evaluation boards ending in the CB designators.
AD7679
Rev. 0 | Page 27 of 28
NOTES
AD7679
NOTES
2003 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective companies.
C0308507/03(0)
Rev. 0 | Page 28 of 28
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