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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
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
a
AD7660*
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
Analog Devices, Inc., 2000
16-Bit, 100 kSPS CMOS ADC
FUNCTIONAL BLOCK DIAGRAM
SWITCHED
CAP DAC
16
CONTROL LOGIC AND
CALIBRATION CIRCUITRY
CLOCK
AD7660
DATA[15:0]
BUSY
RD
CS
SER/
PAR
OB/
2C
OGND
OVDD
DGND
DVDD
AVDD AGND REF REFGND
IN
INGND
PD
RESET
SERIAL
PORT
PARALLEL
INTERFACE
CNVST
FEATURES
Throughput: 100 kSPS
INL: 3 LSB Max ( 0.0046% of Full-Scale)
16 Bits Resolution with No Missing Codes
S/(N+D): 87 dB Min, 90 dB Typ @ 10 kHz
THD: 96 dB Max @ 10 kHz
Analog Input Voltage Range: 0 V to 2.5 V
Both AC and DC Specifications
No Pipeline Delay
Parallel and Serial 5 V/3 V Interface
Single 5 V Supply Operation
21 mW Typical Power Dissipation, 21 W @ 100 SPS
Power-Down Mode: 7 W Max
Package: 48-Lead Quad Flatpack (LQFP)
Pin-to-Pin Compatible with the AD7664
APPLICATIONS
Data Acquisition
Battery-Powered Systems
PCMCIA
Instrumentation
Automatic Test Equipment
Scanners
Medical Instruments
Process Control
GENERAL DESCRIPTION
The AD7660 is a 16-bit, 100 kSPS, charge redistribution SAR,
analog-to-digital converter that operates from a single 5 V power
supply. The part contains an internal conversion clock, error cor-
rection circuits, and both serial and parallel system interface ports.
The AD7660 is hardware factory calibrated and is comprehensively
tested to ensure such ac parameters as signal-to-noise ratio (SNR)
and total harmonic distortion (THD), in addition to the more
traditional dc parameters of gain, offset, and linearity.
It is fabricated using Analog Devices' high-performance, 0.6
micron CMOS process with correspondingly low cost, and is
available in a 48-lead LQFP with operation specified from
40
C to +85C.
PRODUCT HIGHLIGHTS
1. Fast Throughput
The AD7660 is a 100 kSPS, charge redistribution, 16-bit
SAR ADC with internal error correction circuitry.
2. Superior INL
The AD7660 has a maximum integral nonlinearity of 3 LSBs
with no missing 16-bit code.
3. Single-Supply Operation
The AD7660 operates from a single 5 V supply and only
dissipates 21 mW typical. Its power dissipation decreases
with the throughput to, for instance, only 21
W at a 100 SPS
throughput. It consumes 7
W maximum when in power-down.
4. Serial or Parallel Interface
Versatile parallel or 2-wire serial interface arrangement com-
patible with both 3 V or 5 V logic.
*Patent pending.
AD7660SPECIFICATIONS
(40 C to +85 C, AVDD = DVDD = 5 V, OVDD = 2.7 V to 5.25 V, unless otherwise noted.)
REV. 0
2
Parameter
Conditions
Min
Typ
Max
Unit
RESOLUTION
16
Bits
ANALOG INPUT
Voltage Range
V
IN
V
INGND
0
V
REF
V
Operating Input Voltage
V
IN
0.1
+3
V
V
INGND
0.1
+0.5
V
Analog Input CMRR
f
IN
= 25 kHz
70
dB
Leakage Current at 25
C
100 kSPS Throughput
325
nA
Input Impedance
See Analog Input Section
THROUGHPUT SPEED
Complete Cycle
10
s
Throughput Rate
0
100
kSPS
DC ACCURACY
Integral Linearity Error
3
+3
LSB
1
Differential Linearity Error
1
+1.75
LSB
No Missing Codes
16
Bits
Transition Noise
2
0.75
LSB
Full-Scale Error
3
REF = 2.5 V
0.09
0.16
% of FSR
Unipolar Zero Error
3
1
5
LSB
Power Supply Sensitivity
AVDD = 5 V
5%
3
LSB
AC ACCURACY
Signal-to-Noise
f
IN
= 10 kHz
87
90
dB
4
Spurious Free Dynamic Range
f
IN
= 10 kHz
96
dB
Total Harmonic Distortion
f
IN
= 10 kHz
96
dB
Signal-to-(Noise+Distortion)
f
IN
= 10 kHz
87
dB
60 dB Input
30
dB
3 dB Input Bandwidth
820
kHz
SAMPLING DYNAMICS
Aperture Delay
2
ns
Aperture Jitter
5
ps rms
Transient Response
Full-Scale Step
8
s
REFERENCE
External Reference Voltage Range
2.3
2.5
2.7
V
External Reference Current Drain
100 kSPS Throughput
22
A
POWER SUPPLIES
Specified Performance
AVDD
4.75
5
5.25
V
DVDD
4.75
5
5.25
V
OVDD
2.7
5.25
V
Operating Current
100 kSPS Throughput
AVDD
3.2
mA
DVDD
5
1
mA
OVDD
5
10
A
Power Dissipation
5
100 kSPS Throughput
21
25
mW
100 SPS Throughput
21
W
in Power-Down Mode
5, 6
7
W
DIGITAL INPUTS
Logic Levels
V
IL
0.3
+0.8
V
V
IH
+2.0
OVDD + 0.3
V
I
IL
1
+1
A
I
IH
1
+1
A
DIGITAL OUTPUTS
Data Format
Parallel or Serial 16-Bit
Pipeline Delay
Conversion Results Available Immediately
after Completed Conversion
V
OL
I
SINK
= 1.6 mA
0.4
V
V
OH
I
SOURCE
= 500
A
OVDD 0.6
V
TEMPERATURE RANGE
Specified Performance
T
MIN
to T
MAX
40
+85
C
REV. 0
3
AD7660
NOTES
1
LSB means Least Significant Bit. With the 0 V to 2.5 V input range, one LSB is 38.15
V.
2
Typical rms noise at worst-case transitions and temperatures.
3
See Definition of Specifications section. These specifications do not include the error contribution from the external reference.
4
All specifications in dB are referred to a full-scale input F
S
. Tested with an input signal at 0.5 dB below full-scale unless otherwise specified.
5
Tested in parallel reading mode.
6
With all digital inputs forced to DVDD or DGND respectively.
Specifications subject to change without notice.
TIMING SPECIFICATIONS
Symbol
Min
Typ
Max
Unit
Refer to Figures 11 and 12
Convert Pulsewidth
t
1
5
ns
Time Between Conversions
t
2
10
s
CNVST LOW to BUSY HIGH Delay
t
3
15
ns
BUSY HIGH All Modes Except in
t
4
2
s
Master Serial Read after Convert Mode
Aperture Delay
t
5
2
ns
End of Conversion to BUSY LOW Delay
t
6
10
ns
Conversion Time
t
7
2
s
Acquisition Time
t
8
8
s
RESET Pulsewidth
t
9
10
ns
Refer to Figures 13, 14, and 15 (Parallel Interface Modes)
CNVST LOW to DATA Valid Delay
t
10
2
s
DATA Valid to BUSY LOW Delay
t
11
45
ns
Bus Access Request to DATA Valid
t
12
40
ns
Bus Relinquish Time
t
13
5
50
ns
Refer to Figures 16, and 17 (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
0.5
s
SYNC Asserted to SCLK First Edge Delay
t
18
4
ns
Internal SCLK Period
t
19
40
75
ns
Internal SCLK HIGH (INVSCLK Low)
2
t
20
30
ns
Internal SCLK LOW (INVSCLK Low)
2
t
21
9.5
ns
SDOUT Valid Setup Time
t
22
4.5
ns
SDOUT Valid Hold Time
t
23
3
ns
SCLK Last Edge to SYNC Delay
t
24
3
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
t
28
3.2
s
CNVST LOW to SYNC Asserted Delay
t
29
1.5
s
SYNC Deasserted to BUSY LOW Delay
t
30
50
ns
Refer to Figures 18 and 20 (Slave Serial Interface Modes)
1
External SCLK Setup Time
t
31
5
ns
External SCLK Active Edge to SDOUT Delay
t
32
3
16
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
NOTES
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
If the polarity of SCLK is inverted, the timing references of SCLK are also inverted.
Specifications subject to change without notice.
(40 C to +85 C, AVDD = DVDD
= 5 V, OVDD = 2.7 V to 5.25 V, unless otherwise noted.)
REV. 0
AD7660
4
ABSOLUTE MAXIMUM RATINGS
1
Analog Inputs
IN
2
, REF . . . . . . . . . . . . AVDD + 0.3 V to AGND 0.3 V
INGND, REFGND . . . . . . . . . . . . . . . . . . AGND
0.3 V
Ground Voltage Differences
AGND, DGND, OGND . . . . . . . . . . . . . . . . . . . . .
0.3 V
Supply Voltages
AVDD, DVDD, OVDD . . . . . . . . . . . . . . . . . . . . . . . . 7 V
AVDD to DVDD,
AVDD to OVDD . . . . . . . . . . . . . .
7 V
DVDD to OVDD . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7 V
Digital Inputs
Except the Data Bus D(7:4) . . . 0.3 V to DVDD + 0.3 V
Data Bus Inputs D(7:4) . . . . . . 0.3 V to OVDD + 0.3 V
Internal Power Dissipation
3
. . . . . . . . . . . . . . . . . . . 700 mW
Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 150
C
Storage Temperature Range . . . . . . . . . . . . 65
C to +150C
Lead Temperature Range
(Soldering 10 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
C
NOTES
1
Stresses above those listed under Absolute Maximum Ratings may cause perma-
nent 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 Analog Input section.
3
Specification is for device in free air: 48-Lead LQFP:
JA
= 91
C/W,
JC
= 30
C/W.
I
OH
500 A
1.6mA
I
OL
TO OUTPUT
PIN
1.4V
C
L
60pF
1
NOTE:
1
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.
Figure 1. Load Circuit for Digital Interface Timing
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
the AD7660 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.
WARNING!
ESD SENSITIVE DEVICE
ORDERING GUIDE
Model
Temperature Range
Package Description
Package Option
AD7660AST
40
C to +85C
Quad Flatpack (LQFP)
ST-48
AD7660ASTRL
40
C to +85C
Quad Flatpack (LQFP)
ST-48
EVAL-AD7660CB
1
Evaluation Board
EVAL-CONTROL BOARD
2
Controller Board
NOTES
1
This board can be used as a stand-alone evaluation board or in conjunction with the EVAL-CONTROL BOARD 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.
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
NC
DGND
OB/
2C
NC
NC
NC = NO CONNECT
SER/
PAR
D0
D1
D2
BUSY
D15
D14
D13
AD7660
D3
D12
D4/EXT/
INT
D5/INVSYNC
D6/INVSCLK
D7/RDC/SDIN
OGND
OVDD
DVDD
DGND
D8/SDOUT
D9/SCLK
D10/SYNC
D11/RDERROR
NC
NC
NC
NC
NC
NC
NC
NC
NC
INGND
REFGND
REF
PIN CONFIGURATION
48-Lead LQFP
(ST-48)
t
DELAY
t
DELAY
0.8V
0.8V
0.8V
2V
2V
2V
Figure 2. Voltage Reference Levels for Timings
REV. 0
AD7660
5
PIN FUNCTION DESCRIPTIONS
Pin
No.
Mnemonic
Type
Description
1
AGND
P
Analog Power Ground Pin.
2
AVDD
P
Input Analog Power Pins. Nominally 5 V.
3, 6, 7,
NC
No Connect.
4048
4
DGND
DI
Must be tied to digital ground.
5
OB/
2C
DI
Straight Binary/Binary Two's Complement. When OB/
2C is HIGH, the digital output is
straight binary; when LOW, the MSB is inverted resulting in a two's complement output
from its internal shift register.
8
SER/
PAR
DI
Serial/Parallel Selection Input. When LOW, the parallel port is selected; when HIGH, the
serial interface mode is selected and some bits of the DATA bus are used as a serial port.
912
DATA[0:3]
DO
Bit 0 to Bit 3 of the Parallel Port Data Output Bus. These pins are always outputs regardless
of the state of SER/
PAR.
13
DATA[4]
DI/O
When SER/
PAR is LOW, this output is used as the Bit 4 of the Parallel Port Data Output Bus.
or EXT/
INT
When SER/
PAR is HIGH, this input, part of the serial port, is used as a digital select input
for choosing the internal or an external data clock. With EXT/
INT tied LOW, the internal
clock is selected on SCLK output. With EXT/
INT set to a logic HIGH, output data is syn-
chronized to an external clock signal connected to the SCLK input.
14
DATA[5]
DI/O
When SER/
PAR is LOW, this output is used as the Bit 5 of the Parallel Port Data Output Bus.
or INVSYNC
When SER/
PAR is HIGH, 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
DATA[6]
DI/O
When SER/
PAR is LOW, this output is used as the Bit 6 of the Parallel Port Data Output Bus.
or INVSCLK
When SER/
PAR is HIGH, this input, part of the serial port, is used to invert the SCLK sig-
nal. It is active in both master and slave mode.
16
DATA[7]
DI/O
When SER/
PAR is LOW, this output is used as the Bit 7 of the Parallel Port Data Output Bus.
or RDC/SDIN
When SER/
PAR is HIGH, 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 con-
version results from two or more ADCs onto a single SDOUT line. The digital data level on
SDIN is output on DATA with a delay of 16 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 is output on SDOUT only when the conversion is complete.
17
OGND
P
Input/Output interface Digital Power Ground.
18
OVDD
P
Input/Output interface Digital Power. Nominally at the same supply than the supply of the
host interface (5 V or 3 V).
19
DVDD
P
Digital Power. Nominally at 5 V.
20
DGND
P
Digital Power Ground.
21
DATA[8]
DO
When SER/
PAR is LOW, this output is used as the Bit 8 of the Parallel Port Data Output Bus.
or SDOUT
When SER/
PAR is HIGH, 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 AD7660
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:
If INVSCLK is LOW, SDOUT is updated on SCLK rising edge and valid on the next
falling edge.
If INVSCLK is HIGH, SDOUT is updated on SCLK falling edge and valid on the next
rising edge.
REV. 0
AD7660
6
Pin
No.
Mnemonic
Type
Description
22
DATA[9]
DI/O
When SER/
PAR is LOW, this output is used as the Bit 9 of the Parallel Port Data Output
Bus.
or SCLK
When SER/
PAR is HIGH, 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
DATA[10]
DO
When SER/
PAR is LOW, this output is used as the Bit 10 of the Parallel Port Data Output
Bus.
or SYNC
When SER
/PAR is HIGH, 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 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
DATA[11]
DO
When SER/
PAR is LOW, this output is used as the Bit 11 of the Parallel Port Data Output Bus.
or RDERROR
When SER/
PAR is HIGH and 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
DATA[12:15]
DO
Bit 12 to Bit 15 of the Parallel Port Data output bus. These pins are always outputs regard-
less of the state of SER/
PAR.
29
BUSY
DO
Busy Output. Transitions HIGH when a conversion is started, and 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.
RD and CS are OR'd together internally.
32
CS
DI
Chip Select. When
CS and RD are both LOW, the interface parallel or serial output bus is
enabled.
RD and CS are OR'd together internally.
33
RESET
DI
Reset Input. When set to a logic HIGH, reset the AD7660. Current conversion if any is aborted.
34
PD
DI
Power-Down Input. When set to a logic HIGH, power consumption is reduced and conver-
sions are inhibited after the current one is completed.
35
CNVST
DI
Start Conversion. If
CNVST is 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 con-
version. This mode is the most appropriate if low sampling jitter is desired. If
CNVST is LOW
when the acquisition phase (t
8
) is complete, the internal sample/hold is put into the hold state
and a conversion is immediately started.
36
AGND
P
Must be tied to analog ground.
37
REF
AI
Reference Input Voltage.
38
REFGND
AI
Reference Input Analog Ground.
39
INGND
AI
Analog Input Ground.
43
IN
AI
Primary analog input with a range of 0 V to V
REF.
NOTES
AI = Analog Input
DI = Digital Input
DI/O = Bidirectional Digital
DO = Digital Output
P = Power
REV. 0
AD7660
7
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 1/2 LSB
before the first code transition. "Positive full scale" is defined as a
level 1 1/2 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.
FULL-SCALE ERROR
The last transition (from 011 . . . 10 to 011 . . . 11 in two's
complement coding) should occur for an analog voltage 1 1/2 LSB
below the nominal full scale (2.49994278 V for the 0 V2.5 V
range). The full-scale error is the deviation of the actual level of
the last transition from the ideal level.
UNIPOLAR ZERO ERROR
The first transition should occur at a level 1/2 LSB above analog
ground (19.073
V for the 0 V2.5 V range). Unipolar zero error is
the deviation of the actual transition from that point.
SPURIOUS FREE DYNAMIC RANGE (SFDR)
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. It is related to S/[N+D] by the following formula:
ENOB = (S/[N+D]
dB
1.76)/6.02
and is expressed in bits.
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.
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 AND 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
The time required for the AD7660 to achieve its rated accuracy
after a full-scale step function is applied to its input.
REV. 0
AD7660
8
Typical Performance Characteristics
CODE
3
0
INL LSB
16384
2
1
0
1
2
3
32768
49152
65536
TPC 1. Integral Nonlinearity vs. Code
POSITIVE INL LSB
0
0
NUMBER OF UNITS
0.6
5
10
15
20
25
30
1.2
1.8
2.4
3.0
TPC 2. Typical Positive INL Distribution (350 Units)
CODE Hexa
0
COUNTS
8008
8000
7000
6000
5000
4000
3000
2000
1000
0
8009 800A 800B 800C 800D 800E 800F 8010 8011
0
13
9
0
0
7219
7051
1213
879
TPC 3. Histogram of 16,384 Conversions of a DC Input
at the Code Transition
CODE
1.00
0
DNL
LSB
16384
0.00
32768
49152
65536
0.75
0.50
1.00
1.75
1.50
1.25
0.75
0.25
0.50
0.25
TPC 4. Differential Nonlinearity vs. Code
NEGATIVE INL LSB
0
0
NUMBER OF UNITS
0.6
5
10
15
20
25
30
1.2
1.8
2.4
3.0
35
TPC 5. Typical Negative INL Distribution (350 Units)
CODE Hexa
0
COUNTS
10000
8000
6000
4000
0
8009 800A 800B
800C 800D
800E 800F
8010
8011
0
188
0
0
9026
161
3489
3520
2000
TPC 6. Histogram of 16,384 Conversions of a DC Input at
the Code Center
REV. 0
AD7660
9
FREQUENCY kHz
180
0
AMPLITUDE
dB (Full Scale)
10
20
30
40
140
100
60
0
50
4096 POINT FFT
FS = 100kHz
f
IN
= 10.02kHz, 0.5dB
SNR = 89.8dB
SINAD = 89.4dB
THD = 99.3dB
SFDR = 101.6dB
160
120
80
40
20
TPC 7. FFT Plot
FREQUENCY Hz
110
0
THD, HARMONICS
dB
10k
100
90
80
70
60
100k
1M
SFDR
THD
3RD HARMONIC
2ND HARMONIC
60
70
80
90
100
110
SFDR
dB
TPC 8. THD, Harmonics, and SFDR vs. Frequency
INPUT LEVEL dB
90
THD, HARMONICS
dB
140
120
80
60
100
80
70
60
50
40
30
20
10
0
2ND HARMONIC
THD
3RD HARMONIC
TPC 9. THD, Harmonics vs. Input Level
FREQUENCY Hz
75
0
SNR AND S/[N+D]
dB
10k
80
85
90
95
100
100k
1M
SNR
S/(N+D)
13.0
70
13.5
14.0
14.5
15.0
15.5
16.0
ENOB
Bits
ENOB
TPC 10. SNR, S/(N+D), and ENOB vs. Frequency
INPUT LEVEL dB
SNR (REFERRED TO FULL SCALE)
dB
86
88
92
90
60
50
40
30
20
10
0
TPC 11. SNR vs. Input Level
C
L
pF
t
12
DELAY
ns
50
40
0
50
30
20
10
0
100
150
200
OVDD = 2.7V, 85 C
OVDD = 5V, 85 C
OVDD = 2.7V, 25 C
OVDD = 5V, 25 C
TPC 12. Typical Delay vs. Load Capacitance C
L
REV. 0
AD7660
10
TEMPERATURE C
POWER-DOWN OPERATING CURRENTS
nA
0
20
50
25
0
25
50
75
100
40
60
80
100
10
30
50
70
90
AVDD
OVDD
DVDD
TPC 14. Power-Down Operating Currents vs. Temperature
SAMPLING RATE SPS
10000000
0.1
OPERATING CURRENTS
nA
1000000
100000
10000
1000
100
10
1
1
10
100
1000
10000
100000 1000000
OVDD
DVDD
AVDD
TPC 13. Operating Currents vs. Sample Rate
REV. 0
AD7660
11
000...000
000...001
000...010
111...101
111...110
111...111
ADC CODE
Straight Binary
ANALOG INPUT
V
REF
1.5 LSB
V
REF
1 LSB
1 LSB
0V
0.5 LSB
1 LSB = V
REF
/65536
Figure 4. ADC Ideal Transfer Function
Transfer Functions
Using the OB/
2C digital input, the AD7660 offers two output
codings: straight binary and two's complement. The LSB size is
V
REF
/65536, which is about 38.15
V. The ideal transfer charac-
teristic for the AD7660 is shown in Figure 4 and Table I.
Table I. Output Codes and Ideal Input Voltages
Digital Output Code
(Hexa)
Analog
Straight
Two's
Description
Input
Binary
Complement
FSR 1 LSB
2.499962 V
FFFF
1
7FFF
1
FSR 2 LSB
2.499923 V
FFFE
7FFE
Midscale + 1 LSB
1.250038 V
8001
0001
Midscale
1.25 V
8000
0000
Midscale 1 LSB
1.249962 V
7FFF
FFFF
FSR + 1 LSB
38
V
0001
8001
FSR
0 V
0000
2
8000
2
NOTES
1
This is also the code for overrange analog input (V
IN
V
INGND
above
V
REF
V
REFGND
).
2
This is also the code for underrange analog input (V
IN
below V
INGND
).
CIRCUIT INFORMATION
The AD7660 is a fast, low-power, single-supply, precise 16-bit
analog-to-digital converter (ADC). The AD7660 is capable of
converting 100,000 samples per second (100 kSPS) and allows
power saving between conversions. When operating at 100 SPS,
for example, it consumes typically only 21
W. This feature
makes the AD7660 ideal for battery-powered applications.
The AD7660 provides the user with an on-chip track/hold,
successive approximation ADC that does not exhibit any pipe-
line or latency, making it ideal for multiple multiplexed channel
applications.
The AD7660 can be operated from a single 5 V supply and be
interfaced to either 5 V or 3 V digital logic. It is housed in a
48-lead LQFP package that combines space savings and allows
flexible configurations as either serial or parallel interface. The
AD7660 is pin-to-pin-compatible with the AD7664.
CONVERTER OPERATION
The AD7660 is a successive approximation analog-to-digital
converter based on a charge redistribution DAC. Figure 3 shows
the simplified schematic of the ADC. The capacitive DAC consists
of an array of 16 binary weighted capacitors and an additional
"LSB" capacitor. The comparator's negative input is connected
to a "dummy" capacitor of the same value as the capacitive
DAC array.
During the acquisition phase, the common terminal of the array
tied to the comparator's positive input is connected to AGND
via SW
A
. All independent switches are connected to the analog
input IN. Thus, the capacitor array is used as a sampling capaci-
tor and acquires the analog signal on IN input. Similarly, the
"dummy" capacitor acquires the analog signal on INGND input.
When the acquisition phase is complete and the
CNVST input
goes or is low, a conversion phase is initiated. When the conver-
sion phase begins, SW
A
and SW
B
are opened first. The capacitor
array and the "dummy" capacitor are then disconnected from
the inputs and connected to the REFGND input. Therefore, the
differential voltage between IN and INGND captured at the end
of the acquisition phase is applied to the comparator inputs, caus-
ing the comparator to become unbalanced.
By switching each element of the capacitor array between REFGND
or REF, the comparator input varies by binary weighted voltage
steps (V
REF
/2, V
REF
/4 . . . V
REF
/65536). The control logic toggles
these switches, starting with the MSB first, in order to bring the
comparator back into a balanced condition. After the comple-
tion of this process, the control logic generates the ADC output
code and brings BUSY output low.
SW
A
COMP
SW
B
IN
REF
REFGND
LSB
LSB
MSB
32768C
INGND
16384C
4C
2C
C
C
67536C
CONTROL
LOGIC
SWITCHES
CONTROL
BUSY
OUTPUT
CODE
CNVST
Figure 3. ADC Simplified Schematic
REV. 0
AD7660
12
TYPICAL CONNECTION DIAGRAM
Figure 6 shows a typical connection diagram for the AD7660.
C2
R1
D1
D2
C1
IN
OR INGND
AGND
AVDD
Figure 5. Equivalent Analog Input Circuit
Analog Input
Figure 5 shows an equivalent circuit of the input structure of the
AD7660.
The two diodes D1 and D2 provide ESD protection for the
analog inputs IN and INGND. Care must be taken to ensure
that the analog input signal never exceeds the supply rails by
more than 0.3 V. This will cause these diodes to become for-
ward-biased and start conducting current. These diodes can
handle a forward-biased current of 100 mA maximum. For
instance, these conditions could eventually occur when the
input buffer's (U1) supplies are different from AVDD. In such
case, an input buffer with a short circuit current limitation can
be used to protect the part.
This analog input structure allows the sampling of the differen-
tial signal between IN and INGND. Unlike other converters,
the INGND input is sampled at the same time as the IN input.
By using this differential input, small signals common to both
inputs are rejected as shown in Figure 7 which represents the
typical CMR over frequency. For instance, by using INGND to
sense a remote signal ground, difference of ground potentials
between the sensor and the local ADC ground are eliminated.
COMMON-MODE INPUT FREQUENCY Hz
0
0.1k
CMRR
dB
10
1k
10k
100k
1M
10M
20
30
40
50
60
70
80
Figure 7. Analog Input CMR vs. Frequency
During the acquisition phase, the impedance of the analog input
IN can be modeled as a parallel combination of capacitor C1
and the network formed by the series connection of R1 and C2.
Capacitor C1 is primarily the pin capacitance. The resistor R1 is
typically 3242
and is a lumped component made up of some
serial resistor and the on resistance of the switches. The capacitor
C2 is typically 60 pF and is mainly the ADC sampling capacitor.
During the conversion phase, where the switches are opened,
the input impedance is limited to C1. It has to be noted that the
input impedance of the AD7660, unlike other SAR ADCs, is
not a pure capacitance and thus, inherently reduces the kickback
transient at the beginning of the acquisition phase. The R1, C2
makes a one-pole low-pass filter that reduces undesirable
aliasing effect and limits the noise.
100nF
10 F
100nF
10 F
AVDD
10 F
100nF
AGND
DGND
DVDD
OVDD
OGND
CS
RD
SER/
PAR
CNVST
BUSY
SDOUT
SCLK
RESET
PD
IN
INGND
U1
2
REFGND
100nF
C
REF
1
2.5V REF
1
REF
100
D
3
CLOCK
AD7660
ANALOG INPUT
(0V TO 2.5V)
C/ P/DSP
SERIAL
PORT
DIGITAL SUPPLY
(3.3V OR 5V)
ANALOG
SUPPLY
(5V)
DVDD
OB/
2C
NOTES:
1
WITH THE AD780 OR THE ADR291 VOLTAGE REFERENCE, C
REF
IS 47 F
2
THE AD8519 IS RECOMMENDED
3
OPTIONAL LOW JITTER
CNVST
Figure 6. Typical Connection Diagram
REV. 0
AD7660
13
When the source impedance of the driving circuit is low, the
AD7660 can be driven directly. Large source impedances will
significantly affect the ac performances, especially the total
harmonic distortion. The maximum source impedance depends
on the amount of total harmonic distortion (THD) that can be
tolerated. The THD degrades in function of the source imped-
ance and the maximum input frequency as shown in Figure 8.
INPUT FREQUENCY kHz
100
1
100
10
THD
dB
95
90
85
80
75
70
R
S
= 500
R
S
= 100
R
S
= 50
R
S
= 20
Figure 8. THD vs. Analog Input Frequency and
Input Resistance
Driver Amplifier Choice
Although the AD7660 is easy to drive, the driver amplifier needs
to meet at least the following requirements:
The driver amplifier and the AD7660 analog input circuit
have to be able together to settle for a full-scale step the
capacitor array at a 16-bit level (0.0015%). For instance,
operation at the maximum throughput of 100 kSPS requires
a minimum gain bandwidth product of 5 MHz.
The noise generated by the driver amplifier needs to be kept
as low as possible in order to preserve the SNR and transi-
tion noise performance of the AD7660. The noise coming
from the driver is filtered by the AD7660 analog input circuit
one-pole low-pass filter made by R1 and C2. For instance, a
driver with an equivalent input noise of 7 nV/
Hz like the
AD8519 and configured as a buffer, thus with a noise gain of
+1, degrades the SNR by only 0.2 dB.
The driver needs to have a THD performance suitable to
that of the AD7660. TPC 8 gives the THD versus frequency
that the driver should preferably exceed.
The AD8519, OP162, or the OP184 meet these requirements
and are usually appropriate for almost all applications. As an
alternative, in very high-speed and noise-sensitive applications,
the AD829 with an external compensation capacitor of 82 pF
can be used. This capacitor should have good linearity as an
NPO ceramic or mica type. Moreover, the use of a noninverting
+1 gain arrangement is recommended and helps to obtain the
best signal-to-noise ratio.
Voltage Reference Input
The AD7660 uses an external 2.5 V voltage reference. The
voltage reference input REF of the AD7660 has a dynamic
input impedance. Therefore, it should be driven by a low
impedance source with an efficient decoupling between REF
and REFGND inputs. This decoupling depends on the choice
of the voltage reference but, usually consists of a low ESR tanta-
lum capacitor and a 100 nF ceramic capacitor. Appropriate
value for the tantalum capacitor is 47
F with the low-cost,
low-power ADR291 voltage reference or with the low-noise,
low-drift AD780 voltage reference. For applications using
multiple AD7660s, it is more effective to buffer the reference
voltage with a low-noise, very stable op amp like the AD8031.
Care should also be taken with the reference temperature coeffi-
cient of the voltage reference which directly affects the full-scale
accuracy if this parameter matters. For instance, a
15 ppm/C
tempco of the reference changes the full scale by
1 LSB/C.
Power Supply
The AD7660 uses three sets of power supply pins: an analog
5 V supply AVDD, a digital 5 V core supply DVDD, and a
digital input/output interface supply OVDD. The OVDD supply
allows direct interface with any logic working between 2.7 V and
5.25 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 6. The AD7660 is inde-
pendent of power supply sequencing and thus free from supply
voltage induced latchup. Additionally, it is very insensitive to
power supply variations over a wide frequency range as shown in
Figure 9.
INPUT FREQUENCY Hz
PSRR
dB
80
1k
10k
100k
1M
75
70
65
60
55
50
Figure 9. PSRR vs. Frequency
POWER DISSIPATION VS. THROUGHPUT
The AD7660 automatically reduces its power consumption at
the end of each conversion phase. During the acquisition phase,
the operating currents are very low which allows a significant
power saving when the conversion rate is reduced as shown in
Figure 10. This feature makes the AD7660 ideal for very low-
power battery applications. 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 (i.e.,
DVDD and DGND for all inputs except EXT/
INT, INVSYNC,
INVSCLK, RDC/SDIN, and OVDD or OGND for the last
four inputs.
REV. 0
AD7660
14
THROUGHPUT SPS
100
10
POWER DISSIPATION
mW
1
100
0.01
10
1000
10000
100000
0.1
Figure 10. Power Dissipation vs. Sample Rate
CONVERSION CONTROL
Figure 11 shows the detailed timing diagrams of the conversion
process. The AD7660 is controlled by the signal
CNVST which
initiates conversion. Once initiated, it cannot be restarted or
aborted, even by the power-down input PD, until the conver-
sion is complete. The
CNVST signal operates independently of
CS and RD signals.
For a true sampling application, the recommended operation of
the
CNVST signal is the following:
CNVST must be held high from the previous falling edge of
BUSY, and during a minimum delay corresponding to the
acquisition time t8; then, when
CNVST is brought low, a con-
version is initiated and BUSY signal goes high until the completion
of the conversion. 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. For appli-
cations where the SNR is critical, the
CNVST signal should
have a very low jitter. Some solutions to achieve this are to use a
dedicated oscillator for
CNVST generation or, at least, to clock
it with a high frequency low jitter clock as shown in Figure 6.
For other applications, conversions can be automatically initi-
ated. If
CNVST is held low when BUSY is low, the AD7660
controls the acquisition phase and then automatically initiates a
new conversion. By keeping
CNVST low, the AD7660 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 AD7660 could sometimes
run slightly faster than the guaranteed limit of 100 kSPS.
DIGITAL INTERFACE
The AD7660 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
AD7660 digital interface also accommodates both 3 V or 5 V
logic by simply connecting the OVDD supply pin of the AD7660
to the host system interface digital supply. Finally, by using the
OB/
2C input pin, both two's complement or straight binary
coding can be used.
The two signals
CS and RD control the interface. CS and RD
have a similar effect because they are OR'd together internally.
When at least one of these signals is high, the interface outputs
are in high impedance. Usually,
CS allows the selection of each
AD7660 in multicircuits applications and is held low in a single
AD7660 design.
RD is generally used to enable the conversion
result on the data bus.
t
2
t
1
t
3
t
4
t
5
t
6
t
7
t
8
CNVST
BUSY
MODE
ACQUIRE
CONVERT
ACQUIRE
CONVERT
Figure 11. Basic Conversion Timing
t
9
t
8
RESET
DATA
BUSY
CNVST
Figure 12. RESET Timing
REV. 0
AD7660
15
PARALLEL INTERFACE
The AD7660 is configured to use the parallel interface when the
SER/
PAR is held low. The data can be read either after each
conversion, which is during the next acquisition phase, or during
the following conversion as shown, respectively, in Figure 14 and
Figure 15. When the data is read during the conversion, how-
ever, it is recommended, that it is read only during the first half
of the conversion phase. That avoids any potential feedthrough
between voltage transients on the digital interface and the most
critical analog conversion circuitry.
SERIAL INTERFACE
The AD7660 is configured to use the serial interface when the
SER/
PAR is held high. The AD7660 outputs 16 bits of data,
MSB first, on the SDOUT pin. This data is synchronized with
the 16 clock pulses provided on the SCLK pin.
t
1
t
3
t
4
t
11
CNVST
BUSY
DATA BUS
CS = RD = 0
t
10
PREVIOUS CONVERSION DATA
NEW DATA
Figure 13. Master Parallel Data Timing for Reading (Continuous Read)
CURRENT
CONVERSION
BUSY
DATA BUS
CS
RD
t
12
t
13
Figure 14. Slave Parallel Data Timing for Reading (Read after Convert)
PREVIOUS
CONVERSION
t
1
t
3
t
12
t
13
t
4
CS = 0
CNVST, RD
BUSY
DATA BUS
Figure 15. Slave Parallel Data Timing for Reading (Read During Convert)
MASTER SERIAL INTERFACE
Internal Clock
The AD7660 is configured to generate and provide the serial
data clock SCLK when the EXT/
INT pin is held low. The AD7660
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. The output data is valid on both the
rising and falling edge of the data clock. Depending on RDC/
SDIN input, the data can be read after each conversion, or
during the following conversion. Figure 16 and Figure 17 show
the detailed timing diagrams of these two modes.
Usually, because the AD7660 has a longer acquisition phase
than the conversion phase, the data is read immediately after
conversion. That makes the mode master, read after conver-
sion, the most recommended serial mode when it can be used.
In read-after-conversion mode, it should be noted that, unlike
in other modes, the signal BUSY returns low after the 16 data
bits are pulsed out and not at the end of the conversion phase
REV. 0
AD7660
16
which results in a longer BUSY width. In read-during-conversion
mode, the serial clock and data toggle at appropriate instants,
which minimizes potential feedthrough between digital activity
and the critical conversion decisions.
SLAVE SERIAL INTERFACE
External Clock
The AD7660 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. When
CS and RD are both low, the data can be read after
each conversion or during the following conversion. The exter-
nal clock can be either a continuous or discontinuous clock. A
discontinuous clock can be either normally high or normally low
when inactive. Figure 18 and Figure 20 show the detailed timing
diagrams of these methods. Usually, because the AD7660 has a
longer acquisition phase than the conversion phase, the data are
read immediately after conversion.
While the AD7660 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 particu-
larly important during the second half of the conversion phase
because the AD7660 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 is toggling only when BUSY is low or, more importantly,
that it does not transition during the latter half of BUSY high.
t
3
BUSY
CS, RD
CNVST
SYNC
SCLK
SDOUT
t
28
t
29
t
14
t
18
t
19
t
20
t
21
t
24
t
26
t
27
t
23
t
22
t
16
t
15
1
2
3
14
15
16
D15
D14
D2
D1
D0
X
EXT/
INT = 0
RDC/SDIN = 0
INVSCLK = INVSYNC = 0
t
25
t
30
Figure 16. Master Serial Data Timing for Reading (Read after Convert)
EXT/
INT = 0
CS, RD
RDC/SDIN = 1
INVSCLK = INVSYNC = 0
CNVST
BUSY
SYNC
SCLK
SDOUT
t
3
t
1
t
17
t
14
t
19
t
20
t
21
t
24
t
26
t
25
t
27
t
23
t
22
t
16
t
15
D15
D14
D2
D1
D0
X
1
2
3
14
15
16
t
18
Figure 17. Master Serial Data Timing for Reading (Read Previous Conversion During Convert)
REV. 0
AD7660
17
External Discontinuous Clock Data Read after Conversion
This mode is the most recommended of the serial slave modes.
Figure 18 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. The data is shifted out, MSB first, with 16 clock
pulses and is valid on both rising and falling edge of the clock.
Among the advantages of this method, the conversion perfor-
mance is not degraded because there is no voltage transients on
the digital interface during the conversion process.
Another advantage is to be able to read the data at any speed up
to 40 MHz which accommodates both slow digital host interface
and the fastest serial reading.
Finally, in this mode only, the AD7660 provides a "daisy chain"
feature using the RDC/SDIN input pin for cascading multiple
converters together. This feature is useful for reducing compo-
nent count and wiring connections when it is desired as it is, for
instance, in isolated multiconverters applications.
An example of the concatenation of two devices is shown in
Figure 19. Simultaneous sampling is possible by using a common
CNVST signal. It should be noted that the RDC/SDIN input is
latched on the opposite edge of SCLK of the one used to shift
out the data on SDOUT. Hence, the MSB of the "upstream"
converter just follows the LSB of the "downstream" converter
on the next SCLK cycle. Up to twenty AD7660s running at
100 kSPS can be "daisy chained" using this method.
RDC/SDIN
BUSY
BUSY
DATA
OUT
AD7660
#1
(DOWNSTREAM)
BUSY
OUT
CNVST
CS
SCLK
AD7660
#2
(UPSTREAM)
RDC/SDIN
SDOUT
SCLK IN
CS IN
CNVST IN
CNVST
CS
SCLK
SDOUT
Figure 19. Two AD7660s in a "Daisy Chain" Configuration
External Clock Data Read During Conversion
Figure 20 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 16 clock pulses, and is valid on both rising and
falling edges of the clock. The 16 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 RDC/SDIN input should always be
tied either high or low.
To reduce performance degradation due to digital activity, a fast
discontinuous clock of 18 MHz at least is recommended to ensure
that all the bits are read during the first half of the conversion
phase. For this reason, this mode is more difficult to use.
SCLK
SDOUT
D15
D14
D1
D0
D13
X15
X14
X13
X1
X0
Y15
Y14
CS, RD
BUSY
SDIN
EXT/I
NT = 1
INVSCLK = 0
t
35
t
36
t
37
t
31
t
32
t
16
t
33
t
34
X15
X14
X
1
2
3
14
15
16
17
18
Figure 18. Slave Serial Data Timing for Reading (Read after Convert)
REV. 0
AD7660
18
MICROPROCESSOR INTERFACING
The AD7660 is ideally suited for traditional dc measurement
applications supporting a microprocessor, and ac signal pro-
cessing applications interfacing to a digital signal processor.
The AD7660 is designed to interface either with a parallel 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 AD7660 to prevent digital noise from coupling
into the ADC. The following sections illustrate the use of the
AD7660 with an SPI equipped microcontroller, the ADSP-
21065L and ADSP-218x signal processors.
SPI Interface (MC68HC11)
Figure 21 shows an interface diagram between the AD7660 and
an SPI-equipped microcontroller like the MC68HC11. To
accommodate the slower speed of the microcontroller, the
AD7660 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 reading of output data, one byte
at a time, if necessary, could be initiated in response to the
end-of-conversion signal (BUSY going low) using an interrupt
line of the microcontroller. The Serial Peripheral Interface (SPI)
on the MC68HC11 is configured for master mode (MSTR) = 1,
Clock Polarity Bit (CPOL) = 0, Clock Phase Bit (CPHA) = 1
and SPI interrupt enable (SPIE) = 1 by writing to the SPI Control
Register (SPCR). The IRQ is configured for edge-sensitive-only
operation (IRQE = 1 in OPTION register).
IRQ
MC68HC11*
CNVST
AD7660*
BUSY
CS
MISO/SDI
SCK
I/O PORT
SDOUT
SCLK
INVSCLK
EXT/
INT
DVDD
*ADDITIONAL PINS OMITTED FOR CLARITY
OVDD
SER/
PAR
RD
Figure 21. Interfacing the AD7660 to SPI Interface
ADSP-21065L in Master Serial Interface
As shown in Figure 22, the AD7660 can be interfaced to the
ADSP-21065L using the serial interface in master mode with-
out any glue logic required. This mode combines the advantages
to reduce the wire connections and to be able to read the data
during or after conversion at user convenience.
The AD7660 is configured for the internal clock mode (EXT/
INT
low) and acts, therefore, as the master device. The convert com-
mand can be generated by either an external low jitter oscillator
or, as shown, by a FLAG output of the ADSP-21065L or by a
frame output TFS of one serial port of the ADSP-21065L which
can be used like a timer. The serial port on the ADSP-21065L is
configured for external clock (IRFS = 0), rising edge active
(CKRE = 1), external late framed sync signals (IRFS = 0,
LAFS = 1, RFSR = 1) and active high (LRFS = 0). The serial
port of the ADSP-21065L is configured by writing to its receive
control register (SRCTL)--see ADSP-2106x SHARC User's
Manual. Because the serial port, within the ADSP-21065L will
be seeing a discontinuous clock, an initial word reading has to
be done after the ADSP-21065L has been reset to ensure that
the serial port is properly synchronized to this clock during each
following data read operation.
RFS
ADSP-21065L*
SHARC
CNVST
AD7660*
CS
SYNC
RD
DR
RCLK
FLAG OR TFS
SDOUT
SCLK
INVSYNC
INVSCLK
EXT/
INT
RDC/SDIN
SER/
PAR
DVDD
*ADDITIONAL PINS OMITTED FOR CLARITY
OVDD
OR
OGND
Figure 22. Interfacing to the ADSP-21065L Using the
Serial Master Mode
SDOUT
CS, RD
SCLK
D1
D0
X
D15
D14
D13
1
2
3
14
15
16
t
3
t
35
t
36
t
37
t
31
t
32
t
16
CNVST
BUSY
EXT/I
NT = 1
INVSCLK = 0
Figure 20. Slave Serial Data Timing for Reading (Read Previous Conversion During Convert)
REV. 0
AD7660
19
APPLICATION HINTS
Bipolar and Wider Input Ranges
In some applications, it is desired to use a bipolar or wider ana-
log input range like, for instance,
10 V, 5 V or 0 V to 5 V.
Although the AD7660 has only one unipolar range, by simple
modifications of the input driver circuitry, bipolar and wider
input ranges can be used without any performance degradation.
Figure 23 shows a connection diagram which allows that. Com-
ponent values required and resulting full-scale ranges are shown in
Table II.
Table II. Component Values and Input Ranges
Input Range
R1
R2
R3
R4
10 V
1 k
8 k
10 k
84 k
5 V
2 k
8 k
10 k
6.67 k
0 V to 5 V
8 k
8 k
None
0
U1
2.5V REF
ANALOG
INPUT
R2
R3
R4
100nF
R1
C
F
U2
C
REF
IN
INGND
REF
REFGND
100nF
AD7660
Figure 23. Using the AD7660 in 16-Bit Bipolar and/or
Wider Input Ranges
For applications where accurate gain and offset are desired, they
can be calibrated by acquiring a ground and a voltage reference
using an analog multiplexer U2 as shown in Figure 23. Also, C
F
can be used as a one-pole antialiasing filter.
Layout
The AD7660 has very good immunity to noise on the power
supplies as can be seen in Figure 9. However, care should still
be taken with regard to grounding layout.
The printed circuit board that houses the AD7660 should be
designed so 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 AD7660, or, at least, as close as possible to the
AD7660. If the AD7660 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,
which should be established as close as possible to the AD7660.
It is recommended to 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 AD7660 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 AD7660 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 supplies impedance presented to
the AD7660 and reduce the magnitude of the supply spikes.
Decoupling ceramic capacitors, typically 100 nF, should be
placed on each power supplies pins AVDD, DVDD and OVDD
close to, and ideally right up against these pins and their corre-
sponding ground pins. Additionally, low ESR 10
F capacitors
should be located in the vicinity of the ADC to further reduce
low frequency ripple.
The DVDD supply of the AD7660 can be either a separate
supply or come from the analog supply, AVDD, or from the
digital interface supply, OVDD. When the system digital supply
is noisy, or fast switching digital signals are present, it is recom-
mended if no separate supply available, to connect the DVDD
digital supply to the analog supply AVDD through an RC filter
as shown in Figure 6, and connect the system supply to the inter-
face 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 AD7660 has five different ground pins; INGND, REFGND,
AGND, DGND, and OGND. INGND is used to sense the analog
input signal. 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 con-
nected with the least resistance to the analog ground plane.
DGND must be tied to the analog or digital ground plane depend-
ing on the configuration. OGND is connected to the digital
system ground.
Evaluating the AD7660 Performance
A recommended layout for the AD7660 is outlined in the evalu-
ation board for the AD7660. The evaluation board package
includes a fully assembled and tested evaluation board, docu-
mentation, and software for controlling the board from a PC
via the Eval-Control Board.
REV. 0
20
C019282.57/00 (rev. 0)
PRINTED IN U.S.A.
AD7660
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
48-Lead Quad Flatpack (LQFP)
(ST-48)
0.039 (1.00)
REF
TOP VIEW
(PINS DOWN)
1
12
13
25
24
3
6
37
48
0.280 (7.1)
0.276 (7.0) SQ
0.272 (6.9)
0.362 (9.19)
0.354 (9.00) SQ
0.346 (8.79)
0.010 (0.26)
0.007 (0.18)
0.006 (0.15)
0.023 (0.58)
0.020 (0.50)
0.017 (0.42)
SEATING
PLANE
0
MIN
0.007 (0.177)
0.005 (0.127)
0.004 (0.107)
0.006 (0.15)
0.004 (0.10)
0.002 (0.05)
0.028 (0.7)
0.020 (0.5)
0.012 (0.3)
0.067 (1.70)
0.059 (1.50)
0.055 (1.40)
7
3.5
0
0.057 (1.45)
0.055 (1.40)
0.053 (1.35)
CONTROLLING DIMENSIONS ARE IN MILLIMETERS
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