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AD9281

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., 1999

REV. E

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.

Dual Channel 8-Bit

Resolution CMOS ADC

FEATURES
Complete Dual Matching ADC
Low Power Dissipation: 225 mW (+3 V Supply)
Single Supply: 2.7 V to 5.5 V
Differential Nonlinearity Error: 0.1 LSB
On-Chip Analog Input Buffers
On-Chip Reference
Signal-to-Noise Ratio: 49.2 dB
Over Seven Effective Bits
Spurious-Free Dynamic Range: 65 dB
No Missing Codes Guaranteed
28-Lead SSOP

FUNCTIONAL BLOCK DIAGRAM

REFERENCE

BUFFER

QREFB

IREFB

QREFT

IREFT

VREF

REFSENSE

IINA

IINB

"I" ADC

QINB

QINA

"Q" ADC

REGISTER

THREE-

STATE

OUTPUT
BUFFER

AVDD

AVSS

CLOCK

DVDD

DVSS

SLEEP

SELECT

DATA
8 BITS

CHIP
SELECT

AD9281

ASYNCHRONOUS

MULTIPLEXER

PRODUCT DESCRIPTION

The AD9281 is a complete dual channel, 28 MSPS, 8-bit
CMOS ADC. The AD9281 is optimized specifically for applica-
tions where close matching between two ADCs is required (e.g.,
I/Q channels in communications applications). The 28 MHz
sampling rate and wide input bandwidth will cover both narrow-
band and spread-spectrum channels. The AD9281 integrates
two 8-bit, 28 MSPS ADCs, two input buffer amplifiers, an internal
voltage reference and multiplexed digital output buffers.

Each ADC incorporates a simultaneous sampling sample-and-
hold amplifier at its input. The analog inputs are buffered; no
external input buffer op amp will be required in most applica-
tions. The ADCs are implemented using a multistage pipeline
architecture that offers accurate performance and guarantees no
missing codes. The outputs of the ADCs are ported to a multi-
plexed digital output buffer.

The AD9281 is manufactured on an advanced low cost CMOS
process, operates from a single supply from 2.7 V to 5.5 V, and
consumes 225 mW of power (on 3 V supply). The AD9281
input structure accepts either single-ended or differential signals,
providing excellent dynamic performance up to and beyond
14 MHz Nyquist input frequencies.

PRODUCT HIGHLIGHTS

1. Dual 8-Bit, 28 MSPS ADC

A pair of high performance 28 MSPS ADCs that are opti-
mized for spurious free dynamic performance are provided for
encoding of I and Q or diversity channel information.

2. Low Power

Complete CMOS Dual ADC function consumes a low
225 mW on a single supply (on 3 V supply). The AD9281
operates on supply voltages from 2.7 V to 5.5 V.

3. On-Chip Voltage Reference

The AD9281 includes an on-chip compensated bandgap
voltage reference pin programmable for 1 V or 2 V.

4. On-chip analog input buffers eliminate the need for external

op amps in most applications.

5. Single 8-Bit Digital Output Bus

The AD9281 ADC outputs are interleaved onto a single
output bus saving board space and digital pin count.

6. Small Package

The AD9281 offers the complete integrated function in a
compact 28-lead SSOP package.

7. Product Family

The AD9281 dual ADC is pin compatible with a dual 10-bit
ADC (AD9201).

REV. E

AD9281SPECIFICATIONS

(AVDD = +3 V, DVDD = +3 V, F

SAMPLE

= 28 MSPS, VREF = 2 V, INB = 0.5 V, T

MIN

to T

MAX

unless otherwise noted)

Parameter

Symbol

Min

Typ

Max

Units

Condition

RESOLUTION

Bits

CONVERSION RATE

MHz

(32 MHz at +25

DC ACCURACY

Differential Nonlinearity

DNL

0.1

LSB

REFT = 1.0 V, REFB = 0.0 V

Integral Nonlinearity

INL

0.25

LSB

Differential Nonlinearity (SE)

DNL

0.2

1.0

LSB

REFT = 1.0 V, REFB = 0.0 V

Integral Nonlinearity (SE)

INL

0.3

1.5

LSB

Zero-Scale Error, Offset Error

3.2

% FS

Full-Scale Error, Gain Error

1.2

5.4

% FS

Gain Match

0.2

LSB

Offset Match

1.2

LSB

ANALOG INPUT

Input Voltage Range

AIN

0.5

AVDD/2

Input Capacitance

Aperture Delay

Aperture Uncertainty (Jitter)

Aperture Delay Match

Input Bandwidth (3 dB)

Small Signal (20 dB)

240

MHz

Full Power (0 dB)

245

MHz

INTERNAL REFERENCE

Output Voltage (1 V Mode)

VREF

REFSENSE = VREF

Output Voltage Tolerance (1 V Mode)

Output Voltage (2 V Mode)

VREF

REFSENSE = GND

Output Voltage Tolerance (2 V Mode)

Load Regulation (1 V Mode)

VREF

1 mA Load Current

Load Regulation (2 V Mode)

1 mA Load Current

POWER SUPPLY

Operating Voltage

AVDD

2.7

5.5

DVDD

2.7

5.5

Supply Current

AVDD

DVDD

0.1

Power Consumption

225

260

Power-Down

STBY = AVDD, Clock Low

Power Supply Rejection

PSR

0.15

0.75

% FS

DYNAMIC PERFORMANCE

Signal-to-Noise and Distortion

SINAD

f = 3.58 MHz

46.4

49.1

f = 14 MHz

Signal-to-Noise

SNR

f = 3.58 MHz

47.8

49.2

f = 14 MHz

48.5

Total Harmonic Distortion

THD

f = 3.58 MHz

67.5

49.5

f = 14 MHz

Spurious Free Dynamic Range

SFDR

f = 3.58 MHz

49.6

f = 14 MHz

Two-Tone Intermodulation Distortion

IMD

f = 44.9 MHz and 45.52 MHz

Differential Phase

0.2

Degree

NTSC 40 IRE Mod Ramp

Differential Gain

0.08

= 14.3 MHz

Crosstalk Rejection

REV. E

AD9281

Parameter

Symbol

Min

Typ

Max

Units

Condition

DYNAMIC PERFORMANCE (SE)

Signal-to-Noise and Distortion

SINAD

f = 3.58 MHz

47.2

Signal-to-Noise

SNR

f = 3.58 MHz

Total Harmonic Distortion

THD

f = 3.58 MHz

Spurious Free Dynamic Range

SFDR

f = 3.58 MHz

DIGITAL INPUTS

High Input Voltage

2.4

Low Input Voltage

0.3

DC Leakage Current

Input Capacitance

LOGIC OUTPUT (with DVDD = 3 V)

High Level Output Voltage

= 50

2.88

Low Level Output Voltage

= 1.5 mA)

0.095

LOGIC OUTPUT (with DVDD = 5 V)

High Level Output Voltage

= 50

4.5

Low Level Output Voltage

= 1.5 mA)

0.4

Data Valid Delay

MUX Select Delay

Data Enable Delay

= 20 pF. Output Level to

90% of Final Value

Data High-Z Delay

DHZ

CLOCKING

Clock Pulsewidth High

16.9

Clock Pulsewidth Low

16.9

Pipeline Latency

3.0

Cycles

NOTES

SE is single ended input, REFT = 1.5 V, REFB = 0.5 V.

AIN differential 2 V p-p, REFT = 1.5 V, REFB = 0.5 V.

IMD referred to larger of two input signals.

Specifications subject to change without notice.

CLOCK

INPUT

SELECT

INPUT

DATA

OUTPUT

ADC SAMPLE
#2

ADC SAMPLE
#3

ADC SAMPLE
#4

ADC SAMPLE
#5

Q CHANNEL

OUTPUT ENABLED

I CHANNEL
OUTPUT ENABLED

SAMPLE #1-3

Q CHANNEL

OUTPUT

SAMPLE #1-2

Q CHANNEL

OUTPUT

SAMPLE #1-1

Q CHANNEL

OUTPUT

SAMPLE #1-1
I CHANNEL
OUTPUT

SAMPLE #1
Q CHANNEL
OUTPUT

SAMPLE #1
I CHANNEL
OUTPUT

SAMPLE #2
Q CHANNEL
OUTPUT

ADC SAMPLE
#1

Figure 1. ADC Timing

AD9281

REV. E

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 AD9281 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.

PIN CONFIGURATION

TOP VIEW

(Not to Scale)

AD9281

REFT-Q

INB-Q

INA-Q

CHIP-SELECT

VREF

AVDD

REFB-Q

REFB-I

AVSS

REFSENSE

REFT-I

SLEEP

INA-I

INB-I

DVSS

DVDD

(LSB) D0

(MSB) D7

SELECT

CLOCK

NC = NO CONNECT

WARNING!

ESD SENSITIVE DEVICE

ABSOLUTE MAXIMUM RATINGS*

With
Respect

Parameter

Min

Max

Units

AVDD

AVSS

0.3

+6.5

DVDD

DVSS

0.3

+6.5

AVSS

DVSS

0.3

+0.3

AVDD

DVDD

6.5

+6.5

CLK

AVSS

0.3

AVDD + 0.3

Digital Outputs

DVSS

0.3

DVDD + 0.3

AINA, AINB

AVSS

1.0

AVDD + 0.3

VREF

AVSS

0.3

AVDD + 0.3

REFSENSE

AVSS

0.3

AVDD + 0.3

REFT, REFB

AVSS

0.3

AVDD + 0.3

Junction Temperature

+150

Storage Temperature

+150

Lead Temperature

10 sec

+300

*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
sections of this specification is not implied. Exposure to absolute maximum ratings
for extended periods may effect device reliability.

ORDERING GUIDE

Temperature

Package

Model

Range

Description

Options*

AD9281ARS

C to +85

28-Lead SSOP

RS-28

AD9281-EB

Evaluation Board

*RS = Shrink Small Outline.

PIN FUNCTION DESCRIPTIONS

No.

Name

Description

DVSS

Digital Ground

DVDD

Digital Supply

Not Connected

Bit 0 (LSB)

Bit 1

Bit 2

Bit 3

Bit 4

Bit 5

Bit 6

Bit 7 (MSB)

SELECT

Hi I Channel Out, Lo Q Channel Out

CLOCK

Clock

SLEEP

Hi Power Down, Lo Normal Operation

INA-I

I Channel, A Input

INB-I

I Channel, B Input

REFT-I

Top Reference Decoupling, I Channel

REFB-I

Bottom Reference Decoupling, I Channel

AVSS

Analog Ground

REFSENSE

Reference Select

VREF

Internal Reference Output

AVDD

Analog Supply

REFB-Q

Bottom Reference Decoupling, Q Channel

REFT-Q

Top Reference Decoupling, Q Channel

INB-Q

Q Channel B Input

INA-Q

Q Channel A Input

CHIP-SELECT

Hi-High Impedance, Lo-Normal Operation

DEFINITIONS OF SPECIFICATIONS

INTEGRAL NONLINEARITY (INL)

Integral nonlinearity refers to the deviation of each individual
code from a line drawn from "zero" through "full scale." The
point used as "zero" occurs 1/2 LSB before the first code transi-
tion. "Full scale" is defined as a level 1 1/2 LSBs beyond the last
code transition. The deviation is measured from the center of
each particular code to the true straight line.

DIFFERENTIAL NONLINEARITY (DNL, NO MISSING
CODES)

An ideal ADC exhibits code transitions that are exactly 1 LSB
apart. DNL is the deviation from this ideal value. It is often
specified in terms of the resolution for which no missing codes
(NMC) are guaranteed.

AD9281

REV. E

DRVDD

AVSS

DRVSS

AVDD

AVSS

AVDD

REFBS

REFBF

AVDD

AVSS

AVDD

AVSS

AVDD

AVSS

AVDD

AVSS

AVDD

AVSS

AVDD

AVSS

AVDD

AVSS

d. INA, INB

e. Reference

f. REFSENSE

g. VREF

Figure 2. Equivalent Circuits

OFFSET ERROR

The first transition should occur at a level 1 LSB above "zero."
Offset is defined as the deviation of the actual first code transi-
tion from that point.

OFFSET MATCH

The change in offset error between I and Q channels.

EFFECTIVE NUMBER OF BITS (ENOB)

For a sine wave, SINAD can be expressed in terms of the num-
ber of bits. Using the following formula,

N = (SINAD 1.76)/6.02

It is possible to get a measure of performance expressed as N,
the effective number of bits.

Thus, effective number of bits for a device for sine wave inputs
at a given input frequency can be calculated directly from its
measured SINAD.

TOTAL HARMONIC DISTORTION (THD)

THD is the ratio of the rms sum of the first six harmonic com-
ponents to the rms value of the measured input signal and
is expressed as a percentage or in decibels.

SIGNAL-TO-NOISE RATIO (SNR)

SNR is the ratio of the rms value of the measured input signal
to the rms sum of all other spectral components below the
Nyquist frequency, excluding the first six harmonics and dc.
The value for SNR is expressed in decibels.

SPURIOUS FREE DYNAMIC RANGE (SFDR)

The difference in dB between the rms amplitude of the input
signal and the peak spurious signal.

GAIN ERROR

The first code transition should occur for an analog value 1 LSB
above nominal negative full scale. The last transition should
occur for an analog value 1 LSB below the nominal positive full

scale. Gain error is the deviation of the actual difference be-
tween first and last code transitions and the ideal difference
between the first and last code transitions.

GAIN MATCH

The change in gain error between I and Q channels.

PIPELINE DELAY (LATENCY)

The number of clock cycles between conversion initiation and
the associated output data being made available. New output
data is provided every rising clock edge.

MUX SELECT DELAY

The delay between the change in SELECT pin data level and
valid data on output pins.

POWER SUPPLY REJECTION

The specification shows the maximum change in full scale from
the value with the supply at the minimum limit to the value
with the supply at its maximum limit.

APERTURE JITTER

Aperture jitter is the variation in aperture delay for successive
samples and is manifested as noise on the input to the A/D.

APERTURE DELAY

Aperture delay is a measure of the Sample-and-Hold Amplifier
(SHA) performance and is measured from the rising edge of the
clock input to when the input signal is held for conversion.

SIGNAL-TO-NOISE AND DISTORTION (S/N+D, SINAD)
RATIO

S/N+D is the ratio of the rms value of the measured input sig-
nal 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.

a. D0D9 b. Three-State Standby c. CLK

AD9281

REV. E

Typical Characteristic Curves

(AVDD = +3 V, DVDD = +3 V, F

= 28 MHz (50% duty cycle), 2 V input span from 0.5 V to +1.5 V, 2 V internal reference unless otherwise noted)

CODE OFFSET

LSB

16 32 48 64 80 96 112 128 144 160 176 192 208 224 240

Figure 3. Typical INL

CODE OFFSET

LSB

16 32 48 64 80 96 112 128 144 160 176 192 208 224 240

Figure 4. Typical DNL

INPUT VOLTAGE Volts

1.00

1.0

2.0

0.5

 nA

0.5

1.0

1.5

0.80

0.20

0.40

0.60

0.80

0.60

0.40

0.00

0.20

Figure 5. Input Bias Current vs. Input Voltage

INPUT FREQUENCY Hz

1.0E+05

SNR dB

1.0E+06

1.0E+07

1.0E+08

0.5dB

6dB

20dB

Figure 6. SNR vs. Input Frequency

INPUT FREQUENCY Hz

1.0E+05

SNR dB

1.0E+06

1.0E+07

1.0E+08

0.5dB

6dB

20dB

Figure 7. SINAD vs. Input Frequency

INPUT FREQUENCY Hz

30

1.0E+05

THD dB

35

40

45

50

55

60

1.0E+06

1.0E+07

1.0E+08

65

70

0.5dB

6dB

20dB

Figure 8. THD vs. Input Frequency

AD9281

REV. E

CLOCK FREQUENCY Hz

1.00E+06

1.00E+08

1.00E+07

THD dB

Figure 9. THD vs. Clock Frequency

TEMPERATURE C

1.013

1.012

1.008

40

100

20

REF

 Volts

1.011

1.010

1.009

Figure 10. Voltage Reference Error vs. Temperature

CLOCK FREQUENCY MHz

185

POWER CONSUMPTION mW

220

210

200

190

240

230

225

215

205

235

195

Figure 11. Power Consumption vs. Clock Frequency

CODE

1.20E+07

1.00E+07

0.00E+00

N+1

N1

HITS

8.00E+06

6.00E+06

4.00E+06

2.00E+06

12050

10000000

800

Figure 12. Grounded Input Histogram

INPUT FREQUENCY Hz

1.00E+06

AMPLITUDE dB

12

15

21

1.00E+07

1.00E+08

1.00E+09

24

27

18

Figure 13. Full Power Bandwidth

INPUT FREQUENCY Hz

1.00E+05

SNR dB

1.00E+06

1.00E+07

1.00E+08

0.5dB

6dB

20dB

Figure 14. SNR vs. Input Frequency (Single-Ended)

AD9281

REV. E

THEORY OF OPERATION

The AD9281 integrates two A/D converters, two analog input
buffers, an internal reference and reference buffer, and an out-
put multiplexer. For clarity, this data sheet refers to the two
converters as "I" and "Q." The two A/D converters simulta-
neously sample their respective inputs on the rising edge of the
input clock. The two converters distribute the conversion opera-
tion over several smaller A/D sub-blocks, refining the conversion
with progressively higher accuracy as it passes the result from
stage to stage. As a consequence of the distributed conversion,
each converter requires a small fraction of the 256 comparators
used in a traditional flash-type 8-bit ADC. A sample-and-hold
function within each of the stages permits the first stage to oper-
ate on a new input sample while the following stages continue to
process previous samples. This results in a "pipeline processing"
latency of three clock periods between when an input sample is
taken and when the corresponding ADC output is updated into
the output registers.

The AD9281 integrates input buffer amplifiers to drive the
analog inputs of the converters. In most applications, these
input amplifiers eliminate the need for external op amps for the
input signals. The input structure is fully differential, but the
SHA common-mode response has been designed to allow the

converter to readily accommodate either single-ended or differ-
ential input signals. This differential structure makes the part
capable of accommodating a wide range of input signals.

The AD9281 also includes an on-chip bandgap reference and
reference buffer. The reference buffer shifts the ground-referred
reference to levels more suitable for use by the internal circuits
of the converter. Both converters share the same reference and
reference buffer. This scheme provides for the best possible gain
match between the converters while simultaneously minimizing
the channel-to-channel crosstalk.

Each A/D converter has its own output latch, which updates on
the rising edge of the input clock. A logic multiplexer, con-
trolled through the SELECT pin, determines which channel is
passed to the digital output pins. The output drivers have their
own supply, allowing the part to be interfaced to a variety of
logic families. The outputs can be placed in a high impedance
state using the CHIP SELECT pin.

The AD9281 has great flexibility in its supply voltage. The
analog and digital supplies may be operated from 2.7 V to 5.5 V,
independently of one another.

ANALOG INPUT

Figure 16 shows an equivalent circuit structure for the analog
input of one of the A/D converters. PMOS source-followers
buffer the analog input pins from the charge kickback problems
normally associated with switched capacitor ADC input struc-
tures. This produces a very high input impedance on the part,
allowing it to be effectively driven from high impedance sources.
This means that the AD9281 could even be driven directly by a
passive antialias filter.

ADC

CORE

+FS

LIMIT

FS

LIMIT

BUFFER

IINA

IINB

REF

+FS LIMIT =

REF

REF/2

FS LIMIT =
V

REF

REF/2

OUTPUT

WORD

SHA

Figure 16. Equivalent Circuit for AD9281 Analog Inputs

The source followers inside the buffers also provide a level-shift
function of approximately 1 V, allowing the AD9281 to accept
inputs at or below ground. One consequence of this structure is
that distortion will result if the analog input comes within 1.4 V
of the positive supply. For optimum high frequency distortion
performance, the analog input signal should be centered accord-
ing to Figure 27.

The capacitance load of the analog input pin is 4 pF to the
analog supplies (AVSS, AVDD).

Full-scale setpoints may be calculated according to the following
algorithm (V

REF

may be internally or externally generated):

= V

REF

/2)

= V

REF

+ (V

REF

/2)

SPAN

= V

REF

10.0

0.0E+0

SNR dB

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

110.0

2.0E+6

4.0E+6

6.0E+6

8.0E+6 10.0E+6 12.0E+6 14.0E+6

FUND

5TH

6TH

4TH

7TH

3RD

9TH

2ND

8TH

Figure 15a. Simultaneous Operation of I and Q Channels

10.0

0.0E+0

SNR dB

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

110.0

2.0E+6

4.0E+6

6.0E+6

8.0E+6 10.0E+6 12.0E+6 14.0E+6

FUND

5TH

4TH

7TH

3RD

2ND

8TH

6TH

Figure 15b. Simultaneous Operation of I and Q Channels

AD9281

REV. E

The AD9281 can accommodate a variety of input spans be-
tween 1 V and 2 V. For spans of less than 1 V, expect a propor-
tionate degradation in SNR. Use of a 2 V span will provide the
best noise performance. 1 V spans will provide lower distortion
when using a 3 V analog supply. Users wishing to run with larger
full-scales are encouraged to use a 5 V analog supply (AVDD).

Single-Ended Inputs:

For single-ended input signals, the

signal is applied to one input pin and the other input pin is tied
to a midscale voltage. This midscale voltage defines the center
of the full-scale span for the input signal.

EXAMPLE: For a single-ended input range from 0 V to 1 V
applied to IINA, we would configure the converter for a 1 V
reference (see Figure 17) and apply 0.5 V to IINB.

I OR QREFT

I OR QREFB

IINA

IINB

VREF

REF SENSE

0.1 F

10 F

0.1 F

AD9281

0.1 F

10 F

MIDSCALE

VOLTAGE

= 0.5V (1V)

INPUT

Figure 17. Example Configuration for 0 V1 V Single-
Ended Input Signal

Note that since the inputs are high impedance, this reference
level can easily be generated with an external resistive divider
with large resistance values (to minimize power dissipation). A
decoupling capacitor is recommended on this input to minimize
the high frequency noise-coupling onto this pin. Decoupling
should occur close to the ADC.

Differential Inputs

Use of differential input signals can provide greater flexibility in
input ranges and bias points, as well as offering improvements in
distortion performance, particularly for high frequency input
signals. Users with differential input signals will probably want
to take advantage of the differential input structure of the AD9281.
Performance is still very good for single-ended inputs. Convert-
ing a single-ended input to a differential signal for application to
the converter is probably only worth considering for very high
frequency input signals.

AC-Coupled Inputs

If the signal of interest has no dc component, ac coupling can be
easily used to define an optimum bias point. Figure 18 illustrates
one recommended configuration. The voltage chosen for the dc
bias point (in this case the 1 V reference) is applied to both
IINA and IINB pins through 1 k

resistors (R1 and R2). IINA

is coupled to the input signal through Capacitor C1, while IINB is
decoupled to ground through Capacitor C2.

0.1 F

10 F

0.1 F

ANALOG

INPUT

1.0 F

0.1 F

1.5V

0.5V

I OR QREFT

I OR QREFB

IINA

IINB

VREF

AD9281

REFSENSE

Figure 18. Example Configuration for 0.5 V1.5 V ac
Coupled Single-Ended Inputs

Transformer Coupled Inputs

Another option for input ac coupling is to use a transformer.
This not only provides dc rejection, but also allows truly differ-
ential drive of the AD9281's analog inputs, which will provide
the optimal distortion performance. Figure 19 shows a recom-
mended transformer input drive configuration. Resistors R1 and
R2 define the termination impedance of the transformer cou-
pling. The center tap of the transformer secondary is tied to the
common-mode voltage, establishing the dc bias point for the
analog inputs.

0.1 F

10 F

0.1 F

COMMON

MODE

VOLTAGE

0.1 F

10 F

I OR QREFT

I OR QREFB

IINA

IINB

AD9281

QINB

QINA

REFSENSE

VREF

Figure 19. Example Configuration for Transformer
Coupled Inputs

Crosstalk:

The internal layout of the AD9281, as well as its

pinout, was configured to minimize the crosstalk between the
two input signals. Users wishing to minimize high frequency
crosstalk should take care to provide the best possible decoupling
for input pins (see Figure 20). R and C values will make a pole
dependant on antialiasing requirements. Decoupling is also
required on reference pins and power supplies (see Figure 21).

QINA

QINB

IINA

IINB

AD9281

Figure 20. Input Loading

DVDD

I OR QREFT

I OR QREFB

AVDD

0.1 F

10 F

0.1 F

10 F

AD9281

0.1 F

10 F

V ANALOG

V DIGITAL

Figure 21. Reference and Power Supply Decoupling

AD9281

REV. E

REFERENCE AND REFERENCE BUFFER

The reference and buffer circuitry on the AD9281 is configured
for maximum convenience and flexibility. An illustration of the
equivalent reference circuit is show in Figure 26. The user can
select from five different reference modes through appropriate
pin-strapping (see Table I below). These pin strapping options
cause the internal circuitry to reconfigure itself for the appropri-
ate operating mode.

Table I. Table of Modes

Mode

Input Span

REFSENSE Pin Figure

1 V

VREF

2 V

AGND

Programmable

1 + (R1/R2)

See Figure

External

= External Ref

AVDD

1 V Mode (Figure 22)

--provides a 1 V reference and 1 V input

full scale. Recommended for applications wishing to optimize
high frequency performance, or any circuit on a supply voltage
of less than 4 V. The part is placed in this mode by shorting the
REFSENSE pin to the VREF pin.

I OR QREFT

I OR QREFB

IINA

IINB

VREF

0.1 F

10 F

0.1 F

AD9281

0.1 F

10 F

QINB

QINA

REFSENSE

Figure 22. 0 V to 1 V Input

2 V Mode (Figure 23)

--provides a 2 V reference and 2 V input

full scale. Recommended for noise sensitive applications on 5 V
supplies. The part is placed in 2 V reference mode by ground-
ing (shorting to AVSS) the REFSENSE pin.

I OR QREFT

I OR QREFB

IINA

IINB

VREF

0.1 F

10 F

0.1 F

AD9281

0.1 F

10 F

QINB

QINA

REFSENSE

Figure 23. 0 V to 2 V Input

Externally Set Voltage Mode (Figure 24)

--this mode uses

the on-chip reference, but scales the exact reference level though
the use of an external resistor divider network. VREF is wired to
the top of the network, with the REFSENSE wired to the tap
point in the resistor divider. The reference level (and input full
scale) will be equal to 1 V

× (R1 + R2)/R1. This method can be

used for voltage levels from 0.7 V to 2.5 V.

I OR QREFT

I OR QREFB

VREF

0.1 F

10 F

0.1 F

AD9281

REFSENSE

+ 

AVSS

0.1 F

1 F

VREF = 1 +

R2
R1

Figure 24. Programmable Reference

External Reference Mode (Figure 25)

--in this mode, the on-

chip reference is disabled, and an external reference applied to
the VREF pin. This mode is achieved by tying the REFSENSE
pin to AVDD.

EXT

REFERENCE

AVDD

I OR QREFT

I OR QREFB

IINA

IINB

VREF

0.1 F

10 F

0.1 F

AD9281

0.1 F

10 F

QINB

QINA

REFSENSE

Figure 25. External Reference

Reference Buffer

--The reference buffer structure takes the

voltage on the VREF pin and level-shifts and buffers it for use
by various sub-blocks within the two A/D converters. The two
converters share the same reference buffer amplifier to maintain
the best possible gain match between the two converters. In the
interests of minimizing high frequency crosstalk, the buffered
references for the two converters are separately decoupled on
the IREFB, IREFT, QREFB and QREFT pins, as illustrated in
Figure 26.

AD9281

REV. E

QREFT

QREFB

IREFT

0.1 F

10 F

0.1 F

AD9281

REFSENSE

AVSS

0.1 F

10 F

IREFB

VREF

0.1 F

10k

ADC

CORE

INTERNAL

CONTROL

LOGIC

0.1 F

1.0 F

Figure 26. Reference Buffer Equivalent Circuit and
External Decoupling Recommendation

For best results in both noise suppression and robustness
against crosstalk, the 4-capacitor buffer decoupling arrangement
shown in Figure 26 is recommended. This decoupling should

0.5

THD dB

13

23

33

43

53

63

73

0.5

1.5

CML V

a. Differential Input, 3 V Supplies

35

0.5

THD dB

40

45

50

55

60

65

70

0.5

1.5

CML V

2.5

b. Differential Input, 5 V Supplies

Figure 27. THD vs. CML Input Span and Power Supply (Analog Input = 1 MHz)

15

0.5

THD dB

25

35

45

55

65

0.5

1.5

CML V

c. Single-Ended Input, 3 V Supplies

15

0.5

THD dB

25

35

45

55

65

0.5

1.5

CML V

2.5

d. Single-Ended Input, 5 V Supplies

feature chip capacitors located close to the converter IC. The
capacitors are connected to either IREFT/IREFB or QREFT/
QREFB. A connection to both sides is not required.

COMMON-MODE PERFORMANCE

Attention to the common-mode point of the analog input volt-
age can improve the performance of the AD9281. Figure 27
illustrates THD as a function of common-mode voltage (center
point of the analog input span) and power supply.

Inspection of the curves will yield the following conclusions:

1. An AD9281 running with AVDD = 5 V is the easiest to

drive.

2. Differential inputs are the most insensitive to common-mode

voltage.

3. An AD9281 powered by AVDD = 3 V and a single ended

input, should have a 1 V span with a common-mode voltage
of 0.75 V.

AD9281

REV. E

SELECT

When the select pin is held LOW, the output word will present
the "Q" level. When the select pin is held HIGH, the "I" level
will be presented to the output word (see Figure 1).

The AD9281's select and clock pins may be driven by a com-
mon signal source. The data will change in 5 ns to 11 ns after
the edges of the input pulse. The user must make sure the inter-
face latches have sufficient hold time for the AD9281's delays
(see Figure 28).

CLOCK

DATA

I LATCH

CLOCK

DATA

Q LATCH

CLK

DATA

OUT

SELECT

PROCESSING

CLOCK

SOURCE

Figure 28. Typical De-Mux Connection

APPLICATIONS
USING THE AD9281 FOR QAM DEMODULATION

QAM is one of the most widely used digital modulation schemes in
digital communication systems. This modulation technique
can be found in both FDMA as well as spread spectrum (i.e.,
CDMA) based systems. A QAM signal is a carrier frequency
which is both modulated in amplitude (i.e., AM modulation)
and in phase (i.e., PM modulation). At the transmitter, it can
be generated by independently modulating two carriers of iden-
tical frequency but with a 90

phase difference. This results in

an inphase (I) carrier component and a quadrature (Q) carrier
component at a 90

phase shift with respect to the I component.

The I and Q components are then summed to provide a QAM
signal at the specified carrier or IF frequency. Figure 29 shows
a typical analog implementation of a QAM modulator using a
dual 10-bit DAC with 2

× interpolation, the AD9761. A QAM

signal can also be synthesized in the digital domain thus requir-
ing a single DAC to reconstruct the QAM signal. The AD9853
is an example of a complete (i.e., DAC included) digital QAM
modulator.

DSP

ASIC

CARRIER

FREQUENCY

NYQUIST

FILTERS

TO
MIXER

QUADRATURE

MODULATOR

AD9761

IOUT

QOUT

Figure 29. Typical Analog QAM Modulator Architecture

DIGITAL INPUTS AND OUTPUTS

Each of the AD9281 digital control inputs, CHIP SELECT,
CLOCK, SELECT and SLEEP are referenced to AVDD and
AVSS. Switching thresholds will be AVDD/2.

The format of the digital output is straight binary. A low power
mode feature is provided such that for STBY = HIGH and the
clock disabled, the static power of the AD9281 will drop below
22 mW.

CLOCK INPUT

The AD9281 clock input is internally buffered with an inverter
powered from the AVDD pin. This feature allows the AD9281
to accommodate either +5 V or +3.3 V CMOS logic input sig-
nal swings with the input threshold for the CLK pin nominally
at AVDD/2.

The pipelined architecture of the AD9281 operates on both
rising and falling edges of the input clock. To minimize duty
cycle variations the logic family recommended to drive the clock
input is high speed or advanced CMOS (HC/HCT, AC/ACT)
logic. CMOS logic provides both symmetrical voltage threshold
levels and sufficient rise and fall times to support 28 MSPS
operation. Running the part at slightly faster clock rates may be
possible, although at reduced performance levels. Conversely,
some slight performance improvements might be realized by
clocking the AD9281 at slower clock rates.

The power dissipated by the output buffers is largely propor-
tional to the clock frequency; running at reduced clock rates
provides a reduction in power consumption.

DIGITAL OUTPUTS

Each of the on-chip buffers for the AD9281 output bits (D0D9)
is powered from the DVDD supply pin, separate from AVDD.
The output drivers are sized to handle a variety of logic families
while minimizing the amount of glitch energy generated. In all
cases, a fan-out of one is recommended to keep the capacitive
load on the output data bits below the specified 20 pF level.

For DVDD = 5 V, the AD9281 output signal swing is compat-
ible with both high speed CMOS and TTL logic families. For
TTL, the AD9281 on-chip, output drivers were designed to
support several of the high speed TTL families (F, AS, S). For
applications where the clock rate is below 28 MSPS, other TTL
families may be appropriate. For interfacing with lower voltage
CMOS logic, the AD9281 sustains 28 MSPS operation with
DVDD = 3 V. In all cases, check your logic family data sheets
for compatibility with the AD9281's Specification table.

A 2 ns reduction in output delays can be achieved by limiting
the logic load to 5 pF per output line.

THREE-STATE OUTPUTS

The digital outputs of the AD9281 can be placed in a high
impedance state by setting the CHIP SELECT pin to HIGH.
This feature is provided to facilitate in-circuit testing or evaluation.

AD9281

REV. E

ANALOG

CIRCUITS

DIGITAL

LOGIC

ICs

DVSS

AVSS

AVDD

DVDD

LOGIC

SUPPLY

STRAY

GND

= ANALOG

= DIGITAL

ADC

DIGITAL

CIRCUITS

Figure 31. Ground and Power Consideration

These characteristics result in both a reduction of electro-
magnetic interference (EMI) and an overall improvement in
performance.

It is important to design a layout that prevents noise from cou-
pling onto the input signal. Digital signals should not be run in
parallel with the input signal traces and should be routed
away from the input circuitry. Separate analog and digital
grounds should be joined together directly under the AD9281 in
a solid ground plane. The power and ground return currents
must be carefully managed. A general rule of thumb for mixed
signal layouts dictates that the return currents from digital cir-
cuitry should not pass through critical analog circuitry.

Transients between AVSS and DVSS will seriously degrade
performance of the ADC.

If the user cannot tie analog ground and digital ground together
at the ADC, he should consider the configuration in Figure 32.

Another input and ground technique is shown in Figure 32. A
separate ground plane has been split for RF or hard to manage
signals. These signals can be routed to the ADC differentially or
single ended (i.e., both can either be connected to the driver or
RF ground). The ADC will perform well with several hundred
mV of noise or signals between the RF and ADC analog ground.

DATA

ANALOG

GROUND

DIGITAL

GROUND

LOGIC

ADC

AIN

BIN

GROUND

Figure 32. RF Ground Scheme

At the receiver, the demodulation of a QAM signal back into its
separate I and Q components is essentially the modulation
process explain above but in the reverse order. A common and
traditional implementation of a QAM demodulator is shown in
Figure 30. In this example, the demodulation is performed in
the analog domain using a dual, matched ADC and a quadra-
ture demodulator to recover and digitize the I and Q baseband
signals. The quadrature demodulator is typically a single IC
containing two mixers and the appropriate circuitry to generate
the necessary 90

phase shift between the I and Q mixers' local

oscillators. Before being digitized by the ADCs, the mixed
down baseband I and Q signals are filtered using matched ana-
log filters. These filters, often referred to as Nyquist or Pulse-
Shaping filters, remove images-from the mixing process and any
out-of-band. The characteristics of the matching Nyquist filters
are well defined to provide optimum signal-to-noise (SNR)
performance while minimizing intersymbol interference. The
ADC's are typically simultaneously sampling their respective
inputs at the QAM symbol rate or, most often, at a multiple of it
if a digital filter follows the ADC. Oversampling and the use of
digital filtering eases the implementation and complexity of the
analog filter. It also allows for enhanced digital processing for
both carrier and symbol recovery and tuning purposes. The use
of a dual ADC such as the AD9281 ensures excellent gain,
offset, and phase matching between the I and Q channels.

90°C

FROM
PREVIOUS
STAGE

QUADRATURE

DEMODULATOR

ADC

DSP

ASIC

CARRIER

FREQUENCY

NYQUIST

FILTERS

ADC

DUAL MATCHED

ADC

Figure 30. Typical Analog QAM Demodulator

GROUNDING AND LAYOUT RULES

As is the case for any high performance device, proper ground-
ing and layout techniques are essential in achieving optimal
performance. The analog and digital grounds on the AD9281
have been separated to optimize the management of return
currents in a system. Grounds should be connected near the
ADC. It is recommended that a printed circuit board (PCB) of
at least four layers, employing a ground plane and power planes,
be used with the AD9281. The use of ground and power planes
offers distinct advantages:

1. The minimization of the loop area encompassed by a signal

and its return path.

2. The minimization of the impedance associated with ground

and power paths.

3. The inherent distributed capacitor formed by the power plane,

PCB insulation and ground plane.

AD9281

REV. E

R14

(NOT TO SCALE)

AGND

R37

R13

R11

JP21

AVDD

BJ2

BJ1

C42

C40

R38

C38

C41

JP22

R31

R33

R32

JP13

4
TP2

TP1

C15
+

+
C25

TP5

JP14

TP6

JP3

JP10

C1 JP2 JP1

R34

C34

JP7
JP9

R40

R35

C37

JP12JP11

C31
+

AGND

R30

R23

R12

TP3

C32

C21

C12

C11

R16

R17

R24

JP6

JP5

JP4

R10

R18

C19

C24 +

DGND

C30

C49 +

C10

C13

DBVDD

RN2

JP20

RN1

JP17

C47

BJ5

C48

+ C46

JP19

R36

+ C43

C44

BJ6

BJ4

BJ3

C45

TP4

JP16

TP7

JP15

C33

R39

I_IN

STROBE

AGND

AVDD

CLOCK

DGND1

DVDD

DGND2 DBVDD

Q_IN

Figure 38. Evaluation Board Component-Side Silkscreen

(NOT TO SCALE)

Figure 39. Evaluation Board Power Plane Layout

AD9281

REV. E

Figure 40. Evaluation Board

STROBE

AVDD

AD822

ADJ_REF

AVDD

AD822

AVDD

DVDD

DRVDD

VCCB

NC1

GND1

VCCA

T/R

GND2

GND3

0.1

DVDD

74LVXC4245

VCCB

NC1

GND1

VCCA

T/R

GND2

GND3

0.1

DVDD

JP16

HDR3

JP15

HDR3

74AHC14DW

AVDD

U8A

U8B

U8C

C38

0.1

DUTCLK

R39

R-S 50

TP7

CON1

TP4

CON1

R-S TBD

R36

CLK0

C33

0.1

AVDD

R31

500

R32

POT_2k

R33

500

R38

R-S

ADC_CLK

BNC

74LVXC4245

DRVDD

BCLK0

BD0

BD1

BD2

BD3

BD4

BD5

BD7

BD8

BD9

BD6

DRVDD

SLEEP

INA-1

INB-1

REFT-1

REFB-1

AVSS

REFSENSE

VREF

AVDD

REFB-Q

REFT-Q

INB-Q

INA-Q

CHIP-SELECT

DUTCLK

SELECT

DVDD

DVSS

TESTCHIP

D4
D8

DRVDD

JP17

HDR3

RESISTOR

7PACK

CLK

OUT

CON40

74AHC14DW

U8F

74AHC14DW

U8E

74AHC14DW

U8D

C10

0.1

JP20

HDR3

JP19

HDR3

0.1

CLK0

C13

0.1

DUTDATA

[0...9]

DGND

DVDD

FERRITE BEAD

C45

CAP_NP

C44

CAP_NP

C43

10_10V

DVDD

FERRITE BEAD

C48

CAP_NP

C47

CAP_NP

C46

10_10V

DRVDD

BJ3

BANA

BJ4

BANA

BJ5

BANA

BJ6

BANA

AVDD

AVDD VCC

FERRITE BEAD

C42

CAP_NP

C41

CAP_NP

C40

10_10V

BNC

STROBE

R37

R-S 49.9

RN1A

RN1B

RN1C

RN1D

RN1E

RN1F

RN2A

RN2B

RN2C

RN2D

RN2E

RN2F

BNC

CH1IN

R-S 50

HDR3

JP3

TRANSFORMER

JP2

JUMPER

JP2

JUMPER

0.1

10_6V3

TP1

CON1

R-S TBD

TP2

CON1

DCIN1

AGND

R_VREF

JP13

HDR3

C54

1000pF

10_6V3

0.1

R-S 100

MIDSCALE_IN

AD9201/ AD9281

NOT TO SCALE

DRVDD

D[0...9]

DRVDD

C29

CAP_NP

FERRITE_BEAD

DVDD

C53

10pF

C52

10pF

C23

0.1

C25

CAP_P

C26

CAP_NP

C27 0.1

R52

R53

JP6

HDR3

AVDD

INA-Q

INB-Q

JP14

HDR4

R_VREF

TP6

CON1

C55

1000pF

C36

10_6V3

C35

0.1

R35

R-S 1

TP5

CON1

JP11

JUMPER

BNC

JUMPER

C34 0.1

C37

10_6V3

TRANSFORMER CT

R40

R-S 100

JP10

HDR3

R34

R-S 50

CHOIN

JP12

R30

1.5k

R23

POT_10k

C31

10_6V3

C32 0.1

R24

R-S 22

R17

15k

R16

C21

CAP_NP

ADJ_REF

5.49k

DIODE_ZENER

POT_10k

R12

1.5k

C12

0.1k

C11

10_6V3

R10

R-S 10

CAP_NP

15k

JP5

JUMPER

JP7

JUMPER

JP9

JUMPER

C24

10_10V

C19

10_10V

C20

0.1k

C22

0.1k

R18

R-S TBD

R14

R-S TBD

JP4

C16

CAP_NP

C15

CAP_P

C14

0.1

C17

0.1

C50

10pF

C51

10pF

R51

R50

R11

DUTCLK

JP22

AVDD

HDR3

R13

C30

CAP_NP

C49

10_10V

TP3

CON1

APWRIN

GND

BJ1

BANA

BJ2

BANA

DPWRIN

INA-1

INB-1

MIDSCALE_I

JP21

VREF

DCINO

AVDD

R_VREF

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