FEATURES
Complete Dual Matching ADCs
Low Power Dissipation: 215 mW (+3 V Supply)
Single Supply: 2.7 V to 5.5 V
Differential Nonlinearity Error: 0.4 LSB
On-Chip Analog Input Buffers
On-Chip Reference
Signal-to-Noise Ratio: 57.8 dB
Over Nine Effective Bits
Spurious-Free Dynamic Range: –73 dB
No Missing Codes Guaranteed
28-Lead SSOP
PRODUCT DESCRIPTION
The AD9201 is a complete dual channel, 20 MSPS, 10-bit
CMOS ADC. The AD9201 is optimized specifically for applications where close matching between two ADCs is required (e.g.,
I/Q channels in communications applications). The 20 MHz
sampling rate and wide input bandwidth will cover both narrowband and spread-spectrum channels. The AD9201 integrates two
10-bit, 20 MSPS ADCs, two input buffer amplifiers, an internal
voltage reference and multiplexed digital output buffers.
Each ADC incorporates a simultaneous sampling sample-andhold amplifier at its input. The analog inputs are buffered; no
external input buffer op amp will be required in most applications. 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 multiplexed digital output buffer.
The AD9201 is manufactured on an advanced low cost CMOS
process, operates from a single supply from 2.7 V to 5.5 V, and
consumes 215 mW of power (on 3 V supply). The AD9201 input
structure accepts either single-ended or differential signals,
providing excellent dynamic performance up to and beyond
its 10 MHz Nyquist input frequencies.
Resolution CMOS ADC
AD9201
FUNCTIONAL BLOCK DIAGRAM
IINA
IINB
IREFB
IREFT
QREFB
QREFT
VREF
REFSENSE
QINB
QINA
AVDD AVSS
"I" ADC
REFERENCE
BUFFER
"Q" ADC
CLOCK
I
REGISTER
ASYNCHRONOUS
MULTIPLEXER
1V
Q
REGISTER
PRODUCT HIGHLIGHTS
1. Dual 10-Bit, 20 MSPS ADCs
A pair of high performance 20 MSPS ADCs that are optimized 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
215 mW on a single supply (on 3 V supply). The AD9201
operates on supply voltages from 2.7 V to 5.5 V.
3. On-Chip Voltage Reference
The AD9201 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 10-Bit Digital Output Bus
The AD9201 ADC outputs are interleaved onto a single
output bus saving board space and digital pin count.
6. Small Package
The AD9201 offers the complete integrated function in a
compact 28-lead SSOP package.
7. Product Family
The AD9201 dual ADC is pin compatible with a dual 8-bit
ADC (AD9281) and has a companion dual DAC product,
the AD9761 dual DAC.
DVDD DVSS
AD9201
THREE-
STATE
OUTPUT
BUFFER
SLEEP
SELECT
DATA
10 BITS
CHIP
SELECT
REV. D
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.
*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.
13SELECTHi I Channel Out, Lo Q Channel Out
14CLOCKClock
15SLEEPHi Power Down, Lo Normal Operation
16INA-II Channel, A Input
17INB-II Channel, B Input
18REFT-ITop Reference Decoupling, I Channel
19REFB-IBottom Reference Decoupling, I Channel
20AVSSAnalog Ground
21REFSENSEReference Select
22VREFInternal Reference Output
23AVDDAnalog Supply
24REFB-QBottom Reference Decoupling, Q Channel
25REFT-QTop Reference Decoupling, Q Channel
26INB-QQ Channel, B Input
27INA-QQ Channel, A Input
28CHIP-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 transition. “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.
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 AD9201 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.
–4–
REV. D
AVDD
DRVDD
AVDD
AVDD
AVDD
AD9201
AVDD
AVSS
DRVSS
DRVSS
AVSS
a. D0–D9, OTRb. Three-State, Standbyc. CLK
AVDD
AVDD
AVSS
AVSS
IN
AVSS
AVDD
AVDD
REFBS
AVSS
REFBF
d. INA, INBe. Referencef. REFSENSEg. 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 transition 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 number 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 components 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.
AVSS
AVDD
AVSS
AVSS
AVDD
AVSS
AVSS
scale. Gain error is the deviation of the actual difference between 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.
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
REV. D
–5–
SIGNAL-TO-NOISE AND DISTORTION (S/N+D, SINAD)
RATIO
S/N+D 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, including harmonics but excluding dc.
The value for S/N+D is expressed in decibels.
AD9201
INPUT FREQUENCY – Hz
65
60
35
1.00E+05
SNR – dB
50
45
40
55
1.00E+061.00E+071.00E+08
–6dB
–20dB
–0.5dB
–Typical Characteristic Curves
(AVDD = +3 V, DVDD = +3 V, FS = 20 MHz (50% duty cycle), 2 V input span from –0.5 V to
+1.5 V, 2 V internal reference unless otherwise noted)
1.5
1.0
0.5
0
INL
–0.5
–1.0
–1.5
0768128
Figure 3. Typical INL (1 V Internal Reference)
256384512640
CODE OFFSET
8961024
Figure 6. SNR vs. Input Frequency
1
0.5
0
DNL
–0.5
–1.0
0
256384512640
128
CODE OFFSET
768
896 1024
Figure 4. Typical DNL (1 V Internal Reference)
1.00
0.80
0.60
0.40
0.20
0.00
– nA
B
I
–0.20
–0.40
–0.60
–0.80
–1.00
–1.02.0–0.5
00.51.01.5
INPUT VOLTAGE – V
Figure 5. Input Bias Current vs. Input Voltage
65
60
55
50
SINAD – dB
45
40
35
1.00E+05
–0.5dB
–6dB
–20dB
1.00E+061.00E+071.00E+08
INPUT FREQUENCY – Hz
Figure 7. SINAD vs. Input Frequency
–30
–35
–40
–45
–50
–55
THD – dB
–60
–65
–70
–75
–80
1.00E+05
–20dB
–6dB
–0.5dB
1.00E+061.00E+071.00E+08
INPUT FREQUENCY – Hz
Figure 8. THD vs. Input Frequency
–6–
REV. D
–75
CODE
HITS
N–1
1.00E+07
1.20E+07
8.00E+06
6.00E+06
4.00E+06
2.00E+06
0.00E+00
NN+1
10000000
255100
150400
INPUT FREQUENCY – Hz
–12
–15
–30
AMPLITUDE – dB
–21
–24
–27
–18
1.00E+061.00E+071.00E+08
1.00E+09
–9
–6
–3
0
INPUT FREQUENCY – Hz
60
55
35
1.00E+051.00E+08
1.00E+06
SNR – dB
1.00E+07
50
45
40
–0.5dB
–6.0dB
–20.0dB
–70
–65
THD – dB
–60
–55
–50
1.00E+061.00E+071.00E+08
CLOCK FREQUENCY – Hz
AD9201
Figure 9. THD vs. Clock Frequency (fIN = 1 MHz)
1.012
1.011
1.010
– V
1.009
REF
V
1.008
1.007
1.006
–4080–20
0204060
TEMPERATURE – 8C
100
Figure 10. Voltage Reference Error vs. Temperature
Figure 15. Simultaneous Operation of I and Q Channels
(Differential Input)
The AD9201 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. (See Figure 16.)
Each A/D converter has its own output latch, which updates on
the rising edge of the input clock. A logic multiplexer, controlled through the SELECT pin, determines which channel is
passed to the digital output pins. The output drivers have their
own supply (DVDD), 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 AD9201 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 structures. This produces a very high input impedance on the part,
allowing it to be effectively driven from high impedance sources.
This means that the AD9201 could even be driven directly by a
passive antialias filter.
THEORY OF OPERATION
The AD9201 integrates two A/D converters, two analog input
buffers, an internal reference and reference buffer, and an output multiplexer. For clarity, this data sheet refers to the two
converters as “I” and “Q.” The two A/D converters simultaneously sample their respective inputs on the rising edge of the
input clock. The two converters distribute the conversion operation over several smaller A/D subblocks, 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 1023 comparators
used in a traditional flash-type 10-bit ADC. A sample-and-hold
function within each of the stages permits the first stage to operate 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 AD9201 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 differential input signals. This differential structure makes the part
capable of accommodating a wide range of input signals.
–8–
Figure 16. Equivalent Circuit for AD9201 Analog Inputs
The source followers inside the buffers also provide a level-shift
function of approximately 1 V, allowing the AD9201 to accept
inputs at or below ground. One consequence of this structure is
that distortion will result if the analog input approaches the
positive supply. For optimum high frequency distortion performance, the analog input signal should be centered according
to Figure 29.
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
–F
= (V
S
= (V
+F
S
= V
V
SPAN
may be internally or externally generated):
REF
– V
+ V
REF
REF
/2)
/2)
REF
REF
REF
REV. D
AD9201
0.1mF10mF
0.1mF
0.1mF
COMMON
MODE
VOLTAGE
0.1mF
10mF
R1
R2
I OR QREFT
I OR QREFB
IINA
IINB
AD9201
QINB
QINA
REFSENSE
VREF
QINA
QINB
IINA
IINB
AD9201
DVDD
I OR QREFT
I OR QREFB
AVDD
0.1mF
10mF
0.1mF10mF
AD9201
0.1mF
0.1mF
0.1mF10mF
V ANALOGV DIGITAL
The AD9201 can accommodate a variety of input spans between 1 V and 2 V. For spans of less than 1 V, expect a proportionate 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.
1V
INPUT
MIDSCALE
VOLTAGE
= 0.5V
5kV5kV
0V
IINA
I OR QREFT
0.1mF
I OR QREFB
10mF
IINB
0.1mF
AD9201
VREF
REFSENSE
0.1mF
10mF
0.1mF
10mF
0.1mF
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 kΩ resistors (R1 and R2).
IINA is coupled to the input signal through Capacitor C1, while
IINB is decoupled to ground through Capacitor C2 and C3.
Transformer Coupled Inputs
Another option for input ac coupling is to use a transformer.
This not only provides dc rejection, but also allows truly differential drive of the AD9201’s analog inputs, which will provide
the optimal distortion performance. Figure 19 shows a recommended transformer input drive configuration. Resistors R1 and
R2 define the termination impedance of the transformer coupling.
The center tap of the transformer secondary is tied to the common-mode reference, establishing the dc bias point for the analog inputs.
Figure 17. Example Configuration for 0 V–1 V SingleEnded 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.
1.5V
0.5V
INPUT
C2
1.0mF
C1
R1
1kV
C3
0.1mF
IINA
IINB
AD9201
VREF
REFSENSE
ANALOG
Figure 18. Example Configuration for 0.5 V–1.5 V ac
Coupled Single-Ended Inputs
REV. D
REFT
REFB
0.1mF
0.1mF
10mF
0.1mF
Figure 19. Example Configuration for Transformer
Coupled Inputs
Crosstalk: The internal layout of the AD9201, 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).
Figure 20. Input Loading
Figure 21. Reference and Power Supply Decoupling
–9–
AD9201
I OR QREFT
I OR QREFB
VREF
0.1mF10mF
0.1mF
0.1mF
AD9201
REFSENSE
+
–
AVSS
0.1mF
1mF
R2
R1
1V
VREF = 1 +
R2
R1
+
–
1V
EXT
REFERENCE
AVDD
I OR QREFT
I OR QREFB
IINA
IINB
VREF
0.1mF10mF
0.1mF
0.1mF
0.1mF
AD9201
0.1mF
10mF
10mF
1V
0V
QINB
QINA
5kV
5kV
REFSENSE
1V
0V
REFERENCE AND REFERENCE BUFFER
The reference and buffer circuitry on the AD9201 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 appropriate operating mode.
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.
1V
0V
5kV
0.1mF
10mF
5kV
10mF
0.1mF
1V
IINA
IINB
AD9201
VREF
REFSENSE
I OR QREFT
I OR QREFB
QINA
QINB
1V
0V
0.1mF
0.1mF10mF
0.1mF
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.
Figure 24. Programmable Reference
External Reference Mode (Figure 25)—in this mode, the onchip reference is disabled, and an external reference is applied to
the VREF pin. This mode is achieved by tying the REFSENSE
pin to AVDD.
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 grounding
(shorting to AVSS) the REFSENSE pin.
2V
0V
5kV
IINA
IINB
AD9201
VREF
REFSENSE
0.1mF
10mF
5kV
10mF
0.1mF
Figure 23. 0 V to 2 V Input
QINA
QINB
I OR QREFT
I OR QREFB
2V
0V
0.1mF
0.1mF10mF
0.1mF
–10–
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 subblocks 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.
REV. D
0.1mF
ADC
17
16
1kV
VREF
AD8051
3
2
50V
1kV
1kV
6
22V
22V
0.33mF
0.01mF
10pF
10pF
24V
–120
–110
–100
–90
–80
–70
–60
–50
–40
–30
–20
–10
0
10
0.0E+02.0E+6
1.0E+6
4.0E+66.0E+68.0E+610.0E+6
3.0E+65.0E+67.0E+69.0E+6
FUND
2ND3RD
4TH
5TH
6TH
7TH
8TH
AD9201
ADC
CORE
0.1mF
0.1mF
10mF
IREFT
0.1mF10mF
QREFT
0.1mF
10mF0.1mF
REFSENSE
IREFB
VREF
AVSS
10kV
10kV
1V
INTERNAL
CONTROL
LOGIC
QREFB
0.1mF
AD9201
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
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.
DRIVING THE AD9201
Figure 27 illustrates the use of an AD8051 to drive the AD9201.
Even though the AD8051 is specified with 3 V and 5 V power,
the best results are obtained at ±5 V power. The ADC input
span is 2 V.
Figure 27.
Figure 28. AD8051/AD9201 Performance
REV. D
–11–
AD9201
COMMON-MODE PERFORMANCE
Attention to the common-mode point of the analog input voltage can improve the performance of the AD9201. Figure 29
illustrates THD as a function of common-mode voltage (center
point of the analog input span) and power supply.
–30
–35
–40
–45
–50
–55
THD – dB
–60
–65
–70
–75
–80
–0.51.500.51.0
COMMON-MODE LEVEL – V
2V SPAN
1V SPAN
a. Differential Input, 3 V Supplies
Inspection of the curves will yield the following conclusions:
1. An AD9201 running with AVDD = 5 V is the easiest to
drive.
2. Differential inputs are the most insensitive to common-mode
voltage.
3. An AD9201 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.
–10
2V SPAN
–20
–30
–40
–50
THD – dB
–60
–70
–80
–0.500.51.0
COMMON-MODE LEVEL – V
1V SPAN
1.5
c. Single-Ended Input, 3 V Supplies
–30
–35
–40
–45
–50
–55
THD – dB
–60
–65
–70
1V SPAN
–75
–80
–0.52.500.51.0
COMMON-MODE LEVEL – V
b. Differential Input, 5 V Supplies
Figure 29. THD vs. CML Input Span and Power Supply (Analog Input = 1 MHz)
2V SPAN
1.52.0
–10
–20
–30
–40
–50
THD – dB
–60
–70
–80
–0.52.500.51.0
COMMON-MODE LEVEL – V
d. Single-Ended Input, 5 V Supplies
2V SPAN
1V SPAN
1.52.0
–12–
REV. D
AD9201
CLOCK
DATA
I LATCH
CLOCK
DATA
Q LATCH
CLK
DATA
OUT
SELECT
I
PROCESSING
Q
PROCESSING
CLOCK
SOURCE
0
90
DSP
OR
ASIC
10
CARRIER
FREQUENCY
NYQUIST
FILTERS
TO
MIXER
QUADRATURE
MODULATOR
AD9761
IOUT
QOUT
DIGITAL INPUTS AND OUTPUTS
Each of the AD9201 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 AD9201 will drop below
22 mW.
CLOCK INPUT
The AD9201 clock input is internally buffered with an inverter
powered from the AVDD pin. This feature allows the AD9201
to accommodate either +5 V or +3.3 V CMOS logic input signal swings with the input threshold for the CLK pin nominally
at AVDD/2.
The pipelined architecture of the AD9201 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 20 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 AD9201 at slower clock rates.
The power dissipated by the output buffers is largely proportional 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 AD9201 output bits (D0–D9)
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 AD9201 output signal swing is compatible with both high speed CMOS and TTL logic families. For
TTL, the AD9201 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 20 MSPS, other TTL
families may be appropriate. For interfacing with lower voltage
CMOS logic, the AD9201 sustains 20 MSPS operation with
DVDD = 3 V. In all cases, check your logic family data sheets
for compatibility with the AD9201’s Specification table.
A 2 ns reduction in output delays can be achieved by limiting
the logic load to 5 pF per output line.
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 AD9201’s select and clock pins may be driven by a common 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 interface latches have sufficient hold time for the AD9201’s delays
(see Figure 30).
Figure 30. Typical De-Mux Connection
APPLICATIONS
USING THE AD9201 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 31 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 requiring a single DAC to reconstruct the QAM signal. The AD9853
is an example of a complete (i.e., DAC included) digital QAM
modulator.
THREE-STATE OUTPUTS
The digital outputs of the AD9201 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.
REV. D
Figure 31. Typical Analog QAM Modulator Architecture
–13–
AD9201
DATA
ANALOG
GROUND
DIGITAL
GROUND
LOGIC
ADC
AIN
BIN
RF
GROUND
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 32. In this example, the demodulation is performed in
the analog domain using a dual, matched ADC and a quadrature 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 analog filters. These filters, often referred to as Nyquist or PulseShaping 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 AD9201 ensures excellent gain,
offset, and phase matching between the I and Q channels.
I
ADC
DSP
OR
ASIC
Q
ADC
DUAL MATCHED
ADC
CARRIER
FREQUENCY
NYQUIST
FILTERS
LO
DEMODULATOR
90°C
QUADRATURE
FROM
PREVIOUS
STAGE
Figure 32. Typical Analog QAM Demodulator
GROUNDING AND LAYOUT RULES
As is the case for any high performance device, proper grounding and layout techniques are essential in achieving optimal
performance. The analog and digital grounds on the AD9201
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 AD9201. 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.
These characteristics result in both a reduction of electromagnetic interference (EMI) and an overall improvement in
performance.
It is important to design a layout that prevents noise from coupling 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 AD9201 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 circuitry
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 33.
LOGIC
DV
D
SUPPLY
DIGITAL
LOGIC
ICs
GND
D
V
IN
A
A
D
AA
ADC
IC
= ANALOG
= DIGITAL
AVDD
ANALOG
CIRCUITS
A
DVDD
C
STRAY
DIGITAL
CIRCUITS
C
STRAY
I
A
B
I
D
DVSSAVSS
AA
Figure 33. Ground and Power Consideration
Another input and ground technique is shown in Figure 34. 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.
-
Figure 34. RF Ground Scheme
–14–
REV. D
EVALUATION BOARD
The AD9201 evaluation board is shipped “ready to run.”
Power and signal generators should be connected as shown in
Figure 35. Then the user can observe the performance of the Q
channel. If the user wants to observe the I channel, then he
should install a jumper at JP22 Pins 1 and 2. If the user wants to
toggle between I and Q channels, then a CMOS level pulse train
should be applied to the “strobe” jack after appropriate jumper
connections.