AD9201
–9–
REV. D
The AD9201 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
REFSENSE
0.1mF
10mF
0.1mF
0.1mF
0.1mF
AD9201
0.1mF
10mF
10mF
MIDSCALE
VOLTAGE
= 0.5V
1V
0V
INPUT
5kV 5kV
Figure 17. Example Configuration for 0 V–1 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.
0.1mF
10mF
0.1mF
0.1mF
ANALOG
INPUT
C1
C2
1.0mF
C3
0.1mF
R1
1kV
1.5V
0.5V
REFT
REFB
IINA
IINB
VREF
AD9201
REFSENSE
Figure 18. Example Configuration for 0.5 V–1.5 V ac
Coupled Single-Ended Inputs
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 illus-
trates 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 differ-
ential drive of the AD9201’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 coupling.
The center tap of the transformer secondary is tied to the com-
mon-mode reference, establishing the dc bias point for the ana-
log inputs.
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
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).
QINA
QINB
IINA
IINB
AD9201
Figure 20. Input Loading
DVDD
I OR QREFT
I OR QREFB
AVDD
0.1mF
10mF
0.1mF10mF
AD9201
0.1mF
0.1mF
0.1mF10mF
V ANALOG V DIGITAL
Figure 21. Reference and Power Supply Decoupling
