Datasheet

AD9269
Rev. 0 | Page 20 of 40
Input Common Mode
The analog inputs of the AD9269 are not internally dc-biased.
Therefore, in ac-coupled applications, the user must provide a dc
bias externally. Setting the device so that VCM = AVDD/2 is
recommended for optimum performance, but the device can
function over a wider range with reasonable performance, as
shown in Figure 39.
100
95
90
85
80
75
70
65
60
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3
SNR/SFDR (dBFS/dBc)
INPUT COMMON-MODE VOLTAGE (V)
08538-049
SFDR (dBc)
SNR (dBFS)
Figure 39. SNR/SFDR vs. Input Common-Mode Voltage,
f
IN
= 32.5 MHz, f
S
= 80 MSPS
An on-board, common-mode voltage reference is included in the
design and is available from the VCM pin. The VCM pin must
be decoupled to ground by a 0.1 µF capacitor, as described in
the Applications Information section.
Differential Input Configurations
Optimum performance is achieved while driving the AD9269 in a
differential input configuration. For baseband applications, the
AD8138, ADA4937-2, and ADA4938-2 differential drivers provide
excellent performance and a flexible interface to the ADC.
The output common-mode voltage of the ADA4938-2 is easily
set with the VCM pin of the AD9269 (see Figure 40), and the
driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
AVDD
V
IN
76.8Ω
120Ω
0.1µF
33Ω
33Ω
10pF
200Ω
200Ω
90Ω
ADA4938-2
ADC
VIN–x
VIN+x
VCM
08538-007
Figure 40. Differential Input Configuration Using the ADA4938-2
For baseband applications below ~10 MHz where SNR is a key
parameter, differential transformer-coupling is the recommended
input configuration (see Figure 41). To bias the analog input, the
VCM voltage can be connected to the center tap of the secondary
winding of the transformer.
2V p-p
49.9Ω
0.1µF
R
R
C
ADC
VCM
VIN+x
VIN–x
08538-008
Figure 41. Differential Transformer-Coupled Configuration
The signal characteristics must be considered when selecting
a transformer. Most RF transformers saturate at frequencies below
a few megahertz (MHz). Excessive signal power can also cause
core saturation, which leads to distortion.
At input frequencies in the second Nyquist zone and above, the
noise performance of most amplifiers is not adequate to achieve
the true SNR performance of the AD9269. For applications above
~10 MHz where SNR is a key parameter, differential double balun
coupling is the recommended input configuration (see Figure 42).
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone is to use the AD8352 differential driver
(see Figure 43). Refer to the AD8352 data sheet for more
information.
ADC
R0.1µ
F
0.1µF
2
V p-
p
VCM
C
R
0.1µF
S
0.1µF
25Ω
25Ω
SP
A
P
0
8538-010
VIN+x
VIN–x
Figure 42. Differential Double Balun Input Configuration
AD8352
0Ω
0Ω
C
D
R
D
R
G
0.1µF
0.1µF
0.1µF
0.1µF
16
1
2
3
4
5
11
0.1µF
0.1µF
10
14
0.1µF
8, 13
V
CC
200Ω
200Ω
ANALOG INPUT
ANALOG INPUT
R
R
C
ADC
VCM
VIN+x
VIN–x
08538-011
Figure 43. Differential Input Configuration Using the AD8352