Datasheet

AD9608
Rev. 0 | Page 24 of 40
Input Common Mode
The analog inputs of the AD9608 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 43.
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.
0
10
20
30
40
50
60
70
80
90
100
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)
SFDR (dBc)
SNR (dBFS)
09977-056
Figure 43. SNR/SFDR vs. Input Common-Mode Voltage,
f
IN
= 70 MHz, f
S
= 125 MSPS
Differential Input Configurations
Optimum performance is achieved while driving the AD9608 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 AD9608 (see Figure 44), and the
driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
AVDD
VIN
76.8Ω
120Ω
0.1µF
33Ω
33Ω
10pF
200Ω
200Ω
90Ω
ADA4938
ADC
VIN–x
VIN+x
VCM
09977-050
Figure 44. 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 45). 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
09977-051
Figure 45. Differential Transformer-Coupled Configuration
The signal characteristics must be considered when selecting
a transformer. Most RF transformers saturate at frequencies that
are 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 AD9608. For applications above
~10 MHz where SNR is a key parameter, differential double balun
coupling is the recommended input configuration (see Figure 46).
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone is to use the AD8352 differential driver
(see Figure 47). See the AD8352 data sheet for more information.
ADC
R
0.1µF
0.1µF
2V p-p
VCM
C
R
0.1µF
S
0.1µF
25Ω
25Ω
SP
A
P
VIN+x
VIN–x
09977-053
Figure 46. 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
09977-054
Figure 47. Differential Input Configuration Using the AD8352