AD9269(Rev0) View Datasheet(PDF) - Analog Devices
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AD9269 Datasheet PDF : 40 Pages
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AD9269
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
SFDR (dBc)
90
85
80
SNR (dBFS)
75
70
65
60
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3
INPUT COMMON-MODE VOLTAGE (V)
Figure 39. SNR/SFDR vs. Input Common-Mode Voltage,
fIN = 32.5 MHz, fS = 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.
VIN
76.8Ω
0.1µF
90Ω
120Ω
200Ω
33Ω
ADA4938-2 10pF
33Ω
VIN–x AVDD
ADC
VIN+x VCM
200Ω
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
VIN+x
R
C
ADC
R
VIN–x VCM
0.1µF
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.
2V p-p
0.1µF
PA
SS
0.1µF
25Ω
P
25Ω
0.1µF
R
C
0.1µF
R
VIN+x
ADC
VIN–x
VCM
Figure 42. Differential Double Balun Input Configuration
0.1µF
ANALOG INPUT
0Ω 16
1
2
CD
ANALOG INPUT
RD
RG 3
4
5
0.1µF 0Ω
VCC
8, 13
11
AD8352
10
14
0.1µF
0.1µF
0.1µF
0.1µF
R
200Ω
200Ω
C
R
0.1µF
Figure 43. Differential Input Configuration Using the AD8352
Rev. 0 | Page 20 of 40
VIN+x
ADC
VIN–x VCM
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