Input Common Mode
The analog inputs of the AD9648 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.
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3
INPUT COMMON-MODE VOLTAGE (V)
Figure 43. SNR/SFDR vs. Input Common-Mode Voltage,
fIN = 70 MHz, fS = 125 MSPS
Differential Input Configurations
Optimum performance is achieved while driving the AD9648 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 AD9648 (see Figure 44), and the
driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
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. An example is shown in Figure 45. To bias
the analog input, the VCM voltage can be connected to the
center tap of the secondary winding of the transformer.
Figure 45. 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 AD9648. 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.
An example is shown in Figure 47. See the AD8352 data sheet
for more information.
Figure 46. Differential Double Balun Input Configuration
Figure 47. Differential Input Configuration Using the AD8352
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