Analog Devices AD9609 Evaluation Board AD9609-65EBZ AD9609-65EBZ Scheda Tecnica

 

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Optimum performance is achieved while driving the AD9609 in a 
differential input configuration. For baseband applications, the 

, and 

 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 AD9609 (see Figure 37), and the 
driver can be configured in a Sallen-Key filter topology to 
provide band limiting of the input signal. 

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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 38. To bias 
the analog input, the VCM voltage can be connected to the 
center tap of the secondary winding of the transformer.  

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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 AD9609. For applications above 
~10 MHz where SNR is a key parameter, differential double balun 
coupling is the recommended input configuration (see Figure 40). 

An alternative to using a transformer-coupled input at frequencies 
in the second Nyquist zone is to use the 

 differential driver. 

An example is shown in Figure 41. See the AD8352 data sheet 
for more information. 

In any configuration, the value of Shunt Capacitor C is dependent 
on the input frequency and source impedance and may need to 
be reduced or removed. Table 9 displays the suggested values to set 
the RC network. However, these values are dependent on the 
input signal and should be used only as a starting guide. 

0 to 70 

33 

22 

70 to 200 

125 

Open 

A single-ended input can provide adequate performance in 
cost-sensitive applications. In this configuration, SFDR and 
distortion performance degrade due to the large input common-
mode swing. If the source impedances on each input are matched, 
there should be little effect on SNR performance. Figure 39 
shows a typical single-ended input configuration. 

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