interface to input common mode voltage of AD9644

Question

I have a project where I am using the AD9644 and I must remain DC coupled from input to the ADC. 
 I must also have gain control and have initially picked the AD8366 for its DC coupled capability. 
 The problem I'm having is that VCM for the AD9644 is 0.9 volts but I cannot find a suitable amplifier 
 that will allow me to set its VCM output at 0.9V. Is it possible to run a ADA4937 off split supply rails and 
 steer its output to make it compatible with the AD9644? I'd rather do this with the AD8366 but it does 
 not look like it will support split supplies at all. 
 Thanks. 
 R. Traylor

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J.Harris · Answer

Hi Roger, 
 Either of these amplifiers could work. In order to get a suitable common mode voltage for the AD9644 you would just need to use a voltage divider topology in the network between the amplifier and the ADC. In this case, you would use a series R and shunt R on each branch of the analog input to set the common mode voltage division to achieve the 0.9V for the ADC while these Rs simultaneously set the output impedance for the filter between the amplifier and ADC. I've included an example below for the AD8366 which sets the load impedance to 200ohms differentially (as desired by the AD8366) and voltage divides down the AD8366 common mode voltage from 1.6V down to 0.9V. (Left side connects to the amplifier and right to the ADC.) 
 Regards, 
 Jon

J.Harris · Answer

Hi Roger, 
 The signal will get slightly reduced due to the divider, you are correct, it will be need to be compensated for by the gain of the amplifier. 
 The 1.6V was chosen to keep the reduction in gain minimized. it is a push-pull situation where you have to keep the reduction minimized, keep the Vcm value correct, and keep the differential impedance = 200ohms. Larger values of common mode increase the reduction in signal level. 
 In this case you set the amplifier common mode to 1.6V and not use the Vcm of the ADC. You would just need to decouple the Vcm of the ADC to ground with a 0.1uF capacitor. There is no need to try to amplify the ADC common mode voltage. 
 Regards, 
 Jon

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interface to input common mode voltage of AD9644