I am using the eval board ad7961, using the opamps, with Zedboard. Considering the scenarios below I can only get results which make sense in the A.2. I attached the results. The amplitude is 0 to 65535 (2-complement considered). The En pins are driving 1.8V if enable.
Scenario A : Inputs open
1) en[3:0] = 0001 VCM=~0.6V SMA center to GND =~0.6V
2) en[3:0] = 1001 VCM=2.5V SMA center to GND =2.5V
Scenario B : Inputs connected to the same signal (sine at 10KHz) from the function generator
1) en[3:0] = 0001 VCM=~0.07V SMA center to GND =~0.13
2) en[3:0] = 1001 VCM=2.4V SMA center to GND =~0.23
Actually, I was expecting to get almost 0V in all of theses cases. Do I have to isolate the ground of the function generator from the board? Do I have to use some kind of driver between function generator and board? I know it is a fully differential adc and for me it is ok to have the half of full range using only one input.