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AD7961 DC setup

Question asked by bach_2016 on Oct 27, 2017
Latest reply on Nov 17, 2017 by bach_2016

Dear experts,

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.

Regards

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