AnsweredAssumed Answered

Does the voltage of pin Vocm influence the gain of ADA4938?

Question asked by ZhengNice on Jan 12, 2017
Latest reply on Jan 16, 2017 by Emman.A

Hi Emman.A,

   I use the ADA4938 to Drive the ADC(AD9629), the pin Vocm connected to pin VCM of AD9629, and the output voltage of pin VCM is MID AVDD Supply (0.9V).

The schematic is shown below, and the gain of ADA4938 should be 0.5 in theory. 

 

   When the PCB is finished, it's tested with a DC signal,

1.when the Vocm is not provided 0.9V by the VCM of AD9629, the gain is 0.5;

2. but when the Vocm is provided 0.9V by the VCM of AD9629, the gain is not 0.5;

 

I am curious why the gain change when the Vocm is provided 0.9V by the VCM of AD9629.

 

In the schematic, I connect the pin FB to OUT with a external wire,  Could this lead to the problem?

 

Best regards. 

 

Outcomes