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?
I recommend to download the ADIdiffcalc on our web. This will help you to verify your design.
Here is the link: https://ez.analog.com/external-link.jspa?url=http%3A%2F%2Fwww.analog.com%2Fen%2Fdesign-center%2Finteractive-design-tools…
The tool actually recommend not to bring the gain of the amplifier below 0.8 V/V for stability.
The shorted FB and OUT pins should be a problem.
Can you check if the voltage at the VCM pin from the ADC is correct? Especially if it is connected to the VOCM pin of the amplifier.
Also, what is your input voltage? where is it coming from?
When you say "when the Vocm is not provided 0.9V by the VCM of AD9629, the gain is 0.5", what is the connection of the VOCM pin?