I have tested several of these modules, each of them failed after a while and reports any value below 2.5 V (2.5 to -5 V) as 0. To my surprise, it seems to happen whenever the PMOD connection stops to provide voltage (3.3V and GND) while the voltage -2.5V, 0, 7.5 are still on.
I have also found funny little "note" in the schematic file "Ensure 'we' power the ADC up after Pmod Controller board" (just floating piece of text on the 2nd page of schematic file).
I am not sure who the above mentioned 'we' are, perhaps bunch of analog device engineers.
Anyway, could someone prove me right that the module fails to operate properly, once it is subjected to above mentioned situation ? If so, could someone give an explanation, why is this board advertised as plug-in, stand-alone evaluation board, when it misses any documentation, sample design and requires external voltage source ?
Can you share your input range? I Assume you're using PulsarPMODs. Aside from -2.5V,and 7.5V supply. the VCC PMOD should be activated, an enable input pin of U7 and U8(ADP7104). Drive EN high(min 1.0V) to turn on the regulator, drive EN low(max 0.4V) to turn off the regulator.
Please see attached link latest PMODs documentation. Hope this helps.