I got the EVAL_AD9784_PMDZ and Zedboard reading data. It is differential ADC, 2 probes with 1 common ground. The ground is at the ZedBoard GND level (as expected). The voltage input is +7.5, GND, -2.5 V on connector J3 - according to specification and 3.3[V] and GND on Zedboard.
Lately, the probes are giving off voltage of 2.5 [V]. I would like to ask, whether it is faulty behaviour.
I havent measured the difference between probe and ground before, when it seemed to work properly, but I would expect the probe wouldnt force any voltage on measured signal.
Could the troubles come from U8 (second picture in schematics)?
ADDITIONAL INFO (EDIT):
When there is analog signal 2.5 [V] (peak-to-peak), 0 [V] offset, 0.3 Hz, and -VS_EXT is disconnected the board works fine, except when signal is getting closer to 0 [V].
From my point of view, this implies that there is something wrong with the negative voltage supplied to both OP-AMPs on inputs.