pH Electrode Buffer - Offset when solution grounded
pH electrode buffer offset appears when solution is grounded.
I have a pH electrode in some conductive solution (~1000uS). The output (measurement electrode) is buffered by an electrometer type opamp. The opamp rails are powered by an LDO (+/- 3.3V) which in turn are powered from a USB cable to my PC. The reference electrode is referenced to ground, which is shared between USB ground and PE through oscilloscope probe.
The board has been cleaned (ultrasonically in isopropanol and then baked for an hour). The pH electrode has a measurement glass impedance of ~250 MegaOhm and the liquid junction an impedance of around 10 kOhm.
If I measure the pH of liquid in a glass jar the output is relatively stable (<10 mV). However, when I place a grounded (PE) wire in the liquid a negative offset ranging between 50-200mV is introduced. Even if I float the oscilloscope I see the same offsets.
Isolating the buffer circuit solves the issue - but I am trying to understand why the offset happens.
The setup looks something like this (power supplies not show):
I tried to model the power supply (https://cutt.ly/nVuMekg) but I don't see the same offsets. I see 60Hz noise coupled through parasitic capacitance in my isolation transformer (60Hz noise is also present in real circuit but filtered by low pass).
Simulating with common mode noise (to replicate a ground loop) shows considerable 60Hz noise on the output of the buffer but no DC offset. I notice that the output of the buffer is not very stable (very low frequency noise - <<1Hz) when PE is connected to the solution.