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Comments on pH Electrode Buffer - Offset when solution grounded

Post

pH Electrode Buffer - Offset when solution grounded

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Problem

pH electrode buffer offset appears when solution is grounded.

Detail

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):

Image alt text

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.

Simulation:

Image alt text

Output:

Image alt text

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3 comment threads

uS is not a measure of conductivity. Fix your units. (4 comments)
Wet cell perhaps? (1 comment)
A few words about the schematic (1 comment)
A few words about the schematic
a concerned citizen‭ wrote about 2 years ago

The schematic you shared uses a different symbol for the ADA4530-1 -- the symbol appears disconnected. While I could correct the connections since they are fairly straightforward, this should have been done by you: either provide the custom symbol, or make a mention of it (preferably the former). And you're missing the 3.3 V supply (can be added but, ...). Also, the values for the transformer's windings are orders of magnitude lower than the values one would expect from a mains transformer: in your schematic they are uH but, a more likely value would have been Henry (ten thousand times larger).