Comments on pH Electrode Buffer - Offset when solution grounded
Parent
pH Electrode Buffer - Offset when solution grounded
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):
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:
Output:
Post
It seems you have ground loops, and possibly having multiple ground connections shorting out your signals. Your hand-sketched diagram is a good start, but you need to show all the ground connections.
The scope is presumably grounded via its ground lug that plugs into the electrical outlet. Let's call that the "power ground". You show that with the chassis ground symbol in your diagram. But then what's the ground connected to the reference electrode really connected to? Using a different symbol means it's not the same as the power ground, but what is it?
You mentioned that the opamp is powered from a power supply derived from a computer's USB. That is also most likely referenced to power ground. Show that in your diagram.
I'm not familiar with the details of pH meters, so don't know what the significance of the reference electrode is. You don't seem to be using the output of the reference electrode, so it appears it's job is to float the solution at some voltage. However, you also show an explicit connection between the solution and power ground. Something is wrong here.
I would start by eliminating ground loops. Make sure your whole setup is referenced to ground by exactly one path. That should probably be your scope ground lead, since that's the hardest to avoid not being connected to ground.
Use batteries to power the opamp. Not only do they inherently float, but they will also be much less noisy than something derived from USB power.
I can't say what to do about the reference electrode since I don't understand what its function is supposed to be.
I understand the concept of a ground loop - but I don't understand why it creates a DC offset.
It is possible that the multiple ground connections are shorting your reference cell. It's not clear from your diagram, but is hinted at.
1 comment thread
3 comment threads