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I am making an analog front end for a pH probe. I encountered a design where creating an offset on the reference electrode creates an offset on the measure electrode.

How does that work? Does the charge go through water?

Based on TI application notes: https://www.ti.com/lit/an/snoa529a/snoa529a.pdf?ts=1619613280868&ref_url=https%253A%252F%252Fwww.qwant.com%252F

enter image description here

(The temp circuit can be overlooked.)

This circuit basically creates a voltage of 512 mV between the reference electrode and the ground, thus adding an offset of 512 mV on the measure electrode, allowing it to have positive voltage.

I don't understand how the potential of the reference electrode translates to the measure electrode.

I have made a similar circuit and I can indeed observe an offset at the output.

JRE
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Damezumari
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1 Answers1

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The pH probe behaves like a battery, a battery with a very high internal impedance.

One side of the battery is the reference electrode, the other is the measure electrode.

The "battery" voltage is a (fairly linear) function of the pH (and temperature) with 0V output with a neutral solution (pH = 7). Typically the voltage will be something like -400mV near pH = 0 and +400mV near pH = 14. The slope will vary with temperature and with age of the probe.

So the voltage on the measure electrode relative to ground will be the "battery" voltage added to the reference electrode voltage relative to ground.

As to why you might want to use such a configuration- it is so the pH meter can operate from a single supply. By adding a fixed voltage to the bipolar pH probe voltage, everything can be kept positive, which can simplify the circuitry.

Spehro Pefhany
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    Do you think ADCs with differential measurement capability obviate the need for this kind of circuit? Of course you might be able to save some money with a single-ended conversion... – vicatcu Apr 28 '21 at 14:44
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    @vicatcu Most ADCs are nowhere near capable of dealing with the high impedance of a pH probe directly, so you need a buffer amplifier. The buffer amplifier would need a bipolar supply unless the probe is biased. The LMP7721 used above has a typical Ib of +/-3fA at room temperature. – Spehro Pefhany Apr 28 '21 at 14:51
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    cool thanks for clarifying! – vicatcu Apr 28 '21 at 15:29