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I built this pH sensor project on a breadboard and I'm having a big problem. The output on the voltmeter is completely erratic, and I'm pretty sure it's due to RF interference. When I connect the probe (via BNC connector), moving the cable or my hands around or just walking around the room causes the readout on the voltmeter to fluctuate wildly.

How can I fix this? I think I must have to do something about grounding the BNC jack, but I don't know how to proceed. For what it's worth, this is the BNC connector I'm using. The probe is the same one used in the first link and this is how things are setup:

pH sensor project

PeterJ
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karl333
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1 Answers1

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pH probes provide relatively high voltage (hundreds of mV), however they have extremely high impedance - 10's or 100's of M\$\Omega\$ for a typical glass probe.

You must ground the BNC shell to the housing (or an electrostatic shield around the electrometer amplifier), and that should be connected to circuit ground.

It's normal to have a small low-leakage capacitor on the op-amp input, perhaps 1nF NP0 or film, and a series resistor of some M\$\Omega\$, which will filter out any RF that gets past the shielding.

Spehro Pefhany
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    Thanks so much. I quickly tried touching a wire between the BNC jack and the circuit ground and instantly the voltage became stable and within normal range. – karl333 Nov 12 '14 at 05:14