# pH sensor project: trouble with RF interference

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:

• I doubt it's RF. What readings are you getting on the multimeter? Most a picture of your sensor setup. Are you vibrating the table? – EasyOhm Nov 12 '14 at 4:54
• My setup: i.imgur.com/sbgbrcu.jpg – karl333 Nov 12 '14 at 5:12

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.
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.