# How do I optimize my op-amp "guard ring"?

I'm trying to measure the moisture content of wood via a resistance measurement. The moisture content range I'm trying to measure is in the order of 1 MΩ to 1 GΩ, so only ~5 nA current at 5 V DC.

The op-amp I'm using to amplify the current is an MCP6006R. I'm feeding this into the ADC on an Arduino. The datasheet shows a guard ring configuration around the inputs on the op-amp to minimize unwanted signals. Will this be necessary for the circuit to be reliable?

See PCB with guard ring attempt highlighted below:

• Don't be afraid to space out your schematics more. Avoid wires through text (especially component values), always have GNDs 'pointing down', and label nets to make their purpose clear. It helps you in the long run and makes it far easier to understand for those who are unfamliar with your design. Commented Jan 13, 2023 at 10:00
• I think the circuit will not work and as it is now, the guard ring is currently not the thing that makes the circuit reliable. Commented Jan 13, 2023 at 10:00
• Hi Kyotee, could you explain the working principle of this? What's R2's job in all this? How does C1 and the DC current from WOOD_PROBES interact? Commented Jan 13, 2023 at 11:03
• At 1 Gohm max, you are not yet in the guard ring territory. That starts to matter if you need sub-pA leakage control Commented Jan 13, 2023 at 11:12
• Thanks for all the feedback. The working principle is i can measure the voltage rise on the capacitor which is being charged via the current through the wood probes. from the voltage rise i can calculate the resistance of the wood. R2s job is to limit the current that can flow into the op amp as the inputs have a 5mA max. Circuit principle based on "mdpi.com/1996-1944/12/15/2373/htm#" paper Commented Jan 13, 2023 at 22:07