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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: Guard ring is connected to ground and surrounds in- on op-amp

Circuit schematic. relay is used to short capacitor prior to taking a measurement

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    \$\begingroup\$ 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. \$\endgroup\$
    – raaymaan
    Commented Jan 13, 2023 at 10:00
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    \$\begingroup\$ 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. \$\endgroup\$
    – Justme
    Commented Jan 13, 2023 at 10:00
  • \$\begingroup\$ 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? \$\endgroup\$
    – Marcus Müller
    Commented Jan 13, 2023 at 11:03
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    \$\begingroup\$ At 1 Gohm max, you are not yet in the guard ring territory. That starts to matter if you need sub-pA leakage control \$\endgroup\$
    – tobalt
    Commented Jan 13, 2023 at 11:12
  • \$\begingroup\$ 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 \$\endgroup\$
    – kyotee89
    Commented Jan 13, 2023 at 22:07

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Remember why we are using a guard at all: Setting the guard on the same potential as the signal to be guarded reduces the potential difference between the guard and the signal to be guarded to zero and by this the leakage current flowing to and from the signal to be guarded to zero as well, according to:

I = U / R = 0V / R = 0A

So a guard can only work properly when it completely surrounds the signal to be guarded. And because of this, extending the guard in deeper layers of your printed circuit can absolutely make sense.

I do not recommend applying solder mask to guard rings. By removing the solder mask over the guard, you further reduce the paths that leakage currents can take. Note, surface contamination of the PCB can significantly reduce the effectiveness of guard rings. I recommend thoroughly cleaning the PCB before and after assembly.

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