Ground loop protection watermark soil moisture sensor functionality

Question

I am designing a little pcb where I want to read out impedance from an irrometer soil moisture sensor (example). The manufacturer describes preventing ground loops like this:

Since WATERMARK sensors provide a direct path from earth to any device, protection via Zener or TVS diodes should be included on all sensor lines. In the interest of simplicity these are not included in any of examples to follow, but are highly recommended. Any such components will influence the measured readings and will have to be factored in to the reading program.

So I put the diodes like this:

Since this looks good to me I don't know how the diodes work and what they are protecting exactly.

Answer 1

You did this well. The sensors and the wiring outside can catch noise voltages far beyond the supply rails of your circuit. Consider a lightning strike in the vicinity. The Z-diode will conduct if the voltage on the wire is below GND or above VCC with some margin, depending on forward voltage and impedance.

Such a diode will not catch all impacts, adding a series resistor of e.g. 100 ohm in the input path helps to dissipate some energy and a capacitor of e.g. 1 nF parallel to the diode can absorb some energy as well. Not all circuits work with such additional components, but this one would, I guess.

There are TVS diodes with even better protection capabilities. All these strategies try to keep the voltage in the allowed range for the connected IC.

If your wiring receives RF signals from a radio station, this diode may rectify a part of the RF power and might modify the sensor value. To avoid these kind of problems, connect a capacitor of 100 pF or more between each input and GND.