Good day engineers. I'm building a resistivity meter that measures the resistivity of the earth. To make this automated, I'm using pic microcontroller to control the system. But it is needed of the device to be able to measure the spontaneous potential of the earth. This forum has been so helpful with beautiful minds at help. I was able to utilize three resistors and some set of instructions to measure the negative voltage of the earth. But to test it with the Earth, it is not really sensitive. So, doing some research, I came up with a circuit diagram of an instrumentation amplifier. But the problem I'm having is that, the instrumentation amplifier has three pinout to touch the earth surface instead of two. Please help me with a very good idea of how to be able to make this happen. Attached to this post is the circuit diagram of the instrumentation amplifier. With two pinout to the microcontroller and three measuring floating voltages from the ground. But I need just two to the ground. Thanks great minds.