# Measuring positive and negative voltages using ADC

I came across the simple solution for the said problem posted by Transistor (Transistor (https://electronics.stackexchange.com/users/73158/transistor), circuit for measure positive & negative voltage + Vref resolution, URL (version: 2018-06-05): https://electronics.stackexchange.com/q/378320) but when I implemented it, I got some unexpected results. It measures pos and neg voltages very well, however I'm using it in an electrochemical setup measuring the potential between the anode and the cathode and sometimes the polarity swaps (hence my desire to measure pos and neg voltages). The open circuit voltage is constant, and I'm measuring that constant voltage with a logger (Arduino) however when I use the voltage divider set up the OCV is no longer constant and the voltage changes as if the anode and cathode are shortened with a particular resistance between them. Can anyone explain to me what is happening? The voltages are small between 0 - 1V. The R1 and R2 I'm using are 1K Ohm.

The output impedance is about 14.8kOhm. It changed from about 3.3KOhm but settled on 14.8kOhm after some time. See enclosed the schematic.

• What's the output impedance of your sensor compared to the divider impedance? A schematic would help. May 28, 2021 at 16:27
• @ozned edit the question and use the circuit tool to draw a circuit to describe your circuit May 28, 2021 at 17:23
• The divider is loading down the sensor. You'd need to increase the values of the divider resistors by at least an order of magnitude. However, the ADC probably has some maximum input impedance which is typically lower. One solution could be a opamp unity gain buffer. May 29, 2021 at 7:31