Measuring positive and negative voltages using ADC

Question

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.

Answer 1

As hinted to in the comments the output impedance of your source can greatly affect your readings. In general you want the output impedance of your source to be very small compared to the input impedance of the measuring circuit. Usually that is why op-amps are used to "buffer" a source -- besides scaling and translating the voltage of a signal they can also turn a very high impedance source into a very low one.

The input impedance of an ADC pin is very high, and it could be high enough to not load down your electrochemical source signal. If the voltage on your anode is limited in range -- like +/- 1V w.r.t. the cathode -- I would try using a battery to shift the voltage.

For instance, use a couple of AA cells to add around 3 V to your signal:

^{simulate this circuit – Schematic created using CircuitLab}

To determine the anode voltage, just read the voltages at A1 and A2 using two analog inputs and then subtract. A2-A1 will be your anode-cathode voltage. The precise battery voltage isn't critical -- anything that will keep the anode voltage above 0V will work.