# Battery powered device - How to measure external floating differential voltage

I have the following task. As depicted in the sketch, I have a battery powered device which needs to measure the differential voltage VIN between two external nodes, EXT_NODE1 and EXT_NODE2. It is not allowed to have a conducting path between the two nodes while measuring (you can see it as a battery that can have inverted polarity in a way).

simulate this circuit – Schematic created using CircuitLab

I am very confused on how I could do this. In the beginnning I thought an instrumentation amplifier would be perfect but soon realized that wouldn't work. It is impossible to know the common mode voltage of VIN against the "-" electrode of the battery.

What do people usually do in these cases? I mean, there are a lot of handheld data loggers. How do they handle differential signals?

One easy way to go I guess would be to connect one of the external nodes to the "-" of the battery, converting in essence the differential measurement to a single-ended measurement.

With this implementation, there is the problem of how to handle the potentially negative voltage against the node connected to my "ground". I suppose I'll have to be able to connect both nodes to "ground" and measure, every time I want to know how much VIN is.

Is there another or better way to do this?

• The sketch doesn't really depict much. What external nodes? Be clearer as to what you are trying to do. – Andy aka May 29 at 9:33
• there you go, thank you – nickagian May 29 at 10:09
• The external nodes as shown in your picture are very lightly connected to the battery - they have to be. If you can use 1 Mohm resistors then maybe that would achieve what you want? – Andy aka May 29 at 10:18
• But I don't know how to do it, that's why they are not connected. This just describes the problem, not the solution. – nickagian May 29 at 12:17
• The solution is use 1 Mohm resistors from each input to your battery 0 volts. – Andy aka May 29 at 12:19