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).
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?