Do we have to share the ground between a load and a sense-resistor based current meter?

Question

I see in the datasheets of some current-meter chips, that the LOAD ground is also the same as current-meter chip (inAMP for example) ground. For example AD8293 inAMP:

Or this model (INA3221) from ti:

While it makes sense to share the same ground, I was wondering why the chip couldn't do that:

Without a shared ground, we don't know the exact amount of voltage, but we can say that for example, one leg of that shunt resistor is X volts, the other leg is at (x+0.1) volts. because getting the value of X needs a similar reference, (the GND) but getting the difference, wouldn't. Also, a simple multimeter, just reads the current using 2 probes which can be high side or low side, without a shared ground.

So is it possible to read current without a shared ground? (for example in those 2 aforementioned ICs?)

Answer 1

In theory, no, like you can do with a multimeter, the multimeter can measure difference of two points without reference.

However, if you are standing on a ground and trying to measure an electric like at 400 kilovolts, something is going to break because the common mode voltage is too large.

For the INA3221, it can measure +/- 26V between the two terminals, but each terminal must be between -0.3V and +26V with regards to chip ground, or something is going to break.

For AD8293, it also must have each input voltage between positive and negative supplies.

So in practice, the chips you ask, need to have a common voltage reference between measured voltages and ICs.

Answer 2

So is it possible to read current without a shared ground? (for example in those 2 aforementioned ICs?)

Yes it's possible. The ground voltage of the device needs to be near or at 0V too keep the terminals in the 0 to 5V range of the inst amp. Think of this: If the AD8293 circuit floated to a high voltage value vs the circuit you are measuring, like more than 5V then the AD8293 will not function correctly. You also risk the protection diodes turning on.

Also, a simple multimeter, just reads the current using 2 probes which can be high side or low side, without a shared ground.

For that you would need an input amplifier that has beyond the rails capability just in case there is a voltage difference between the current sense circuit and the amplifier circuit. However, if you can guarantee that the voltage will be close to each other then you might consider it. I would suggest grounding or selecting a different amplifier