Measuring small differential voltages with common mode above the 10V input rating of a DAQ without any external components

[Károly]

I have a task where I need to measure a small differential voltage about 0,1V to 0V in magnitude over a shunt resistor. Problem is that the resistor is connected to a 12V voltage rail while the "maximum working voltage for analog inputs" rating of my card (PCIe-6353) is ±11 V. My supervisor keeps suggesting to just disconnect everything from the aiGND and keep it floating and connect just the differential inputs from each leg of the resistor and it should work, however I cannot get accurate results and it just keeps clipping for me. From my understanding this will never work as even though the differential voltage is very small the common mode of the two inputs is above the 11V rating. Am I misunderstanding something?

The problem is described in much greater detail with very useful contributions from some forum members in the thread Getting incorrect voltage measurements overt resistor divider bridge but I figured it is better to start a separate thread just to help others who might have this exact same question in the future. 

[santo_13]

You better get a high-side current shunt amplifier for the job and measure the output voltage of that shunt amplifier using your DAQ.

This is a good article - https://www.analog.com/en/technical-articles/highside-currentsense-measurement-circuits-and-principl...

[Károly]

Thank you for the answer. If you check the thread that I've linked above actually many others have recommended the use of external amplifiers. It is still unresolved so maybe you can join and contribute to the discussion there too if you feel like it.

This question was more about how to interpret the meaning of "Maximum working voltage for analog inputs". Is my supervisor right that this metric is not relevant to my application since I'm only measuring a differential voltage of a few mV (even though the actual inputs to the inverting and non inverting terminals are >11V)?

[santo_13]

Since it is not a fully isolated DAQ, the common mode with respect to earth still cannot go above 11V whether you connect AIGND or not.

No, I would not agree with your supervisor. As the PCIe DAQ is still referenced to Earth and so does your 12V rail, so the common mode is very close to 12V and is operating outside of DAQ specification and potentially damage it permanently.