Avoiding ground loops with common-ground DAQ
What component or circuit exists that performs this function?


Given that the OP has revealed that the DAQ is the DT9816-S, in order to get the most from converting its very good single ended input to a differential input, I would suggest using the AD8221 instrumentation amplifier as a decent choice. I've used them myself when designing DAQs for test beds and found them to be pretty good: -



Because of the need to protect sensitive inputs I would use series resistors with each input. They will also allow for a little RFI filtering: -



The gain set resistor can be left open circuit for G = 1 operation.