At work we have a DAQ system that is measuring a 0-10vdc signal from a fully isolated signal conditioner, i.e. the signal should be floating. The analog input card has differential inputs, with a reported 50Mohm input impedance. The manufacturer, along with other resources, recommend that a 10K or 20k resistor be used to provide a return path from the low side of the signal to an AGND on the input board. As you might guess, previous designers on this system did not bother to provide the return path. However, everyone says that everything is reading fine, with no problems ever observed or reported. I'm pretty sure I have a solid understanding of why the return path is needed, which makes me wonder how this setup is working despite the apparent design flaw. The designer DID connect the low side of the signal conditioner's power supply to AGND. But, I don't see how this accomplishes anything, as the power supply is supposed to be isolated from the signal output by a ridiculous 1500Vrms. I guess there could be an unintended return path somewhere, or maybe the circuit has never run long enough for a problem to show up. Anyway, I'd appreciate any thoughts on what could be going on.