Currently I use a BNC 50 Ohm Plug to “shorten” the unused minus input when single ended connection is used on the plus side. So, what is the best resistor value for lowest noise? Should be somewhat between for lowest voltage noise and current noise. Or is just zero ohm best?
The voltage spectral noise density is given by sqrt (4 * k * T * R), which is 0.9nV/√Hz for 50 ohms at room temperature. I think that’s sufficiently far below the QA403’s noise floor as to be negligible, given its -115dBV noise specification.
But yes 0 ohms is always quieter!
Current noise isn’t usually relevant at such low impedances.