Remarks
Use the [SENSe:]CURRent:AC:NULL:VALue command to set the null value.
The CONFigure:CURRent:AC and MEASure:CURRent:AC? commands turn
off the null state.
A Factory Reset (*RST command) or an Instrument Preset
(SYSTem:PRESet command) returns the null state to off.
