User`s guide

Remarks
To obtain a stable measurement, send the FETCh? command first to ensure
the measurement is complete.
You must enable dc voltage measurements before you can use this
command.
Before sending this command, turn on the peak measurement state with
the [SENSe:]VOLTage[:DC]:PEAK:STATe command.
This command will return "not a number" (+9.91000000E+37) if no valid
measurement is available and there is no valid measurement in progress.