User`s guide

Remarks
Due to internal quantization, the actual aperture that you set may be
slightly different than your specified value. The increment is approximately
20 µs. Use the query command to determine the exact aperture that is set.
For example, if you send "VOLT:DC:APER 500 ms", and then send the
query "VOLT:DC:APER?" the actual aperture is returned (for example:
"+5.00000753E-01").
Setting an aperture with this command changes any integration time
selected with the [SENSe:]VOLTage[:DC]:NPLC command.
To obtain best normal mode (line frequency noise) rejection, set an
integration time equivalent to an integral number of power line cycles with
the [SENSe:]VOLTage[:DC]:NPLC command.
The instrument disables aperture and sets the integration time to 1 PLC
after a Factory Reset (*RST command) or an Instrument Preset
(SYSTem:PRESet command).