HP 54721A User Reference page 16

Example
Other devices
Channel Setup Menu
Probe
Because the mainframe's CAL signal is a voltage source, you cannot use it to
calibrate to the probe tip when units is set to Ampere, Watt, or unlmown.
Instead, you set the external gain and external offset to compensate for the
actual characteristics of the probe or device, If you do not know the actual
characteristics, you can refer to the typical specifications that came with the
probe or device. The following steps show you how to do this,
1 SettheAtten units softkey to ratio.
2 Set the Probe atten softkey to Is 1.
3 Set the Units softkey to Ampere, Watt, or unknovm.
4 Set the Ext gainsoftkey to the actual gain characteristics of the
probe or device.
5 SettheExt off set softkey to the offsetthat is introduced bythe
probe or device.
Calibrate probe
Connect a voltage probe from the plug-in to the CAL signal on the mainframe,
then press the Calibrate probe softkey. The oscilloscope calibrates to the tip
of the probe by setting the probe attenuation to the actual attenuation ratio
of the probe. The oscilloscope also automatically compensates for any offset
that the probe may introduce.
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