Reference
P6330 3.5 GHz Differential Probe Instruction Manual 21
A quick way to assess the magnitude of CMRR error when the
common-mode signal is not sinusoidal is to connect both leads to the
samepointinthecircuit.Theoscilloscopewill display only the
common-mode component which is not fully rejected by the probe.
Whilethistechniquemaynotgiveyouentirelyaccuratemeasure-
ments,itdoesallowyoutodetermineifthemagnitudeofthe
common-mode error signal is significant.
Input Impedance Effects on CMRR
The lower the input impedance of the probe relative to the source
impedance,thelowertheCMRR.SeeFigure12onpage28.
Significantdifferences in the source impedance driving the two
inputs will also lower the CMRR.
Extending the Input Leads
Attimesitmay be necessarytoextendtheprobeinputswith wires or
a probe tip adapter. When you do this, you should minimize the lead
lengthstooptimizecommon-moderejection and twist the input leads
together as shown in Figure 10.
Twisting the input leads together does increase capacitance that may
degrade high-frequency performance. You should take into account
anyeffectscaused by the extended leads when you take a measure-
ment.