Input Impedance Effects On Cmrr; Extending The Input Leads - Tektronix P6246 Mode D'emploi

Reference
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
same point in the circuit. The oscilloscope will display only the
common-mode component which is not fully rejected by the probe.
While this technique may not give you entirely accurate measure-
ments, it does allow you to determine if the magnitude of the
common-mode error signal is significant.

Input Impedance Effects on CMRR

The lower the input impedance of the probe relative to the source
impedance, the lower the CMRR. See Figure 8 on page 28.
Significant differences in the source impedance driving the two
inputs will also lower the CMRR.

Extending the Input Leads

At times it may be necessary to extend the probe inputs with wires or
a probe tip adapter. When you do this, you should minimize the lead
lengths to optimize common-mode rejection and twist the input leads
together as shown in Figure 7.
Twisting the input leads together does increase capacitance that may
degrade high-frequency performance. You should take into account
any effects caused by the extended leads when you take a measure-
ment.
CAUTION. To avoid damaging the input connectors, do not insert
anything larger than a 0.025-inch square pin into the inputs.
22
P6246 & P6247 Instructions