Twisting The Input Leads; Probe Loading - Tektronix P5210 Instructions Manual

Operating Basics
Common mode rejection decreases as the input frequency increases.
Figure 4 on page 21 is a plot of typical CMRR of the probe versus
input frequency. For example, if you apply a 60 Hz line voltage of
500 V to both input leads of the probe, the probe rejects the signal
P-P
by 80 dB (typical) and the signal appears as only a 50 mV signal
P-P
on the oscilloscope screen.

Twisting the Input Leads

Twisting the input leads as shown in Figure 3 helps to cancel noise
that is induced into the input leads and to improve the high-frequen-
cy response of the inputs.
Figure 3: Twisting the Input Leads

Probe Loading

When you touch your probe tip to a circuit element, you are
introducing a new resistance, capacitance, and inductance into the
circuit.
Frequency and impedance of the source determine how much the
probe loads the circuit you are measuring. As the frequency of the
source starts to increase beyond 1 kHz, the input impedance of the
probe begins to decrease. The lower the impedance of the probe
relative to that of the source, the more the probe loads the circuit
under test. For a graph of frequency versus input impedance, refer to
Figure 5 on page 22. As the graph shows, the probe has virtually no
loading effect on sources with relatively low impedance and low
frequency.
P5210 Instructions
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