Common-mode rejection
The common-mode rejection ratio (CMRR) is the specified ability of a probe to reject signals that are common to both inputs. More
precisely
, CMRR is the ratio of the differential gain to the common-mode gain. The higher the ratio, the greater the ability of probe to reject
common-mode signals.
Common mode rejection decreases as the input frequency increases. For example, if you apply a 60 Hz line voltage of 500 V
p-p
to
both input leads of the probe, the probe rejects the signal by 80 dB (typical) and the signal appears as only a 50 mV
p-p
signal on the
oscilloscope screen.
Twisting the input leads
Twisting the input leads helps to cancel noise from high-EMI environments that is induced into 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 the Specifications section. As the graph shows, the probes have virtually no loading ef
fect on
sources with relatively low impedance and low frequency.
Operating basics
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