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First, the probe itself has an input resistance. The same theory as
testing voltage by multimeter, in order to minimize the impact of
the to be tested circuit, requires the input resistance (Rprobe) of
the probe itself as much as possible. However, because it's impossible
for Rprobe to be infinity, shunting will be generated with the to be
tested circuit, and the actual measured voltage may not be the
the real voltage on probe point, which is ofter encountered on some
power or amplifier circuit test. To avoid the impact of the load resistance
of the probe, there is the level required of Rprobe-10 times more than
Rsource and Rload. Most of the probe's input impedance is between
10K ohms and 10M ohms.
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