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Figure 7
Waveform 1
Waveform 2
Equation 2
Resistive Loading Effects
The two major effects caused by resistive loading are amplitude distortion and
changes in dc bias conditions in the circuit under test.
Amplitude Distortion
Amplitude distortion is depicted in figure 7, where waveform 1 is the signal
before probing and waveform 2 is the signal while probing. (The baselines of
these signals have been overlayed to show the amplitude change. If the
baseline of a signal is not at zero volts it will shift when the signal is probed.)
Oscilloscope Display Showing Amplitude Distortion
The cause of the error is the voltage divider developed between the source
resistance of the device under test and the input resistance of the probe being
used. Equation 2 calculates the error caused by the voltage divider.
R
source
----------------------------------------
Error(%) =
R
+
R
source
A probe with an input resistance ten times that of the source resistance of the
device under test causes a 9.09% error in the measurement. It is best to use a
probe with an input resistance at least ten times that of the source resistance.
×
100
probe
Chapter 2: Probing Considerations
Resistive Loading Effects
23
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