Probe Capacitive Loading
The capacitance of the probe tip to ground forms an RC circuit with the
output resistance of the circuit under test. The time constant of this RC
circuit will slow the rise time of any transitions, increase the slew rate, and
introduce delay in the actual time of transitions. The approximate rise time
of a simple RC circuit is 2.2RC. Thus, for an output resistance of 100 W and a
probe tip capacitance of 8 pF, the real rise time at the node under test cannot
be faster than approximately 1.8 ns. Without the probe, the rise time might
be faster.
If the output of the circuit under test is current-limited (as is often the case
for CMOS), the slew rate will be limited as shown by equation 2 as shown in
figure 13-20 for an example.
Equation 2
dV
⁄
dt
=
I
⁄
C
Effects of Probe Capacitance
Figure 13–20
Measurements
Time-interval measurements
13–44
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