How the Instrument Works
System Architecture
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 slows the rise time of any transitions, increases the slew rate,and
introduces delay in the actual time of transitions. The approximate rise time
of a simple RC circuit is 2.2 RC. Therefore, for an output resistance of 100
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, although it might be faster
without the probe.
If the output of the circuit under test is current-limited (as is often the case
for CMOS), the slew rate is limited by the relationship dV/dT = I/C. Refer to
Figure 13-9.
Figure
13-9. Eects
of probe
capacitance.
13-15
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