HP 54645A, User'S And Service Manual

Page: 124 / 301

Probe Grounding
A probe ground is the low-impedance path for current to return to the source
from the probe. Increased length in this path will, at high frequencies, create
large common mode voltages at the probe input. The voltage generated
behaves according to the equation:
V
=
L
di
dt
Increasing the ground inductance ( L ), increasing the current ( di ) or
decreasing the transition time ( dt ), will all result in increasing the voltage
(
V
). When this voltage exceeds the threshold voltage defined in the
oscilloscope, a false data measurement will occur.
Sharing one probe ground with many probes forces all the current that flows
into each probe to return through the same common ground inductance of
the probe whose ground return is used. The result is increased current (
di )
in the above equation, and, depending on the transition time ( dt ), the
common mode voltage may increase to a level that causes false data
generation.
Common Mode Input Voltage Model
Figure 4-16
Run/Stop/Single/Autostore/Erase Operation
Using Digital Channels to Probe Circuits
4-41