Digital Channels 17
InfiniiVision 7000B Series Oscilloscopes User’s Guide 377
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.
Figure 39 Common Mode Input Voltage Model
In addition to the common mode voltage, longer ground returns also
degrade the pulse fidelity of the probe system. Rise time is increased, and
ringing, due to the undamped LC circuit at the input of the probe, is also
increased. Because the digital channels display reconstructed waveforms,
they do not show ringing and perturbations. You will not find ground
problems through examination of the waveform display. In fact, it is likely
you will discover the problem through random glitches or inconsistent
data measurements. Use the analog channels to view ringing and
perturbations.
Z
in
Z
in
i
n
i
2
+i
2
i
1
i
1
+i
n
+i
n
Z
in
Probe 1
L (GND)
Probe 2
Probe N
V
n
(Common Mode)
Probe
Ground
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