Agilent Technologies MSO6012A User Manual page 332

6000 Series Oscilloscope User's Guide
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.
Probe 1
L (GND)
Probe
Ground
Probe 2
Probe N
Figure 52 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
i
1
V (Common Mode)
n
i +i +i
1 2 n
i +i
2 n
Z
in
i
n
Z
in
Reference 8
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