Agilent Technologies InfiniiVision 5000 Series User Manual

Digital Channels 13
InfiniiVision Oscilloscopes User’s Guide 387
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 as if this path were an
inductor according to the equation:
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
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
VL
di
dt
-----
=
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

Agilent Technologies InfiniiVision 5000 Series Specifications

General

Brand	Agilent Technologies
Model	InfiniiVision 5000 Series
Category	Test Equipment
Language	English