4-40
MegaZoom Concepts and Oscilloscope Operation
Using Digital Channels to Probe Circuits
Probe Grounding
A probe ground is the low-impedance path for curren t to return to the source
from the probe. Increased length in this path will, at high frequencies, create
lar ge co mm on m od e v ol ta ge s at th e p ro be i np ut . T he vo l ta ge ge ne ra te d be ha ve s
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 vo ltage (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 curr ent that flows
into each p robe to r eturn th rough th e same com mon grou nd induc tance 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
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
Vn (Common Mode)
Probe
Ground