Agilent Technologies 22A, 54621D, 24A, 42A, 54621A, 41A manual Probe Grounding

MegaZoom Concepts and Oscilloscope Operation

Using Digital Channels to Probe Circuits

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:

di V = L----

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.

Probe 1

Probe Ground

Probe 2

Z in

L (GND)

i1+i2+ in

Z in

i1

Vn (Common Mode)

i2 + in

in

Probe N

Zin

Common Mode Input Voltage Model

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