Re: [SI-LIST] : Noise Voltage levels vs. EMI levels

Larry Smith (ldsmith@lisboa.eng.sun.com)
Tue, 27 Jul 1999 15:12:47 -0700 (PDT)

Ron - I think you are saying that you can make a measurement by
touching the hot side of the probe to a node and just not worry about
the ground side of the probe. You end up making a measurement of that
node with respect to some "electrical mass".

I would agree with you to some extent and have used the same technique
myself. It is good enough to find out if a signal is alive and
approximately what it is doing. Depending on where the "electrical
mass" is, it might even be an indication of what that node is doing WRT
the center of the earth or spice node 0. But the measurement is not
very well controlled. The FET in the high impedance probe will measure
a voltage WRT it's local circuit ground. The local circuit ground is
not connected to the circuit that you are trying to measure but is
heaviliy influenced by capacitance to your hand, surrounding metal and
a highly inductive path back down the cable that attaches the probe to
the measurement instrument.