[SI-LIST] Re: Scope probe attenuation
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxxxxxx>
- To: SI <si-list@xxxxxxxxxxxxx>
- Date: Fri, 3 Aug 2001 11:37:30 -0400
In the old days with hi-Z probes that had a X1/X10 attenuation switch on
them, there was a link between the probe and the scope that would tell the
scope when you had it in the X10 (divide-by-10) position. It would
automagically change the displayed vertical scale on the scope, so that you
would get the correct reading of volts per division at the probe tip.
But a 500 ohm or 1K probe is just a series resistor, maybe with a bit of
added compensation, and no switch, and probably no signal to tell the scope
that you've got an attenuating probe on the end of the coax. It's easy to
make one yourself (see, for example, the Johnson & Graham book). The result
is that you must manually scale the waveforms displayed on the scope,
according to the probe's actual attenuation.
I am assuming that this is what you mean by "the amplitude is considerably
less than it should be." Either that, or the 1K probe input impedance is
loading down the circuit you are probing too much.
Andy
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