[SI-LIST] Re: FET Probe

> Does anyone have a basic schematic for Tektronix FET probe?  ...
> 
> The reason that I am asking is that I have a board that will not come
> up
> properly when the positive terminal of the probe is hung off of a DDR
> strobe
> line.  No ground attachment required.
 
Think about that.  What you have is several meters of antenna wire
connected to a signal pin.  Or maybe a very large loop going back to the
AC mains power.  It's a nice way to inject noise on your signal.

The FET probe's equivalent input circuit is relatively meaningless if
the probe's ground isn't connected, except at very low frequencies.

At the least, a FET probe has capacitance, which may alter the shape
and/or timing of your signals.  Some also have a moderately small
resistance.  Read what's printed on the probe.

The Cin and Rin that are printed on the probe, apply at DC.  As the
frequency increases, many probes get more lossy, meaning that Rin drops,
sometimes by orders of magnitude.  Different probes vary.

Some FET probes have a small dynamic range of only a few volts, which
limits their usefulness when looking at 5V or 3V signals.  I don't know
whether they also incorporate clamp diodes in the probe tip; if they do,
this could definitely alter the signal itself.

Bottom line is, FET probes come in a number of different types.

Andy



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