[SI-LIST] Re: FET Probe
- From: Jim Roberts <jgroberts@xxxxxxxxxx>
- To: Andrew.Ingraham@xxxxxx
- Date: Fri, 01 Nov 2002 17:40:02 +0100
Hi,
I agree with Andrews statement. A short ground and signal connection are
required to reduce direct coupling from the board.
However, also be aware that when you connect the ground of the probe to
ground of the unit you are also making an "earth-loop".
This can generate as much or more interference dependent how the scope is
grounded
relative to the unit.
I can recall a very important customer/public demo at Munich where the
whole system collapsed because the earthing of a probe was on a different mains
to the system and yet another distrbution for the LAN/debug port.
Easy to say, but difficult to always remember.
My personal preference is to use differential probes!!
--
Regards, __________ James G Roberts
/___ ____ | jrobert@xxxxxxxxxxxxxxxxxxxxx
Jim __ / /___/ / jgroberts@xxxxxxxxxx
/ /_/ /---| | Room: BE436, Hilversum
\____/ /_/ Tel: +31 35 687 4308 Fax: 5976
=============================================================================
"Ingraham, Andrew" wrote:
>
> > 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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