FreeLists / si-list / [SI-LIST] Re: FET Probe

[Doug Smith <doug@xxxxxxxxxx>]

Hi Jim and the group,

Even the inductance of a 1 cm diameter loop formed by the probe tip 
and ground lead (or both probe tips in the case of a differential 
probe) will cause many GHZ+ active probes to reach an input impedance 
as low as a few tens of ohms in the upper octave of their stated 
bandwidth. Without the ground lead at all, of course, the loop is 
huge. The fairly low input impedance of the probe (capacitance) will 
form an interesting circuit with all that cable.

Those probes that use a ferrite core in the ground lead (usually a 
short, hinged stiff wire) to damp resonance are very susceptible to 
external interference. In one case, the addition of the ferrite caused 
a 2 cm ground lead to appear as it if was 15 cm long as far as picking 
up external interference. To see that data look at:

http://emcesd.com/tt2001/tt090101.htm

Doug

Jim Roberts wrote:
> 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!!


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