[SI-LIST] Re: Probe impedance effect at high frequency
- From: Doug Smith <doug@xxxxxxxxxx>
- To: TerenceHsieh@xxxxxxxxxxxx
- Date: Wed, 15 Mar 2006 03:41:29 -0800
Hi Terence,
I am in Seoul, Korea at the moment so I don't have any of my material
at home, but here are a few points:
1) On the surface, the probe may not have a high enough impedance, and
none may. It is very difficult to get an input impedance higher than
300 Ohms in the GHz range. The input impedance in the GHz range of
most high speed active probes I have seen is determined to a great
extent by their input damping resistors, the location of these
resistors relative to the probe tips, and parasitics of the probe
connection to the circuit.
2) The way any high speed probe is connected to the circuit is at
least as important to input impedance as the probe itself. TEK should
supply a SPICE model of the probe and connection hardware which you
can use to determine the effect on your circuit. A simple plot, in my
opinion, of input impedance is generally not enough information to
effectively use a modern high speed probe.
I tried to get this information to give you a better answer but the
Tek site requires me to register. I have too many registrations
already and can't remember any more so I didn't register from here.
These days I do not use many websites that require registration unless
I plan to use the site at least several times per week (like
accuweather.com for me) so I can remember the logon info.
3) In general, for many probes I have seen of all types, published
information is for the best case which may not be how you are using
the probe. Another good reason to use SPICE models to predict performance.
Doug
TerenceHsieh@xxxxxxxxxxxx wrote:
> I have a problem about differential probe. When measure a DUT with low
> output impedance, it is appropriate to use high impedance probe. If the
> probe impedance is ten times than DUT, the probe impedance could ignore
> and the measurement result will more accurate. I checked the technical
> note of differential probe P7380. The DC impedance of P7380 is 100K ohm
> and this is larger enough for most DUT. However, the impedance is just
> about 300 ohm at about 2GHz. If I want to measure a DUT that operate at
> 1.25GHz (PCIE Gen.1) and the differential impedance of DUT is 100 ohm.
> Could P7380 measure accurate result? How the low AC Loading of probe
> affect the result? Thanks
>
> Terence
>
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