From: Win Hill
Subject: Re: How to measure C+stray to 0.01pf
Date: 3 Sep 2002 15:24:57 -0700
Organization: Rowland Institute
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firstname.lastname@example.org (Tom Bruhns) wrote...
> email@example.com (Adam Seychell) wrote
>> Agilent have seem to make the impossible possible. The Agilent 4285A
>> Precision 75KHz - 30MHz LCR meter for example claims 10aF resolution!
>> Can somoene explaing the purpose of such a ridiculous reading ?
> Well, let's see. In the work I mentioned in my earlier posting, we
> worried about drift on the order of 25 parts per million or so. Since
> we were dealing with nominally 3pF capacitors, that would amount to
> about 75aF. So yes, I can imagine some folk would care.
0.01fF = 10aF is a mighty small capacitance; very impressive for
an automated instrument. However it isn't ground-breaking. :-)
My old General Radio model 1615-A Capacitance Bridge measures to
this same accuracy, 0.01fF or 10aF on its 11pF full-scale range.
I have used it at this level and was amazed at how repeatable the
results were. Aha! But then I acquired a General Radio model 1616
Precision Capacitance Bridge. Because this is a "precision" model,
it can measure 100x better, to 0.1aF. Now _that_ is impressive!
I haven't yet used in to that level, so I can't attest to the last
digit, but it's reputed to be up to the task.
Someone should point out that all we serious capacitance-measuring
folks :-) use three-terminal capacitance measurements, where the
capacitance to ground scarcely matters at all.
I'm holding in my hand an old 0.001pF standard capacitor that is
accurate to 1% (made by General Radio - who else? - model 1403-V).
This means as a reference standard, it's accurate to +/-10aF.
OK, are we becoming believers yet? :-)
Remember my thread "Measuring 0.02fF (right, that's 0.00002pF)" on
s.e.d. a few years ago? It was complete with schematics of my nice
electronic fast-response capacitance meter, used for high-resolution
STM atomic-scale position measurements. With a companion thread,
"fF capacitive measurements," it makes for some interesting reading.