RE: [SI-LIST] : Heisenberg's uncertainty principle and circuits

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From: Thomas Jackson (tjackson@fmi.fujitsu.com)
Date: Tue Apr 03 2001 - 09:40:40 PDT


Ah, you young whipper-snappers!

Back in the Dark Ages when I went to school, it was standard to have one or
more laboratory courses that included measurements on a circuit that had
been "tuned" so that the impedance of the measuring device (oscilloscope and
probe, voltmeter and probe, etc.) caused very different results than those
predicted (by the students). The instructors/professors would always laugh
as each group of students tried to figure out what went wrong with their
experiment.

Later on in life, I was a metrologist, the people who make precision
measurements and comparisons to known standards. There are situations in
which measurement loading effects must be accounted, such as when one is
trying to measure a small effect with a large background, like noise and/or
DC offset.

The important thing is to not take these things for granted. A
"back-of-envelope" calculation can usually determine when the effect of the
measurement is significant.

Tom Jackson

Thomas L. Jackson, P.E.
Staff VLSI Design Engineer
Network Access Development
Systems Solutions Group
FUJITSU MICROELECTRONICS, INC.
3545 North First Street
San Jose, CA  95134-1804
telephone: (408) 922-9574
facsimile: (408) 922-9618
http://www.fujitsumicro.com

-----Original Message-----
From: Douglas C. Smith [mailto:doug@emcesd.com]
Sent: Monday, April 02, 2001 7:43 PM
To: emc-pstc; si-list
Subject: [SI-LIST] : Heisenberg's uncertainty principle and circuits

Hi All,

Often, when making measurements, the loading on the signal by the
measurement is not taken into account. In addition to probe bandwidth,
its input impedance is just as important. Many high performance probes
on the market are useable to only a fraction of their stated bandwidth
because of very low input impedances, and this includes many FET probes.
That spec of less than a picofarad of input capacitance seems to imply
low circuit loading, but this is not always the case.

This month at http://www.dsmith.org the Technical Tidbit is on error
caused by probe loading. It is a real eye opener. But this is just the
"tip of the iceberg" on measurement limitations.

Doug

-- 
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