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:d...@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 -- ------------------------------------------------------- ___ _ Doug Smith \ / ) P.O. Box 1457 ========= Los Gatos, CA 95031-1457 _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799 / /\ \ ] / /\ \ Mobile: 408-858-4528 | q-----( ) | o | Email: d...@dsmith.org \ _ / ] \ _ / Website: http://www.dsmith.org ------------------------------------------------------- **** To unsubscribe from si-list or si-list-digest: send e-mail to majord...@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu ****
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