From: John Woodgate
Subject: Re: Audio noise in diff amps
Date: Thu, 19 Dec 2002 17:32:11 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Thu, 19 Dec 2002 19:34:16 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Rick wrote
(in ) about 'Audio noise in diff
amps', on Thu, 19 Dec 2002:
>I'd love to hear what you think is inside this clever Fluke meter that
>measures noise voltage WITHOUT drawing any current whatsoever. It would
>have to be a non-inverting follower with infinite input impedance,
>which is rumoured to be a tricky one to construct!
As it happens, many of these DMMs have an input impedance on the low
ranges of around 1 gigohm. But in any case, the meter is scaled so that
0 dBu = 0.775 V, in accordance with the definition of 'dBu'. It does not
In audio, we measure noise as voltages, and don't consider power unless
the calculations demand it, in which case we often effectively use V^2/R
rather than an explicit power term.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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