From: "Frank Bemelman"
Subject: Re: linear power supply noise questions
Date: Wed, 13 Nov 2002 00:23:01 +0100
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Organization: EuroNet Internet
NNTP-Posting-Date: 12 Nov 2002 23:23:16 GMT
"Walter Harley" schreef in bericht
> This weekend I was fooling around with a simple noninverting audio opamp
> circuit. The feedback divider was AC-coupled to ground, rather than
> DC-coupled, to avoid amplifying the DC offset of the opamp along with the
> audio. However, in order to avoid using a nonpolar electrolytic, I
> to couple it to the negative supply, rather than to ground per se,
> that since the AC source impedance of the negative supply should be very
> it wouldn't make any difference.
> Bad plan! There was somewhere around 0.5mV rms of broadband noise (not
> sitting on the supply rail, and that of course coupled right through to
> output. In fact, the way I discovered it was by pulling the opamp from
> circuit and observing that the noise persisted, unless I turned off the
> power switch. I changed the circuit to couple to ground instead of the
> negative supply, and got a 30dB improvement in noise at the output.
The cap will have a very low DC voltage across it, so I don't see
why you want to avoid that polarized cap or even worry if the
polarity is actually reversed ?
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