Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: linear power supply noise questions
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Tue, 12 Nov 2002 19:56:40 -0000
NNTP-Posting-Date: Tue, 12 Nov 2002 20:05:37 GMT
"Walter Harley" wrote in message
> This weekend I was fooling around with a simple noninverting audio
> circuit. The feedback divider was AC-coupled to ground, rather than
> DC-coupled, to avoid amplifying the DC offset of the opamp along with
> 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
> it wouldn't make any difference.
> Bad plan! There was somewhere around 0.5mV rms of broadband noise
> sitting on the supply rail, and that of course coupled right through
> output. In fact, the way I discovered it was by pulling the opamp
> circuit and observing that the noise persisted, unless I turned off
> power switch. I changed the circuit to couple to ground instead of
> negative supply, and got a 30dB improvement in noise at the output.
> I have three questions:
> First, I'm using an LM337T as the negative regulator. It's bypassed
> input with a 0.1uF ceramic, and at output with a small tantalum (don't
> remember value offhand). Am I right that the next thing I should do
> reduce the supply noise is bypass the ADJ pin, presumably with a
> polypropylene cap? Or would I be better off putting a poly or ceramic
> across the output (in parallel with the tantalum), or should I do
By and large, if a power supply has broadband noise, there is not a lot
you can do about it. The output is already at a very low impedance, so
putting a cap on it aint going to help much.
> Second, is this problem typical,
>and is that why these circuits usually show
> a cap to ground instead of a supply rail?
Always decouple signal leads to ground.
> It seems like it would be better
> engineering to use a cheaper, more widely available part in place of
> nonpolar cap, but not if it means increased noise.
> Third, my Hitachi V212 oscilloscope's max calibrated sensitivity is
> division; with a x10 probe, that's 50mV per division, which means I
> barely see the 0.5mV supply noise (instead, I discovered it with an
> Precision analyzer). Do people generally use scopes with higher
> sensitivity, or outboard preamplifiers, or is a scope just not the
> tool, or should I get my eyes checked?
To measure PS noise, its not uncommon to build a 60db amp.
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