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From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: LM317 or 7808 is better performance for 9V output ?
Date: Wed, 11 Dec 2002 12:34:11 -0800
NNTP-Posting-Date: Wed, 11 Dec 2002 12:34:12 EST
Organization: Bell Sympatico
John Woodgate wrote:
> I read in sci.electronics.design that Mike Monett wrote
> (in <3DF7505B.4DF@Spam.Bots>) about 'LM317 or 7808 is better performance
> for 9V output ?', on Wed, 11 Dec 2002:
> > I'm sorry I didn't include your favorite prefilter circuit.
> > Certainly this would help reduce ripple, but it would not affect the
> > regulator noise.
> It's not clear to me how much noise passes *through* the regulator from
> the raw supply (and I mean broadband noise) and how much is generated in
> the regulator itself.
> Regards, John Woodgate
Presumably, the regulator is able to reject incoming noise near the ripple
frequency the same way as it rejects the ripple. It's not clear how the
regulator responds to high frequency noise, but it may follow the same
general performance as op amps where the attenuation degrades at higher
frequencies. So it passes along whatever it receives.
The internally generated noise can be quite large, especially in some LDO
regulators like the LM2940. According to the spec, it could be as much as
1000 uV rms. Much worse than the 723. I assume the noise is measured with
clean power supplying the regulator.
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