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From: Winfield Hill
Subject: Re: LM317 or 7808 is better performance for 9V output ?
Date: 10 Dec 2002 05:40:23 -0800
Organization: Rowland Institute
References: <3DEDABCA.906FB592@fanwap.com> <email@example.com> <firstname.lastname@example.org> <3DF5DF73.1FB2@Spam.Bots>
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>Bob Wilson wrote:
>> Noise especially. 3-terminal regulators will do nothing to clean up noise.
> The regulators can contribute a great deal of noise, which can be a problem
> in pll's and low-level amplifiers. I recently analyzed four different methods
> of reducing the noise and posted the results to
Anyone using a LM317 who's concerned with noise will certainly add the
suggested filter cap from the ADJ pin to ground; You left this important
part out of your drawings and analysis. The same advice goes for Bob's
comments. Adding this cap lowers the 10Hz to 10kHz noise from the 317's
spec'd 30ppm of Vout, to 30ppm of 1.2V = 36uV. By comparison a 7812 is
spec'd with a massive 75uV of noise. So there, all you LM317 nay-sayers!
Mike, further to your various ripple-reducing schemes, you have failed to
include my favorite highly-effective filter, which is a simple FET or BJT
2nd-order filter on the INPUT of the regulator. Add it just after the
rectifier energy-storage filter capacitor, like this:
. ,--||----------, _____
. | | | |
. ---+--------|---- D S --+--+---| |-----+--- out
. | | G | |_____| |
. | | | === | ===
. '--/\/\--+--/\/\--+ | gnd |
. | gnd gnd
It can be much more effective to eliminate the problem at its source,
rather than to struggle with it after the regulator.
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