Regulator Noise

[Don Tillman]

   Date: Wed, 17 Sep 1997 12:33:09
   From: Bob Zimmer <bzimmer at voicenet.com>

   A short while back, there was a discussion on the noise output of three in
   regulators.  I did some looking in a databook and found some interesting
   numbers:

   Fixed Regulators:
   [Noise: 40 - 150 uV]

   Variable Regulators:
   [Noise: 0.003%]

   If my math is correct, .001% on a 15V output from a variable regulator
   translates to 150uV, while .003% on a 15V output translates to 450uV!

That's why it's always important to bypass the lower resistor of the
LM317 circuit with a cap; that way the noise is a fraction of the
reference voltage (1.25V) and not the output voltage (15V), bringing
the noise voltage down to about 37.5uV, which is better.

  -- Don