Regulator Noise

[Bob Zimmer]

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:

    Regulator      Voltage          Current    Noise Level
    LM309            +5V              1A         40uV
    LM320          -5V, -12V, -15V    1.5A       150uV
    LM323            +5V              3A         40uV
    LM325           +-15V             100mA      150uV
    LM340          +5V, +12V, +15V    1.5A       40,75,90 uV
    LM345           -5V               3A         150uV
    LM78xx         +5, +8, +12, +15   1.5A       75uV
    LM79xx         -5, -8, -12, -15   1.5A       125uV

Variable Regulators:

    Regulator      Voltage          Current    Noise Level
    LM317          +1.2 - +37         1.5A       .003%
    LM333          -1.2 - -32         3A         .003%
    LM337          -1.2 - -37         1.5A       .003%
    LM338          1.2 - 32V          5A         .003%
    LM350          1.2 - 33V          3A         .001%
    LM723(w/5uF)   2 - 37             150mA      2.5uV

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!

Seems that (with the exception of the 723) fixed are significantly quieter
than variable units!  The LM723 is the winner by far even though it
requires more components including a pass transistor!

I had been using LM317 & LM337 units since they allow 15dB tighter
regulation on the supply line!  After seeing this, I'm wondering if having
3x the noise level should make me reconsider!

Any thoughts on just how high a level is acceptable in synths?


  >=== Bob Zimmer -- Phila, PA     bzimmer at voicenet.com ===<
  >===         http://www.voicenet.com/~bzimmer/        ===<
  >=== "Oat-bran noise is more likely to be an issue in ===<
  >=== situations where cereal data ia concerned [:-)]" ===<
  >===         Analog Dialogue - Analog Devices         ===<