[time-nuts] Low noise voltage regulators
Charles P. Steinmetz
charles_steinmetz at lavabit.com
Fri Feb 26 02:05:38 UTC 2010
>With the 723, you can make the reference noise as low as you want, by
>heavy RC filtering. This applies whether you use its own reference or a
>better external reference.
>The 723 also seems to work quite happily with a feedback capacitor from
>the output to the inverting input, reducing the AC gain to unity. The
>output noise will then just be the buffer amplifier's input noise
If the 723 is "great" it is because its functional blocks are
available externally -- it allows you to use the full range of design
tricks to bring the noise down pretty close to the internal
amplifier's input noise. If one needs even lower noise, there is an
embarrassingly rich assortment of op-amps available today with noise
figures in the 1 nV/rtHz neighborhood. But if you go this route,
make sure to keep circuit impedances low so you get the benefit!
One other thing that I don't think has been mentioned -- when using
RC or LC networks to filter the reference noise, make sure your C
doesn't have a lot of leakage noise, as many electrolytic capacitors
do. I have seen many "noise filter" capacitors in commercial
products that actually made the overall circuit noise worse by 3 dB
or more, with all components operating nominally.
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