[time-nuts] re low noise regulators

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Dec 13 23:56:13 EST 2007


Jeffrey Pawlan wrote:
> I checked both datasheets, the 723 and the LT1762.  It was specsmanship that
> rated the 723 as having lower noise. Here's how:
>
> The internal comparison opamp in the 723 was OK for its day back in Fairchild's
> history but by today's standards, it is noisy and has poor gain and BW. So in
> order to make the regulator low noise, a 5uF capacitor had to be placed across
> the ref comparison input.
>
>   
Nonsense, if the amplifier was the largest contributor to the noise this
would have virtually no effect as it only filters the reference noise.
> The low noise, and even the standard linear regulators today are much quieter
>   
Rubbish look at the output noise from any 78xx regulator.
> and few people need 5uV of power supply noise,
Just those who want low flicker phase noise RF amplifiers.
>  especially given that the
> circuitry we place on the power rails will almost certainly spoil that.
> So no one specs putting a 5uF cap on the ref line. The LTC LT1762 does suggest a
> Cbyp of .01uF will provide a noise of 20uV broad bandwidth as you correctly
> stated. 
At 5V output, again specmanship at work.
> I am certain that putting a higher value cap and perhaps even a RC
> filter on the reference input will lower this substantially.
Without some idea of the internal circuit configuration how can you be
sure of this?
It may well make the noise worse than when using a single capacitor or
event cause the device to oscillate. the performance as there's nowhere
to put it.
It would appear from the 1761 datasheet that it merely filters the
1761's noisy internal reference or at least ensures that the midband
gain from the reference to the output is unity.

>  But again, I
> suggest that it will become meaningless to all but Super-Audio Snake Oil
> people because any circuitry powered from the produced Vcc line will add more
> noise than the regulator. Even with the now ancient history LM317 style
> adjustables, I always put a tantalum cap across the reference to ground . It
> does wonders for getting rid of traces of hum.
>
> Just for your collection I will send John two App notes in PDF form to post.
> They cover the subject of reducing spikes and switching regulator noise.
>
> regards and 73,
>
>
> Jeffrey Pawlan  WA6KBL
> Pawlan Communications
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>
>   




More information about the time-nuts mailing list