[time-nuts] Super Regulator links

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Dec 14 06:07:30 EST 2007


Grant Hodgson wrote:
> Torex make some very nice low noise regulators; the XC6204 series has 
> been around for some years now and appears to offer much lower noise 
> levels than the 723 :-
>
> http://www.torex-europe.com/products/range/125
>
> Click on 'Technical Diagrams' to see the noise plot of the 3V part - sub 
> 1uV /rtHz at all frequencies above 100Hz.  One has to assume that the 
> noise specification for the uA723 is per rtHz, in which case the Torex 
> part has lower noise.  The uA723 has higher ripple rejection though.
>
> The Torex XC6204 is specifically intended for noise-critical 
> applications such as oscillators, PLLs and the like - it's not really a 
> general purpose regulator like the 723.
>
> Torex parts are available in the UK (and maybe elsewhere) from Farnell, 
> other distributors also stock them.
>
> I suspect that lower noise regulators are now available.
>
> regards
>
> Grant
>   
Grant

Congratulations, you've been seduced by the quoted noise without
considering what it actually means.
This is just the sort of reaction the marketers hope to induce.
This is a natural reaction, you should be more skeptical of first
impressions.

All I see is the usual obfuscation, this isnt a particularly low noise
regulator.
There is no provision for reducing the output noise (like low pass
filtering the reference) other than the somewhat ineffective technique
of using the largest output capacitor available.
To make valid comparisons the output voltage of al regulators being
compared has to be the same.
Alternatively divide the noise specifications by the output voltage.
Compared to a buried zener reference this part is noisy before scaling
its even worse after.

Taking the noise plot on page 36 (weird axis label - "ripple frequency")
and dividing it by the output voltage the noise is 333nV/V/rtHz at low
frequencies and 33nV/V/rtHz at high frequencies.
A good buried zener can achieve 28nV/V/rthHz at 10Hz and 14nV/V/rtHz at
high frequencies. The buried zener is significantly better even before
low pass filtering which is not possible with the Torex regulator.

Dont assume that the 723 noise is specified in this way it isnt, there
is no indication of its frequency dependence.
Even though the noise without reference filtering may perhaps appear
larger this is irrelevant as reference filtering is feasible with the 723.

Bruce



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