[sdiy] A question about regulator noise figures

Miles Stevens milesstevens89 at gmail.com
Tue Mar 16 14:56:33 CET 2021

```My cheat sheet for this kind of thing, may be of some use to others.

Agree with the above about spectral density being an interesting
measurement, I love my Rohde Schwarz UPL to death for that kind of thing

On Tue, 16 Mar 2021 at 17:30, Florian Teply <usenet at teply.info> wrote:

> Hi All,
>
> Am Mon, 15 Mar 2021 23:57:28 +0000
> schrieb Tom Wiltshire <tom at electricdruid.net>:
>
> > Hi All,
> >
> > I’ve been reading data sheets for linear regulators, and I’ve got a
> > question.
> >
> > Some regulators specific the output noise voltage from 10Hz to 100KHz
> > as an RMS level, usually uV. However, the Microchip MCP1702 specifies
> > an output noise figure as 8 uV/Hz. What does this mean? What’s the
> > relationship between uV/Hz and RMS uV?
> >
> Ah, that noise stuff.
> Actually It's not that difficult once you know what you need to do with
> that figure in order to get something useful. The value given is noise
> density and reads V/sqrt(Hz). Converting to RMS is somewhat easy:
> Multiply the value give with the square root of the bandwidth of
> interest, here 100 kHz - 10 Hz which more or less equal to 100 kHz.
> Square root of 1e5 Hz is approximately 3e2 sqrt(Hz).
> 8 uV / sqrt(Hz) times 300 sqrt(Hz) yields 2400 uV or 2.4 mV. It's
> already RMS, so comparison to Volts RMS is easy.
>
> There are pitfalls however: The simple calculation give above only
> gives reasonable results when the noise density is constant across
> the bandwith of interest (that means white noise and would exclude
> flicker noise). And: The datasheet states voltage noise density
> under specific conditions. For one thing, it doesn't say how that number
> changes when you divert from those conditions given. Change Capacitance
> or current draw and you're on your own. It also doesn't tell anything
> about flicker noise. Chances are it's not going to be too shabby with a
> 1/f noise corner below 100 Hz, but you never know beforehand: the
> mnufacturer didn't specify either because they didn't bother to measure
> it (that's not super easy in automated testing as it takes quite a
> while to do) or because they decided it's so bad they rather not report
> it publicly.
>
> There's one more pitfall: some manufacturers tend to specify noise as
> peak-to-peak value, and conversion between peak-to-peak and RMS depends
> on what number the manufacturer settled for as peak-to-average ratio
> for noise, which usually is not given in the datasheet...
>
> HTH,
> Florian
>
> > Here’s the datasheet:
> >
> >
> https://4donline.ihs.com/images/VipMasterIC/IC/MCHP/MCHPS03366/MCHPS03366-1.pdf?hkey=6D3A4C79FDBF58556ACFDE234799DDF0
> >
> > Thanks for any guidance!
> >
> > Tom
> >
> > ==================
> >        Electric Druid
> > Synth & Stompbox DIY
> > ==================
> >
> >
> >
> >
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