[sdiy] A question about regulator noise figures

[Florian Teply]

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
> ==================
> 
> 
> 
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Use marketplace at synth-diy.org