[time-nuts] measuring noise of power supplies (was: For those that insist on using switching power supplies)

David davidwhess at gmail.com
Fri Oct 14 23:25:55 EDT 2016

I have done this and it works great; the breakpoint between the
chopper amplifier and the low noise amplifier can be adjusted to
combine the wideband noise from the low noise amplifier and the 1/f
noise and drift of the chopper amplifier.

Jim Williams wrote a couple of different application notes where this
was used with both integrated and discrete amplifiers.

On Sat, 15 Oct 2016 00:53:25 +0200, you wrote:

>The low frequency range is what we usually call the 1/f region,
>although the long term stability also belongs to it. But unlike
>the long term region you don't have to sacrifice a virgin to
>get decent measurment data. Jim William's appnote[1] has lots of
>details how to measure noise in this region. There are slightly
>more modern circuits by Todd Owen/Amit Patel[2] and Gerhard Hoffmann[3].
>I recently stumbled over a similar amplifier by Enrico Rubiola
>and Franck Lardet-Vieudrin[4]. Both [4] and [5] explain why for
>low impedance sources (like power supplies) a BJT input stage
>would be a better choice than jFETs and also cover the influence
>of temperature on the measurement. [6] gives some additional info
>on how to design the differential input stage.
>I wonder how an active offset voltage cancelation scheme for
>the differential pair input stage using one of the chopper stabilized
>opamps (eg LTC2057) would change the temperature dependence and long
>term stability  (aka 1/f^a noise) of the circuit, but I have not seen
>any measurements of a system like this yet.

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