[volt-nuts] Practical power supply noise testing

David davidwhess at gmail.com
Fri Jul 1 12:38:40 EDT 2016

In the past when I did this, I used a high resolution digital
voltmeter like a Fluke 8505A in sample mode with a simple 10 Hz low
pass filter.  I took samples for 10 seconds to get down to 0.1 Hz and
calculated the standard deviation to get RMS noise.  I made a recent
post about this in connection with very low noise DC amplifiers.

To measure output ripple, I use a DSO set to line triggering.
Averaging removes all noise which is not synchronous with the AC line

A low noise preamplifier with offset capability or a very low cutoff
frequency is still needed for lower level noise measurements.  I have
a Tektronix 7A22 now but its AC coupled low pass cutoff is only
specified as better than 2 Hz and its offset capability is somewhat
limited.  It works great to help with higher frequency bandpass
measurements though.  A 7A13 has slightly better offset capability but
is also higher noise.

You said you want to avoid building a custom low noise amplifier
however for more sensitive measurements, even a simple operational
amplifier based design will help considerably.  I always hack together
an AC coupled amplifier using a low noise JFET operational amplifier
as needed; the quality of the AC coupling capacitor is what limits low
frequency performance.

Check out Linear Technology application notes 83, 124, and 159 for
some ideas and a discussion of the AC coupling capacitor problem:


On Fri, 1 Jul 2016 08:47:18 -0400, you wrote:

>I have several supposedly low noise bench power supplies that I want to check out.  They seem to work well but are quite old, so of course one wonders about the caps, etc.
>Can someone suggest a practical test regime to verify power supply DC noise performance?  Preferably one that doesn't require building up custom low noise amps?

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