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

David davidwhess at gmail.com
Tue Oct 18 06:25:35 EDT 2016

On Tue, 18 Oct 2016 11:27:05 +0200, you wrote:

>On Fri, 14 Oct 2016 22:25:55 -0500
>David <davidwhess at gmail.com> wrote:
>> 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.
>Yes, there are several appnotes and papers that list this method.
>But I am not aware of any noise measurement below 0.1Hz for such
>an amplifier setup.
>			Attila Kinali

Below 0.1 Hz it is not all that interesting; the noise is just the
noise of the chopper amp and flat below the chopping frequency.  I
think Jim Williams mentioned in one of his articles that at low
frequencies, noise and drift are effectively the same thing so thermal
EMF becomes a large if not the largest contributor.

When I did it, I extended the single ended design to a fully
differential gain of 1000 amplifier using a pair of LT1028s with a
pair of LTC1150s for correction.  I used the noise curves to estimate
what the integrator gain should be and after adjusting it for minimum
noise from about 0.1 to 10 Hz, it was very close to the actual
crossover point in the datasheet specifications.  RMS noise was
measured by taking the standard deviation of the DC values from a
Fluke 8505A over 10 seconds.

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