Electronics > Metrology

Noise cancellation circuits for voltage references

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Kleinstein:
The AC coupling circuit does not care much about a little (e.g. 100 µV) range residual offset. As long as it is reasonable constant the leakage of the capacitor is not that bad. The search for low leakage is mainly be cause leakage often also comes with extra noise, not because of the DC leakage on it's own. Similart amplifier input current by itself is only causing a little extra leakage and only the current noise that usually comes with the bias current is really a problem.

The the low pass filter leakage add offset and this can be a problem.

How much lower the noise with the filter actually is, depends on how the reference is used / measured. With the high end references one mainly cares about the low frequency part and measures / compared the reference over some time, e.g. with a DMM. The measuring instrument already does quite some filtering.  Much of the noise a simple filter can remove may be suppressed by the DMM anyway. Still many DMM as not perfect in noise suppression - e.g. the usual 10 PLC AZ mode lets some of the 2.5 Hz frequency range noise pass even with averaging many readings. So the filter can still help, but that effect can be more limited than one may think.

miro123:
I think that the tread is moving in magic circle.
I think that we must put the requirement on the table. I have learned /from my little experinace/ that Vref requirements are contradictory
 - low white noise
 - low low mid-low frequency - 1..0.1Hz - creates top on low, and ultra low frequency.
 - high stability
 - low TC
To make it clear - Low noise high stability and low TC Vref does not exist - come compromise should be made. This statement is based on traditional voltage reference design- the modern digital approaches are not taken into account
As far as I understand this tread discus reducing white noise and low refuency noise in lets say 1 min base - as result of it other parameters get worse - eg. stability TC mid low freqency noise

David Hess:
Low drift and low low frequency noise are essentially the same thing because at low frequencies they are indistinguishable.  A reference with one effectively has the other.

High frequency noise can be ignored because it is both easy to filter out, and integrating ADCs will ignore it anyway.  It matters of course for a sampling ADC.

Flicker noise is especially pernicious because its amplitude increases below the low pass filter's cutoff frequency, which is why it is better to avoid generating it than to try and filter it out.

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