Electronics > Metrology

Noise cancellation circuits for voltage references

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branadic:
Hi everyone,

we have voltage references and we have low noise amplifiers to characterize their noise, mostly multistage inverting topologies, but has someone ever tried to combine both with a sum amp to create a Noise Cancellation Circuit? Even the best voltage reference we have today is not limited by the opamps used, but the reference itself. On the other hand we have high-performance chopper amps for the sum amp too. So at least in theory it could be possible to achieve lower noise at the output, right?

I'm happy to hear about any approach tested by now.

Just in case someone argues, LNA's with frequencies down to 4 mHz were alread published, so we could benefit from that very low frequencies. On the other hand I found, while testing multiple different brands and caps for an LNA that Yageo 85 °C caps showed <5 nA of leakage after 24 h, while 105 °C suffered from way larger leakage in general.

Edit: Added a first example to give a brief idea how such NCC could look like and as better basis for discussion.

-branadic-

iMo:
What would be the propagation delay through the LNA?

dietert1:
There is a difference: Standard usage of a LNA is to look at the AC output, while the DC precision isn't that important. With a cancellation circuit you will have the DC output in the result.
Then you arrive at the same problem as those engineers trying to make a LP filter for noise suppression: Leakage, dielectric absorption and temperature dependence of the capacitor(s).

Regards, Dieter

Kleinstein:
The main part of the LNA is the AC coupling as a kind if high pass filter. So the use for noise reduction is using a low pass filter.

I have a low pass fitler for the reference in my ADC boards (some 5 K and 4.7 µF). It does help noticible with the noise of the LM399 reference, though part of the effect in my case is from the 50 kHz range that would be filtered with less capacitance too. The dielectric absorption gives a little longer settling time on turn on, but not very much - at least not with a polyester cap. It could be an issue for using electrolytic capacitors. It is more the normal warm up that effects the settling.
Similar the temperature effect on the capacitor should not be that bad, except for the electrolytic ones.
This does not mean a filter with electrolytic capacitor would be bad, but it takes extra care in avoiding temperature variations and mechanical stress and finding a suiteable. It can slow down the settling quite a bit.
With many references we don't really care about the high frequency noise, the troublesome part is the low frequency part. With some ADCs/DMMs  there can be an extra sensitivity to noise at frequencies around 25 Hz or maybe 2.5 Hz from the AZ cycle. So it may be worth filtering for this frequency range. This is the motivation in my case.

dietert1:
All pass minus high pass gives a low pass. This is used in digital loudspeaker crossovers. The all pass includes a delay line in order to get a low pass filter of good steepness. Without the delay the resultant low pass has only 6 dB/oct.
I think the idea was to make a low pass with a corner frequency of 0.1 Hz using one of the standard 0.1 Hz to 10 Hz LNAs. This will be considerably more difficult than a 13 Hz low pass.
Recently i happened to use some 1uF/100V film caps we had for filtering a ADR1399 with 16 Hz corner frequency. I got a warmup time of several hours due to DA and an extra TC of 0.4 ppm/K due to a 4 ppm temperature dependent leakage loss. Then i found a 2.2uF/400V cap that behaves much better. It's an old part from a CRT display horizontal deflection.

Regards, Dieter

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