Noise cancellation circuits for voltage references

[RoGeorge]

Reading through this thread, it seems the capacitor's imperfections are introducing some limitations when used to separate out the DC, or when used to implement low frequency filters.

I wonder why not using transformers instead (to remove the DC), and coils instead of capacitors for filtering?

[Kleinstein]

Inductors are also not ideal (unless you can use superconductors) with series resistance. The large inductance values would need iron cores and there one can have some kind of delayed hysteresis leading to similar and possible worse effects the dielectric absorbtion.
To avoid the trouble from DC current one would more combine an inductor with a capacitors instead of a resistor. Still inductors in the kH range are rare (and large). So one would still need a relatively large capacitor, but it could still be an option, as the inductor could live with a little more leakage.
So inductors are usually not used for low frequencies. They get more popular in the MHz range, buit not in the mHz range.

[miro123]

The link provide by Dieter gives the fundamental.  http://www.scholarpedia.org/article/1/f_noise
Mathematicians says that you cannot reduce the 1/f with simple integrator or differentiator.
Engineers knows that soldering of high value RLC components around  voltage reference makes thinks worse.
@Diether - I was trying to characterize several LM399 and ADR1399 - I sow that pop-corrn characteristics of those devices has changed over time. I have not idea is there any long term correlation with external factors  - e.g. ambient temperature, humidity - The artificial noise is reduced using batteries and optical interface

[dietert1]

Yesterday some Wima MKP4 10uF 250V arrived here and the one i tested first is showing 7 pA residual current at 14.1 V.
In comparison to the 70 pA input current of the OPA189 used as buffer, that isn't bad. These capacitor aren't that large. One could use two of them with 5 KOhm resistors to implement the Sallen-Key lowpass and errors should stay below 1 uV.
The ADR1399 evaluation kit already got a temperature isolated "shoe box" around it. That will likely become a thermostat when ready. Total consumption could be 1 W including the 600 mW of the ADR1399 heater.

Regards, Dieter

[RoGeorge]

The large inductance values ...
... in the mHz range.

For very big inductances, maybe a multiplier (with gyrators) to emulate a big coil. 
Never tried, but it should work.  Same idea might be advantageous for capacitors, too, using a small capacitor multiplied instead of a leaky electrolytic.

Even more, either inductors or capacitors, they are not used here as energy storage, but to form filters.  They can do the d/dt operation for voltage or current, and thus they can form filters.  However, nowadays number crunching and ADC/DAC are cheap, might make sense to go digital and filter there.

Putting aside how practical will be to implement any of that, either analog or digital, for active noise cancellation, two things are needed:
- 1. to decide what to subtract from the input mix of Vref+noise (usually using RC filters)
- 2. to correct the output (usually an adder)

However, point 1. takes time, so we will also need
- 3. a delay line for the original signal, so the extracted Vnoise and the Vref+noise are in sync in the cancellation adder from point 2.

Speaking while writing this, once we have a delay line "component" we can use that to implement filters, and get rid entirely of big capacitors or big inductors.

I have zero hands-on experience with voltage references, and to implement such low frequency filters I would be tempted to go digital to process the noise (before adding it back to the analog circuit for cancelling).


Heaving an analog delay line would help filtering out the shot noise, too. 

We can detect when a jump in voltage occurs (by comparing the current value with the most expected averaged value), and when a jump in voltage occurs, we disregard the current (wrong) voltage, and instead, copy to the analog output a previously known good voltage (we take the previously known good voltage from a tapped delay line, where each delay tap is connected to the input of an analog multiplexer, and we decide which input is considered as correct, or as heaving no pop-noise in it at that moment in the tapped delay line).

[iMo]

The issue here is the amplitudes of the jumps are small, like 2-4uV for 399 and even much less with 1000. And the jumps are buried in white and 1/f noise too. While looking at the measurements above made by a 7.5digits dmm you clearly see the jumps, but that "background" noise is filtered out by the dmm. Not having the dmm you would not see the jumps in the background wide band noise. Therefore, for example, my above idea with amplifying the "edges" of the jumps would not work in practice..

Delay line - that is my concern too - you are summing up two signals where one of them will be delayed by the high pass filter in the LNA (on the picture in the first post)..

What would be the propagation delay through the LNA?

[RoGeorge]

A voltage reference is a very particular case of signal, where the time lag between the physical live/noisy Vref and the clean output Vref does not matter.  In fact, the output of a Vref should never change (ideally).  We know it is expected to be constant, and that is the extra info we must take advantage of.

That is why, if we have long term memory (just like the measuring instrument has had while measuring/averaging for those 7.5 digit plots) we can filter out any pop-noise before it arrives at the output, here by outputting a recorded value of the Vref, recorded from a recent period of time when no pop-noise was present.

About the instrument having many digits, true that, but to detect pop noises we only need a similar high resolution measurement, but without the need of high accuracy, while a proper 7.5 digits instrument has to have both.  So our task is simpler, we only need to see sudden changes.  Should be doable.

As a side note, I'm aware that plenty of smart people thought a lot already about how to get a clean Vref, and low hanging fruits are long gone, so the only hope from now on is to try very different/stupid approaches, like the proposed digital filtering + analog delay line. 

[iMo]

..About the instrument having many digits, true that, but to detect pop noises we only need a similar high resolution measurement, but without the need of high accuracy, while a proper 7.5 digits instrument has to have both.  So our task is simpler, we only need to see sudden changes.  Should be doable..

I jumped on that idea in my earlier post above - to detect the sudden changes. My idea has been to high-pass the signal and to extract the "change" - represented by the rising and falling edges of the popcorn jumps. It works great when the background noise/signal contains lower spectral components only, ie. like frequencies limited to 1kHz max. You set the high-pass to 100kHz and you get perfectly every edge, even though when the signal amplitude to the jump amplitude ratio is X hundreds. But how to do it practice? When you limit broadbanded vref signal with a low pass filter, for example, you limit the jump's edges amplitudes as well (the edges are "high freqs") and you are not able to extract the edges that way - as they are not steep edges anymore.. And without limiting it you'll get a broadbanded signal and you are not able to detect the jump's edges as the signal contains unlimited number of "edges"..

PS: an example below - Vref signal contains max 1kHz (1-1000Hz sine generators used), the total background noise is 400uVpp, the popcorn jumps are 1uV on 7V with 1sec duration and 1us edges, every change/edge has been detected properly.

[MasterT]

The large inductance values ...
... in the mHz range.

1. --- For very big inductances, maybe a multiplier (with gyrators) to emulate a big coil. 
Never tried, but it should work.  Same idea might be advantageous for capacitors, too, using a small capacitor multiplied instead of a leaky electrolytic.

Even more, either inductors or capacitors, they are not used here as energy storage, but to form filters. 


2. ---- We can detect when a jump in voltage occurs (by comparing the current value with the most expected averaged value), and when a jump in voltage occurs, we disregard the current (wrong) voltage, and instead, copy to the analog output a previously known good voltage (we take the previously known good voltage from a tapped delay line, where each delay tap is connected to the input of an analog multiplexer, and we decide which input is considered as correct, or as heaving no pop-noise in it at that moment in the tapped delay line).

1. I think using small capacitors or gyrators is bad idea. Since more OPAmp amplification required and consequently more OPAmp noise getting in the path.  Capacitors served exactly to this purpose, as energy reservoir, the bigger energy - less entropy.

2. Reminds me median filter, where highest / lowest values are kicked out of the consideration. Median is the best (IMHO) filter to process spiky interference - noise.
Seems, we are arriving to the point that "classic" linear filter is not fit for reference voltage, doesn't matter if its 1-st order RC or 2-nd Sallen-Key.
 Going "median" way requires S/H circuits - to keep "most reliable value" till analog processing /evaluation new data completed. Processing may include linear continuous filtering over some periods of time, like below average pop-corn frequency, than comparator new/ previous values and some adjustment circuits if new value accepted as "noise-free" but slightly off compare to current.

[dietert1]

In my understanding the edges to detect result from the integration of flicker noise events. And a low pass with lower corner frequency supresses most of them, but will eventually show a similar picture from more rare events of higher amplitude.
So one can use an analog low pass filter to limit noise bandwidth in oder to simplify subsequent digitial processing for the purpose of noise cancellation.

Regards, Dieter

[DeltaSigmaD]

Passive noise filter for references

Active noise filter circuits substitute reference noise by noise of the operational amplifier, which has much lower noise. However, there is also a theoretical possibility to filter output noise of the reference with a passive filter while a low output impedance is provided. The attached reference filter circuit has a 2nd order Bessel characteristic with 1 Hz cut-off frequency and 0.25 Ohm series resistance. Such circuit could provide a reference voltage almost free of noise above the cut-off. This could be very interesting for data acquisition, e.g. noise measurement, in this frequency range.

The practical realisation would be not easy. The total leakage current of the capacitors should be reduced to less than 400 nA. Active circuits to reduce the 0.25 Ohm output impedance add OP noise, so that the passive filter is useless (I tested several topologies). The inductor can generate several kinds of noise. First of all, the inductor is working as antenna for EMI, and magnetic shielding of the filter is required. Second, Barkhausen noise is generated if the inductor current or the surrounding magnetic field of the inductor is changed. Third, ferrites generate noise due to loss mechanisms of the ferrite. A larger part of this inductor noise could be suppressed by the passive filter itself.

There is no other filter which can - theoretically - obtain lower noise (excluding superconducting circuits). Has anyone experience with the noise properties of inductors?

[dietert1]

As far as i understand, interaction with external fields is proportional to stray inductance. So one could try a toroid. For example a toroidal common mode choke with both coils in series, e.g. SCR-020-0R55A250J. Its two coils add up for 0.4 Ohm.

The capacitors may be more difficult.

Regards, Dieter

[Kleinstein]

With the relatively low resistance from the inductor(s) is can be attractive to use electrolytic capacitors or super capacitors, possibly even without extra compensation for the leakage. There is the DA part, but even that is partially attenuated.
For the damping part is could be possible to use a resistor parallel to the inductor instead of the extra RC element.
I would consider even more inductance. With a somewhat larger core one can get to the 1 H and maybe even 100 H range. An iron core (e.g. torroidal transformer) may not be that bad.

For the noise, there can also be Barkhausen jumps caused by mechanical stress / noise. The jumps are small and may be small enough to not be a serious problem.
The mechanical stress can limit the symmetry and this way cause more leakage inductance in a toroidal transformer than one may think.

[DeltaSigmaD]

Thanks for the information on mechanical stress. That's one source for possible external influences more to be considered.

A damping resistor parallel to the inductor leads to a filter transfer function which has a bandpass contribution. The load step response shows therefore strong oscillations or, with increased damping, the filter order degrades to 1st order. The filter topology shown in the picture can have oscillation-free step response by choosing e.g. critical damping characteristic.

[MegaVolt]

I'll bring up the topic.

A standard voltage reference noise meter contains: series capacitor - 1-5 kΩ resistor per ground -> amplifier.
And we trust this RC circuit to separate AC from DC. Those. on R only alternating current on C only direct current.
Let's swap them. Let's put R first and C on the ground. And we will take the signal from C. It should have much less noise.
If that doesn't work then the noise measurement circuit shouldn't work either
What am I missing?

[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.