Noise cancellation circuits for voltage references

[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

[MasterT]

HPF is 1-st order.  I think, that 2-nd order Sallen Key LPF build around ada4523 would be more efficient

[n_haku]

Some time ago, I run into clever idea of reducing voltage regulator' noise by introduce a resistor in series on whitch noise current can be short to ground with transconductance amplifier. Probably, this idea also will work for ref buffer, especially if provide feedback after that resistor to dc accuracy.

[Kleinstein]

Some time ago, I run into clever idea of reducing voltage regulator' noise by introduce a resistor in series on whitch noise current can be short to ground with transconductance amplifier. Probably, this idea also will work for ref buffer, especially if provide feedback after that resistor to dc accuracy.

It is a nice idea for higher freuquency noise of a voltage regulator, but I am afraid it would not work for a reference:
The main problem is the DC accuracy. The series resistor and noise compensation current would ruin the DC accuracy, unless one would use a large capacitor to couple the correction current. Than one is essentially back at a more classical low pass filter.

[David Hess]

Instrumentation and difference amplifiers use this idea to AC couple a differential input without error prone differential AC coupling which would compromise common mode rejection in the transition band.  The single ended output is integrated and fed back into the reference input.

I do not see any advantage when the same components being used for the high pass filter could be used for the low pass filter.  Leakage is still creating low frequency noise, so the capacitor should be bootstrapped for zero volts across it reducing the leakage.

There is a DC accurate alternative where the high pass filter is AC coupled at its input *and* output which removes its contribution to DC offset, but this requires two large filtering capacitors.  Old digital multimeters used this for their DC accurate low pass filtering.

[openloop]

There's a known trick to get rid of leakage and absorption - stacked capacitors. Basically it's two RC lowpass filters one sitting on top of the other with joined input, output from the top one. Thus DC voltage across the top capacitor is essentially zero. So nothing to leak and nothing to absorb.

Thermal things are still a problem though. Mostly from the bottom capacitor.
I think smearing it with thermal grease and sticking into a tight-fitting block of aluminum should slow it down enough.

[Kleinstein]

The filter configuration used for the DMM input or in the Fluke 57xx calibrators needs at least 2 large capactors, but both are used as a 2nd order filter. So this is no extra parts needed just starting with a 2nd order filter and the capacitors are fully used.

The filter part (without active amplifier) is rather low power. So there is little need for extra thermal past or similar. Just some thermal insulation and may be additional thermal ballast would be enough to get a reasonable stable temperature. Temperature variations slower than the filter transition frequency would not matter that much. It would be mainly about filtering the faster thermal fluctuations.

Instead of using a lot of effort on a filter, there is also the option to use a second refrence and average.

If reference noise is a problem, one would in most case record data over a longer time and average - this way the DMM is doing much of the filtering. The integrating type conversion filters out most the higher frequencies above some 50/60 Hz and avearging over multiple readings filters from one over twiche the avearging time to half the reading rate. So much of the reference noise would be filtered out by the DMM. There can be a small frequency band from the auto zero mode and only integrating the input half of the time (most DMMs with multislope ADC) for reference noise passing. That would be something around 2.5 Hz for 10 PLC mode or 25 Hz for 1 PLC conversion mode. The analog filter could make some sense for that frequency window. Especially when using 1 PLC mode the cross over does not have to that low. The rest of the filtering will be done by the DMM digitally. So I see relatively little need to get a filter cross over of much below 1 Hz.

[tggzzz]

Wouldn't help much with any of a reference's popcorn noise

[macaba]

I have tried simulating a few different non-conventional filters, and the main problem comes down to exactly what Dieter said - "Leakage, dielectric absorption and temperature dependence of the capacitor(s)."

So any complex circuit (more complex than passive RC) needs to address this issue (IMO).

I attach 2 circuit ideas here as food for thought. Both of them will filter, but both of them are not good enough (IMO) for DC precision (i.e. stability over temperature) that would allow for voltage reference filtering.

Circuit 1: Directly compensating for the leakage with an error amplifier
Circuit 2: Reducing the voltage drop over the critical capacitor(s) to nearly 0V.

[MasterT]

Using supercapacitors to filter LF noise is not a panacea, 66uA leakage current is roughly translates into 66 mV (!) of noise over 1 kOhm resistor.
 I recently discovered that Nichicon has UKL - Low Leakage Current series product. About 10 times lower current compare to normal

[David Hess]

Filtering is useful for midband noise, and high frequency noise is easy to filter out, but it may be better to use multiple references in parallel to reduce low frequency and flicker noise.

[Kleinstein]

Using supercapacitors to filter LF noise is not a panacea, 66uA leakage current is roughly translates into 66 mV (!) of noise over 1 kOhm resistor.
 I recently discovered that Nichicon has UKL - Low Leakage Current series product. About 10 times lower current compare to normal

Leakage current times series resistor does not directly translate to noise. Primarily this gives an offset, which for a reference is even worse.
The leakage current has however a good chance to be also accompanied with noise as the leakage current may have instabilities (current noise) and possibly shot noise.

The circuit from macaba looks a bit suspicious: the extra correction amplifier should also effect the cross over frequency and may effectively reduce the resistor by the gain.

[macaba]

The circuit from macaba looks a bit suspicious: the extra correction amplifier should also effect the cross over frequency and may effectively reduce the resistor by the gain.

https://doi.org/10.1063/1.4870248 has more information.

[macaba]

Circuit 3: Error sensing

Using the error sense output; adjust the input DAC value within very slow digital control loop (slower than the RC constant) to compensate.

[dietert1]

One could make R2 somewhat bigger to amplify the error signal and simplify digitizing it. You get a noise measurement for free.
Probably i will try a 7 to 10V gain stage that is a Sallen-Key low pass filter at the same time, like shown in PDF.
In a drawer i found Vishay MKP1848 40 uF/900V and Kemet C4DE 380uF/400V capacitors. Both tested with less than 50 pA leakage at 20V. MKP power caps seem to be a reasonable choice for this application.

Regards, DIeter

[miro123]

I have been playing with Macaba Circuit #1. While it filter high and mid-low frequency it introduces utralow frequency that creates even bigger headache.

[MasterT]

I have been playing with Macaba Circuit #1. While it filter high and mid-low frequency it introduces utralow frequency that creates even bigger headache.

My experience similar. Circuits is dated back to 1990,
https://www.ti.com/lit/an/sbva002/sbva002.pdf?ts=1669056577129&ref_url=https%253A%252F%252Fwww.google.com%252F
I observed a "bump" on the spectrum around cut-off frequency

Using supercapacitors to filter LF noise is not a panacea, 66uA leakage current is roughly translates into 66 mV (!) of noise over 1 kOhm resistor.
 I recently discovered that Nichicon has UKL - Low Leakage Current series product. About 10 times lower current compare to normal

Leakage current times series resistor does not directly translate to noise. Primarily this gives an offset, which for a reference is even worse.
The leakage current has however a good chance to be also accompanied with noise as the leakage current may have instabilities (current noise) and possibly shot noise.

Not directly, I say roughly. The point is, if AC component of the leakage current is only 1% - flicker noise, than it still 0.66 mVrms - about x30 times higher than noise level presented from voltage reference itself, 20uVrms or so.
 It is good practise to assume that AC may reach 100%

[KT88]

The challenge with very low frequency (analog-) filters is capacitor leakage obviously.
I played a bit with leakage mitigation strategies. Here is an approach with a simple Sallen-Key filter extended with a little servo that keeps the DC voltage across C1 at near zero. Note that the servo amplifier itself needs a second buffer to mitigate leakage through it's capacitors. I haven't completely optimized the frequecy response but it demonstrates the principle.
I chose Rp values from an AVX data sheet to make it realistic. Of course other effects like temperature, mechanical stress is not modled.
However the servo would cancel slow changes of Rp of C9, C6, C5 and C3. C1 and C2 are not exposed to any DC voltage more that a few uVolts...

Cheers

Andreas

[maat]

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).

I can only second that. I have been there and tried this and that until I finally gave up, did the maths and realized, that one either relies on the stability of some resistor (or capacitor) ratios or the stability of a filtering capacitor, which is a fallacy, but let me explain.

While the white noise component of a reference can be filtered fairly well, once you get into the 1/f territory you will hit a brick wall. To understand this, one needs to look at the spectral properties of 1/f noise. i have attached a little simulation, that shows the 1/f noise in the time domain, its PSD and the Allan deviation (I will get to that latter).

The time domain plot and the PSD are likely the plots people are most familiar with. The PSD already shows what it happening. As you go to lower frequencies, the noise power increases linearly with f (no sh** Sherlock). At the same time (pun intended) the settling time of any filter goes with 1/f as well.  So you will always have the same noise power at your output. Simply speaking, the filter will never settle. This brings me to the adev plot. I beautifully sums it all up. No matter how long I wait, the deviation will be the same.

Filtering at frequencies below the corner frequency makes matters worse. You will have have the same noise power and on top that, there is no filter apart from maths, that has the theoretical transfer function, i.e. is flat from DC to the corner frequency and then rolls off. Every filter introduces its own 1/f noise a low frequencies. So the harder you try, the worse you will make it. This is simple physics or maths.

So is there a way to cheat fate? Yes and no. 1/f noise has an Achilles' heel. It is correlated. So in other words, it does give you some information about its future. This can be used by a predictive filter like a Kalman filter (https://en.wikipedia.org/wiki/Kalman_filter) to suppress is. The catch? - Its all digital.

The only analogue thing you can do, is to sum several references. This will get the noise floor down by 1/sqrt(N), if the noise is uncorrelated. This is important to keep in mind, because as you start summing references you will invariably introduce correlation (like thermal EMF on the board, or supply ripple, etc.), so you will hit a dead end there as well. Life is a bitch...

Basically, the only way to get rid of 1/f noise is to not introduce it in the first place. If you want low noise pick the lowest noise Zeners (by hand) and sum like 4 of them. Then call it a day. It's all mother nature will give you. Ever!

Addendum: The stuff one adds usually has some temperature dependence, like caps, which results in a random-walk behaviour. A random walk is 1/f². So things will get out of hand pretty quickly.

[KT88]

On the issue of settling time it is possible to use smaller resitors to charge the caps and let the DA settle (for still a long time though). These smaller resistors could be switched on and off with analog switches or relays.
Marcoreps used large higher voltage electrolytics in his LNA. A lot of them may give good (enough) results with leakage mitigation...

[maat]

On the issue of settling time it is possible to use smaller resitors to charge the caps and let the DA settle (for still a long time though). These smaller resistors could be switched on and off with analog switches or relays.
Marcoreps used large higher voltage electrolytics in his LNA. A lot of them may give good (enough) results with leakage mitigation...

Unfortunately, no.
Let's keep it simple and neglect pesky physical properties for a second. Assume you can sample the voltage perfectly and transfer that to the ideal capacitor of the filter. Now this "measured" value will have an uncertainty. If you would like to improve upon that uncertainty by filtering (or one may say integrating over) the source, this is where the trouble starts. This is what the adev plot is showing, that for all integration periods, the uncertainty will stay the same. You will never be the wiser. The Allan deviation does not make an assumption over the starting point of integration phase. So no matter how "well" defined your starting point may be, its uncertainty will not improve, no matter what type of filter or cut-off frequency you use. Digital or analogue - it won't help you.

There is no cure to 1/f noise.