Electronics > Metrology

DC-accurate Low-pass-filter

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Roehrenonkel:
Hi voltnuts,
 
for my voltage-reference(s) i'd like to add a DC-accurate Low-pass-filter. (See PDF)
Okay, we have to deal with the OP-Amp-offset-voltage, i know.

Here we have the standard Sallen-Key Butterworth 2nd order LP
(fc10Hz) modified to cancel the leakage-current through C2.
Step-response is 0.2 seconds.

Meassured leakage of foil-caps is 100...200 pA at 45 to 50 Volts
that result in 225...500 Gigaohms.
And there are my precious ppms escaping to ground. 8-O
In the modified LP there is no DC over C1 and C2 anyway, nice. :-)


What do you think?
Is this a good idea, or did i overlook something?
Should i make a 1Hz- or even 100mHz-Version?

Best regards

macaba:
Have you investigated the attached topology? I use this in practical DC accurate circuits a lot.
Attached image shows a 3rd order response, it can be extended to further orders but my experimentation shows it tends to be better to chain multiple 3rd order stages instead.

trtr6842:
I have been working on a DC accurate low-pass filter too!  I actually just got my prototype boards made, but I have yet to test them.
I got the idea when I was working on my LNAs.  I figured if you can measure the noise on a voltage reference, then you can cancel it out!

Obviously a low-pass filter will not help long term stability, but I do think it could have great value in reducing measurement times, or helping measurements that have to be fast (i.e. scanner cards) be a little less noisy.

The main challenge is making it such that the filter doesn't add more noise than it filters out...
@Roehrenonkel I did a quick simulation of your design, and it appears that it would have a noise floor of approximately 19nV/√Hz.  This would be about 600nVpp in the 0.1-10Hz range, even with a shorted input, which is great for bandgap or LM399 references, but falls short of LTZ/ADR1000 territory.  To reduce that you have to increase all capacitances and reduce all resistances, that's the only way around it.  This simulation even ignores opamp input voltage and current noise too.


I also simulated @macaba's topology, and it has similar noise results, about 500nVpp form 0.1-10Hz.


I attached the schematic for the active filter I'm working on, I decided to call it "PARF" for Precision Active Reference Filter, just because I thought it was kindof funny. 
Based on simulations I think it should get to about 175nVpp of input referred noise, including contributions from the opamps, and it is effectively a single pole 15mHz low-pass filter.
I essentially took my LNA design, modified the input for a 15mHz high-pass cutoff, then feed it's inverted output back into the buffered input to cancel out noise.  Since the noise is amplified, I can AC couple it back into the DC path using ceramic caps and a voltage divider.  The maximum attenuation depends on how well the gains are matched between the LNA gain and the AC injection divider, but with most voltage references it doesn't take much attenuation before you reach the noise floor of the low-pass filter, so you don't need a lot.  This circuit is definitely a bit over-the-top, and I admit I have no idea if it will have any practical applications, but I'm working on it anyways!




Anyways, I haven't yet powered it on and tested with it, but I'll provide updates when I do!
I'm also well stocked with LNA's to measure the input vs output noise!

Roehrenonkel:
Hi,
 
thank you both for your inputs.
@macaba: "Gyrator" was the first think that came to my mind.
How do you calculate it? Is there more info about it?
One could even use the buffer U1 to amplify 7V-->10V.

@trtr6842: Good job on the LNA, Kudos.
Noise-cancelation only works if you're in time to be 180° out of phase.
Resistor-noise within the LP-filter: Yes these are critical.
22k seems to be the upper limit with regards to noise.
But, i'll definetly will scale the resistors down.
Let's see what else i'll find in my stock:
500* 1u 63V 15mm Pinspacing.
That look's promissing. :-))
Now we have to wait what the SMU says about leakage (See you in a few hours).

I have got over 18.000 Mica-caps, but not ONE Teflon-/PTFE-cap! Someone wants to trade?

Ciao4now

Roehrenonkel:
Hi again,
 
turns out, that distance is king for low-leakage.
The best cap so far with some old Wima MKS-3.
Fast setling in regards to my other samples (ERO 1818 & 1826 / Wima MKS).
Picture shows sample-# and leakage-current, 10 seconds between samples.

Could become a big pcb, but i'm a 19"-guy anyway. ;-)

Ciao4now

Edit: It's an ERO (Emil Roederstein) MKC-cap.

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