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.
![](https://i.imgur.com/4Mh2H5A.png)
I also simulated @macaba's topology, and it has similar noise results, about 500nVpp form 0.1-10Hz.
![](https://i.imgur.com/qpGjZPn.png)
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!
![](https://i.imgur.com/lkkTktF.png)
![](https://i.imgur.com/c2BiBgq.png)
![](https://i.imgur.com/AN8Do32.jpeg)
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!