Electronics > Metrology

DC-accurate Low-pass-filter

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Kleinstein:
The inductors get better (less resistance) the larger they get. The chokes are usually for a certain DC current, for the ref. filter one could use a core without a gap as there is essentially no DC current. A mains transformer primary or an evenen higher voltage winding could be a first start. No need for really special material for low frequency, but a hi µ material (e.g. grain oriented iron in a toroid or Nonoperm, maybe metglass) can help to get high inductance.

CurtisSeizert:

--- Quote from: Roehrenonkel on July 12, 2024, 11:49:24 am ---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.

--- End quote ---

When I was working on an LNA with a high input impedance I tested a number of caps for leakage current and DA. For film caps, the major mechanism of apparent leakage is DA based on a reference cited in AOE3 that measured over a year. My measurement setup just used two points spaced a couple hours apart (the ones here were 2.4 to 4 h except the PTFE, which was 8.75 h) and assumed linearity because I was measuring a lot of samples, the FOM I was looking at was self discharge time constant, which I recorded in ks. Here are a couple highlights:

C0G 470nF (Murata): 162 ks
PPS 470nF: 203 ks
PE 6.8uF (WIMA MKS2): 246 ks
PTFE 470nF (Soviet surplus): 39400 ks
PP 1uF (Panasonic): 3840 ks
PP 4.7uF (Panasonic): 6850 ks
PP 12uF (Kemet): 14400 ks

I did not do a ton of measurements with PE because PP was so much better, but with PP, the time between points and whether a sample was pre-charged for 24 hours prior to the measurement were big factors, which is consistent with most of the apparent leakage being a result of DA.

That said, I have successfully used bootstrapped SMD tantalum caps for voltage reference filtering down to a breakpoint of 2.7 Hz (single pole). In the case I tested most thoroughly with an ADR1000 reference, the concern was more TC than absolute accuracy, and settling time was not a major concern. The higher voltage cap was biased with about 10 mV because the R of the filter was part of a divider, and I wanted a bit of margin on the bootstrap node to ensure there was a positive bias on that cap. The ratio TC I got before and after the filter was negligible, i.e., <10 ppb/K. I have generally used this in preference to RC LP filters using Rubycon multilayer film caps, which you can get in fairly large sizes as SMD components (up to 22 uF I think) because it's cheaper and the solution size is less than a single 2220 cap. I like using fairly small (i.e., 3k3 or lower) R values because of the current noise characteristics of my preferred buffer amp, the OPA2205. Also, if I am going to this trouble, I use the standard RC filtered buffer amp topology at the output with something like 10R/47uF and appropriate feedback R and C values to maintain stability. I attached a screenshot of a schematic where I implemented this sort of thing for providing a 5V reference to an AD4030-24.

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