Measuring nanoamps and below like a Ninja

[SilverSolder]

Hopefully the sun will shine again one day, but right now the skies are very dark. 

Three steps forward and two back,  is the history of the world.

[Vgkid]

Looking forward to seeing this in use.

[SilverSolder]

Vgkid, device will be completed in next month... _{If world not be destroyed in this period...}

Haha better work fast, the world is powered by misdirected testosterone at the moment!

[MacIntoshCZ]

Sanctions makes russia stronger  ...
Good luck

[macaba]

Watching with interest

Couple of things:
1. If you have a chart showing step response of all the capacitors you tested, zoomed into the first few seconds, I would like to see it. It is an interesting compromise between the physical size (Teflon > polypropylene > C0G) and DA.
2. I prototyped an integrating electrometer (like this project but in reverse!) with LMC662 and about 10pF of parallel AVX MLO capacitors. There is a PDF that shows these have extremely low DA, and I could see this on the output of the electrometer circuit - after a step change of 2V, it settled to less than near-zero fA of leakage in 10-20 minutes, much faster than the C0G caps I tested. Might be an interesting capacitor to try for the main current-generating capacitor.

[macaba]

I'm planned use 0.01pF to 10pF capacitors with air or nitrogen dielectric, to generate current. They must be extremely low DA.
About 100pF and 1000pF i'm not sure... maybe its be K71-7 or K72P-6 or some else...

It is understandable that you want to use air or nitrogen, I am looking at the same thing myself, maybe using 2 PCBs as parallel plates capacitor (with guarding, and low DA Rogers 4350B PCB laminate) but the uncertainty due to thermal/moisture expansion probably makes that a bad idea.
It still might be worth trying AVX MLO in your prototype as it is cheap & the DA seems extremely low (not as low as air/nitrogen... but no manufacturing required!). There is also possibly a volume advantage; small 0603 is not going to capture as many cosmic rays!

What is your air/nitrogen capacitor plan?

[macaba]

Near-zero because I don't have the ability to measure it any better! 

[TimFox]

Are you concerned about noise injected into the power supply from the neon bulb?

[TimFox]

Of course, a neon bulb run on 120 V or 240 V rms needs a large series resistor, but the current waveform is "rough", certainly when compared with an incandescent bulb or LED, due to the interesting behavior of the glow discharge.

[mawyatt]

Gas ionization is a good source of broadband noise, even with a large series resistor the distributed capacitance will charge up to the ionization voltage and very quickly discharge into the tube. This will produce a very quick and sharp current peak/spike rich in spectral content.

Edit: Instead of a neon bulb why not use an LED (maybe multiple LEDs) on one, or more of the DC outputs, just use the series bleed resistor(s) already in place.

Why such large capacitance on the transformer secondaries? This isn't necessary and even increases power losses and these large capacitors have a large effective inductance which hinders their ability to provide a low impedance shunt at higher frequencies where they help reduce noise feedthru.

BTW, those custom toroidal transformers look nice!!

Best,

[mawyatt]

And what test result would that be??

Best,

[mawyatt]

What type of "noise performance" tests did you conduct?

I'm curious since I can't recall OEMs using such large capacitance values directly on the secondary of mains transformers which are used for linear type operation (read not SMPS), at least I haven't seen this on the equipment used here in US and may have missed something!!

Best,

[Kleinstein]

The capacirtance on the secondary can serve 3 purposes:
1) They limit the speed on how fast the voltage changes and this way limit / avoid reverse recovery spikes at the rectifiers. This may need quite some capacitance if the transformer has tight coupling / low series impedance.
2) Together with the transformers parasitic series inductances they act as higher frequency fitlering for EMI from the grid.
3) The capacitors can provode some of the magnetizing current for the transformer. This can lower the no load current and actually redue the losses a little - though not much as the resistive loss is only a small part of the no load loss. Transformed back to the primary side the 1 µF at a 25 V winding is only a little more than 10 nF.  The question is more why use so large capacitors for C1 and C2 - here I would consider less (e.g. 100 nF), so that the circuit is safe even when the bleeder resistor fails and the link to the transformer is lost.

The capacitors on the secondary are not standard, as they add costs. This is more like volt nut terotory where low costs don't matter that much.
One finds such capacitors sometimes in audio equipment, not just audio-foolery.

[mawyatt]

I've never seen a 4.7uF directly on the secondary of a transformer for linear type use, usually much smaller like 100~470nF. As OP just stated added series resistance per Horowitz does make some sense as this gives a known impedance for the shunt cap to work into when acting as a HF bypass shunt, and also provides a startup surge impedance for the transformer, rectifiers and filter caps and must be sized for such.

Best,

[macaba]

Yes I'm understand, but 10-20m to "near-zero fA" may increased to hours with "near-zero aA".
Anyway... when my smugglers buy some sanctioned toys for me, i try get a couple MLO caps for testing.

I attempted to measure it as best I could by using the integrator circuit itself. After 0.6V step change, it is 1fA after 5 minutes, 100aA after 1 hour, 10aA after 3 hours.

More detail here

[Kleinstein]

The residual settling after the step still looks quite significant. 100 aA is not a large current, but the charge transferred over 1 hous is still not that small compared to the initial jump.  It is still impressive, but a low DA capacitor should be way better than that. So chances are the observed settling is more something other than the the capacitor, e.g. the PCB, case of the LMC662.  Epoxy and the usual FR4 PCB material is more like poor DA performance.

[macaba]

chances are the observed settling is more something other than the the capacitor, e.g. the PCB, case of the LMC662.

I agree. I used air wiring on the input pin & consideration of cleanliness during construction, so this could be package related (and probably not contamination or FR4).

[Echo88]

Were there ever leakage measurements done on MEMS switches (ADGM1004 for example), just to get a rough estimate in reality compared to the usually conservative leakage spec?
Are there any other MEMS switches that are capable of DC-range with interesting specs compared to normal analog switches?

https://www.analog.com/media/en/technical-documentation/data-sheets/adgm1004.pdf

[TimFox]

I couldn't read the dial:  what temperature did you set the oven to?

[TimFox]

An amusing coincidence for us in the Fahrenheit domain is that Sn50-Pb50 alloy melts at 212^o C.

[tggzzz]

Were there ever leakage measurements done on MEMS switches (ADGM1004 for example), just to get a rough estimate in reality compared to the usually conservative leakage spec?
Are there any other MEMS switches that are capable of DC-range with interesting specs compared to normal analog switches?

https://www.analog.com/media/en/technical-documentation/data-sheets/adgm1004.pdf

https://menlomicro.com/products/signal-relays

0.5mm pitch BGA package, claims 25pA at 150V, 90fF, both typical, note hot switching and floating limitations.

[HighVoltage]

You find the most interesting stuff
What was the purpose for these vacuum capacitors, if you know?
At +/- 5% they are most likely not for reference.

[bsw_m]

10pF and 100pF air capacitors with sapphire insulators that used in differentiator of a EK1-6 and NK4-1 low current calibrators.
A 10pF capacitor is used for ranges down to 1E-13A.
A 1pF and 0.1pF capacitors are used for ranges down to 1E-17A (sorry no photo).

[Kleinstein]

The dielectric absorption makes the capacitance to go down when the frequency goes up.
The measurement at 50Hz is somewhat faster than the use with the ramp.

So part of the difference can be due to the DA of the PS cap. The usual tabulated amount of DA for PS is at some 0.05%, which may be a coincidence because of the different time scales and not all PS caps are the same.

[Kleinstein]

0.05 % accuracy for 1 nA looks already quite good. This is especialy knowing that there a known effect to cause a bias in that direction.