Measuring nanoamps and below like a Ninja

[ZhuraYuk]

I think DA means dielectric absorption, but D on LCR meter stands for Dissipation factor. Which is a different thing

[fcb]

The mkp4 seems to be “bad”. DA sb around 0.0002

My 10 all measure similarly.  Also my Russian K71-7 is lower ( but larger ) as you demonstrate.

Randall

How are you measuring DA?  My understanding is this is not a trivial thing to measure and not available on your typical LCR meter?

[TimFox]

There is a mil-spec measurement for dielectric absorption (DA):  a specific sequence of applying a certain voltage, disconnection, waiting, and re-measuring voltage.  It is good as a "figure of merit" to compare different capacitors with each other, but I don't understand how to use that standardized value to predict the effect in a practical circuit, with different voltages and times, since it is a non-linear effect (analogous to ferromagnetism).
"D" = 1/Q is closer to being a linear effect, but depends on frequency.  For non-linear dielectrics, such as Z5U ceramic and electrolytic capacitors, it also varies with voltage.

[TimFox]

The US standard test for DA is similar.
Do you know any method for using that value to predict the error in a different circuit with different timing?

[Kleinstein]

There is a mil-spec measurement for dielectric absorption (DA):  a specific sequence of applying a certain voltage, disconnection, waiting, and re-measuring voltage.  It is good as a "figure of merit" to compare different capacitors with each other, but I don't understand how to use that standardized value to predict the effect in a practical circuit, with different voltages and times, since it is a non-linear effect (analogous to ferromagnetism).
"D" = 1/Q is closer to being a linear effect, but depends on frequency.  For non-linear dielectrics, such as Z5U ceramic and electrolytic capacitors, it also varies with voltage.

With film capacitors the dielectric absorbtion is usually still linear. So more voltage gives more recovered valtage. The DA itself may be nonlinear mainly with electrolytic and class 2 /3 ceramic.

The standard DA test measure the effect for a certain time window (time constands from some 5 seconds to some 3 minutes for the russian version and a little longer for the US version). They still don't seprate it for the different times and a calculation is thus only approximate.

Form the tests I have done so far the PP and PS caps the DA seems to be distributed over a wide range of time constants and usually not just a few discrete ones.  An about even distribution on a log time scale seems to be a reasonable assumption to extra polate to shorter and maybe slightly longer times.

To do a calculation of can use the equivalent model with extra RC elements in parallel, e.g. with 1 or 2 capacitors per decade (in time/frequency). The effect is quite broad in the frequency and thus no need for more elements, as we don't have more specific information anyway. The standard tests covers a little less than 2 decades in the time/frequency. So the size of the extra capacitors would be about 60% times the measured standard DA value for 1 RC element per decade and than sereis resistors 1 decade. Alternative half the size with 2 RC elements per decade. One could than use this equivalent ciruit in a spice simulation of the circuit in question.

[TimFox]

to Kleinstein:
In my career, DA was something to avoid rather than quantify, so I don't have personal experience with the parasitic R-C networks that are often used to discuss DA.
However, in that model, which is linear, DA would not cause any harmonic distortion in the voltage across a practical capacitor driven from a pure-frequency sinusoidal current.
If DA is from ferroelectric hysteresis (presumably true with electrolytics and the crummy ceramic dielectrics), it becomes a non-linear problem.
Respectable audio literature has many examples of careful measurements of audio-frequency THD from capacitors, possibly the result of DA.  One series of measurements showed 0.01% THD on some metallized-polyester devices, where the same measurement on a polystyrene capacitor or a C0G ceramic showed unmeasurable THD.  The same author, M. Blencowe, points out that silver-mica capacitors have mediocre DA but do not show THD in audio tests.  I remember reading the General Radio literature on the choice between polystyrene and mica in standard capacitors and decade boxes, where the micas show an increase in apparent capacitance at low frequencies, which GR attributed to "interfacial polarization" at the mica-silver interface.
Before retirement, my main concern with DA involved the hold capacitor in sample-hold circuits.  Lacking a good quantitative handle on the problem, we started with polystyrene back in the early 1980s with TH boards, later changing to polypropylene, but had to settle for C0G ceramic in the later SMT boards.  (We could not justify using PTFE.)

[Mickle T.]

GOST 28885, DIN 41328-4 etc are useful for capacitors selection, but almost useless for any other thing, like precision circuits design. However, there are a large number of publications (mainly by American and Soviet authors) that describe other methods for modeling dielectric absorption and measuring the parameters of its approximation models.

[TimFox]

Thanks for the references.  I quickly read the third one with its methodology for estimating the parasitic R-C networks, but it was confined to a linear model (since it was based on vector network analysis).  I still am interested in the effect of hysteresis.

[Kleinstein]

I looked at low DA capacitors for my multi-slope ADC design. There  I tested  polystyrene, polypropylene and different C0G versions. Found these I found the lowest DA with TDK brand C0G caps, while the other C0Gs I tested were similar the polystyrene and PP.  The good C0Gs were about 1/5 the DA of PS or PP capacitors. So the need to change to SMD may not be so bad.  I did most of the testing with a shorter time scale (e.g. 1-40 ms ) like relevant for the ADC. So things may be a bit different in the more standard tests.

With a modern and thus relatively fast reading DMM/electrometer one can do the initial test also at a shorter time scale.  The faster test is less sensitive to voltmeter input bias current and is thus more practical also for small capacitors.
If one shortens the discharge time one can get data for quite some time range from a single run. The expected curve for a longer discharge time can be calculated quite easy: just take the voltage at that time as "Zero".

A consequence of the DA is that the capacitance goes up for lower frequencies, as more of those RC elements in the equivalent circuit contribute to the capacitance and not only to the losses. The loss and frequency dendence of the capacitance are linked, by the Kramers Kronig relation. The DA can be the main part of the loss in a capacitor. So capacitors with low dissipation factor are also low DA. The differences is mainly in the frequency range. D is often given for 1 MHz or 1 kHz, while the DA is more like 1 mHz range.

[TimFox]

The exception to "capacitors with low dissipation factor are also low DA" seems to be silver-mica, which has low D but highish DA.

[mawyatt]

Doesn't Keithley use ceramic capacitors in their latest DMMs?

Best,

[Kleinstein]

Essentially all the newer MS- ADCs in the modern DMMs (e.g. from K2000 or 34401/3458 on) use ceramic (likely C0G) capacitors. The ones I use are about 1/5 the DA of PP caps and AFAIK were some 10 cents each. So nothing expensive, one just needs to know which ones are good and which ones are not (about the same as PP and should still be good enough for the HP/KS meters). The HP3458 has a PTFE cap for the S&H stage, but not sure of they really need it there - the coax form may have additional effects and could be more than just a capacitance.

I don't know how good the higher capacitance G0Gs are - they may still be an alternative with 100 nF available in relatively small size (e.g. 1206 and small THT).

[mawyatt]

Essentially all the newer MS- ADCs in the modern DMMs (e.g. from K2000 or 34401/3458 on) use ceramic (likely C0G) capacitors. The ones I use are about 1/5 the DA of PP caps and AFAIK were some 10 cents each. So nothing expensive, one just needs to know which ones are good and which ones are not (about the same as PP and should still be good enough for the HP/KS meters). The HP3458 has a PTFE cap for the S&H stage, but not sure of they really need it there - the coax form may have additional effects and could be more than just a capacitance.

I don't know how good the higher capacitance G0Gs are - they may still be an alternative with 100 nF available in relatively small size (e.g. 1206 and small THT).

Before retiring a few years ago, we used mostly TDK and AVX ceramic chip capacitors, one of our concerns were stress induced effects (vibration sensitivity) as well as very low ESR and ESL. These brands were "discovered" way back in 80~90s, and they seemed to still be the best overall for our use much later. Samsung may be another brand with good overall characteristics?

Some of our work involved telemetry from an active shell in flight which induced well over 50,000Gs when fired

Best,

[Kleinstein]

Measuring the step response is a good idea. It should get a little better (less noise from the DMM / source) when looking at a step down from some 10 or 20 V back to zero.

[KT88]

It would be interesting to see how a vacuum capacitor behaves...

[TimFox]

Is the noise from your large Teflon capacitors due to their physical size (presumably pickup from the environment)?

[Kleinstein]

I did a quick (did not take the 4 hous discharge to serious - it probably was) DA test on a relatively cheap epoxy dipped 1 µF PP caps from Panasonic. It turned out a bit higher in DA with around 0.07% (5 sec. to 3min like russian standard). So not the best choice if low DA is needed.

The higher noise with the large PTFE cap is likely due to hum pick-up and maybe other capacitive coupling.

The noise seen for the PP cap is already at about the level expected for LM399 references.

Settling to 1 ppm after about 15 seconds is still not that bad. With the slope test the excitation would be a bit less than a full jump.

[SilverSolder]

I never realized that blue electrical tape outperforms the black version - good to know it is worth a ppm or two! 

[TimFox]

In high-impedance circuits, I used to use the polyethylene tape that is sold for sealing the caps on reagant chemical bottles.
Unfortunately, Cole-Parmer only stocks this in expensive case lots:  https://www.coleparmer.com/p/polyethylene-tape/4130

[Kleinstein]

Lowerring the impedance makes some sense, as long as the capacitors don't get extraordinary large. However I see little effect of the DA if the cross over frequency is kept constant. The effect of DA gets less important (faster settling) if the cross over frequency goes up. It depends on the update frequency what cross over frequency is actually needed. The initial plan 1 Hz were quite slow. the 2.8 Hz from the next step look more reasonable and one may be able to go a little higher, like 10 Hz. However I am afraid much higher may not give enough attenuation of the DAC steps for very slow ramps.

The ideal filter frequency may depend on the ramp speed, as faster ramps would need faster settling, but less critical filtering, while slow ramps would also need more filtering, but have plenty time for settling.  So one could consider a relay to swich 2 speeds.
Another option would be to generate the very small ramps with a divider after the filter.

I may be worth looking at the filter like used in the Fluke and Datron PWM based DACs in the calibrators. They can get higher order (e.g. 5 th order) fitlers with only 1 DC critical OP and they may only need 2 critical capacitors. The other capacitors in these filter type would not see much DC voltage and could be less critial with DA. The other OPs in the circuit are less critical and could be normal +-15 V supply.

[KT88]

Bob pease once published a compensation circuit for DA. Unfortunately I wasn't able to find it anymore - maby someone recalls that RAP stuff anyhow....

[Echo88]

Couldnt find the exact RAP-document, but heres the circuit mentioned by KT88:
https://electronics.stackexchange.com/questions/289312/how-do-i-find-capacitors-for-low-dielectric-absorption

[MegaVolt]

Bob pease once published a compensation circuit for DA. Unfortunately I wasn't able to find it anymore - maby someone recalls that RAP stuff anyhow....

[KT88]

But we are in to digital era, wasn't it?

Yeah, digital capacitors without DA - what is the DigiKey order-No? 

[SilverSolder]

But we are in to digital era, wasn't it?

Yeah, digital capacitors without DA - what is the DigiKey order-No? 

You could run a model of the RAP compensation circuit in software, instead of realizing it as a physical circuit?   Yay, love digital!