PROJECT: Micro-Voltmeter Design

[dietert1]

This is the most basic linear extrapolation: 10 nA / 70 000 mV * 1 mV = 0,14 pA. Due to cascade effects that exist at 70 V but not at 1 mV it will probably be even lower than that. Yet as far as i understand you have a setup ready to check whether leakage stays below 1 pA at elevated temperature. Much better than discussing the datasheet.

Regards, Dieter

[Lesolee]

This is the most basic linear extrapolation: 10 nA / 70 000 mV * 1 mV = 0,14 pA.

I am confident that the supplier has more expertise in this field than me. Of the 3 data sheets to hand, only Diodes Inc give measured leakage data with voltage. Over the range from 70 V down to about 5 V the leakage current drops by a factor of perhaps 2. The linear extrapolation method predicts a factor of 14 reduction to 5 V whereas the data sheet says a factor of 2.

I think you are being very 'brave' in your estimation of the current from 1 data point. On the other hand I am saying to use a part with no spec at all! I am working on the basis of an area factor. A smaller junction should give a pro-rata lower leakage compared to a similar junction of larger size. Therefore a smaller collector current device should theoretically give a lower leakage. Such transistors do exist, and I would have bought one, but they come in reels of thousands from distribution -- which exceeds my interest.

There is always a problem with extrapolating manufacturer’s beyond what they are willing to state. They are, after all, the experts. If they were confident that such extrapolation was reasonable, I am sure they would be keen to publish it, even as a typical value. There then comes a question of whether the designer is being clever or unreasonable.

As long as you get away with it you can consider yourself clever. If it fails, then you go from hero to zero overnight.  

[dietert1]

The Diodes Inc curves have a knee at 7 V, where they state about 550 pA typ at 85 °C. From then on leakage appears to drop exponentially, roughly a factor 2 for a 20 % of voltage drop. Again those are typical values that one would measure when looking at a few pieces like you do. But OK, we can stop this until you have a BAV199. Then you can can contribute something.

I'd like to recommend once more having a look at the Fluke 845A input circuitry to learn where to place the protection diodes and where not to place them. In that circuit the protection diodes never see more than some 10 or 20 nV. As far as i understand then you can use almost any diode.

Regards, Dieter

[Lesolee]

I'd like to recommend once more having a look at the Fluke 845A input circuitry to learn where to place the protection diodes and where not to place them.

So readers can easily find the relevant schematics, here is a link:
https://xdevs.com/fix/f845ab/

I was surprised that this is claimed to be a "high impedance" null detector, given that on the 1 mV range and below it is evident that there is a 1M resistor directly across the input. Then I found the Issue 3 Errata of 7/93 (on the same site) where they correct the input resistance to 1M on the 1 mV range and below. 

[Kleinstein]

Extrapolating the leakage current to lower voltage is tricky. One can do that with the ideal equations, but the problem with leakage are those parts that don't work as expected and have more leakage. It is very hard to tell how those bad guys behave. For the good ones that follow the typical curve, everything is fine.  One may be able to test at a slightly higher temperature and hope things get better at lower temperature also for the bad examples.

[Lesolee]

In doing some full range testing on the µVM  I noticed that the settling time was unreasonably long. It is ok at fractions below a tenth of full range, but still not ideal. The obvious culprit is the 2µ2 polyester capacitor across the 100K feedback resistor. I hoped it would be good enough on dielectric absorption, but with 5 digits resolution it is noticeably non-ideal. 

So I made up a dielectric test set today, which was far less straightforward than I would have expected. The LM555 timers just like internally reseting themselves when they switch the relay loads. 



Anyway it is working now, so here is a taster of tomorrow’s testing. I have 4 caps to test. First is a 2µ2 ceramic as the rubbish one to get started. Then there is the existing polyester, a new PPS capacitor, and a new PP X2 beast. Details tomorrow.



Rather than the pure integrator approach, I am continuously discharging the capacitor via 1M. This makes the amplifier easier (its just a scope input). It also more readily shows the shape of the dielectric effect. Just a peak value does not tell you the time constants involved, so this test should be more discriminating against long time constant absorption.



I just checked, and a 20 minute charge does give a slightly higher peak (83.6 mV compared to 81.3mV) but I am hoping for larger changes between the capacitors than that.

[CDN_Torsten]

You may want to place a diode across the relay coil to 'absorb' the energy of the collapsing magnetic field...

[2N3055]

Bipolar 555 needs good decoupling on Vcc. Very unstable otherwise. Try 100nF ceramic and 10uF elco.. And yes, a blocking diode on relay like Torsten said..

[splin]

Alex Nikitin measured the leakage of various diodes in this thread:

https://www.eevblog.com/forum/metrology/measuring-nanoamps-and-below-like-a-ninja/msg1114374/#msg1114374

He measured various BAV199s - see reply #64 - but didn't specify the manufacturers. He hinted at the best, TI and NXP in another post however:

https://www.eevblog.com/forum/metrology/diy-cable-for-picoammeter/msg2798298/#msg2798298

Also have a look at this thread:

https://www.eevblog.com/forum/projects/forward-leakage-of-a-diode/

[David Hess]

If you want an inexpensive but tested low leakage diode, then the 10 picoamp 2N4117 and the 1 picoamp 2N4117A low leakage JFETs are the best option.

Probably the price went up when you weren't looking.

£11.94 each (from Mouser.co.uk) is not inexpensive 

The leakage is also only specified at room temperature. I think this sort of part is no longer popular, and they are being discontinued, or re-manufactured at a significant premium.

As Kleinstein points out, get the plastic packaged equivalents.  Mouser sells Fairchild MMBF4117s in the SOT23 case for 47 cents each and at 20 volts, they are 10 picoamps at 25C and 25 nanoamps at 150C.  The TO-92 version is $2.42.

I have never had problems extrapolating leakage using the doubles every 10C rule.

[Lesolee]

You may want to place a diode across the relay coil to 'absorb' the energy of the collapsing magnetic field...

It would have been easy to miss, but the relays are bistable (latching) single coil types. The coil polarity is reversed in normal use, so a flywheel diode is not possible across the coil.

[Lesolee]

Bipolar 555 needs good decoupling on Vcc. Very unstable otherwise. Try 100nF ceramic and 10uF elco..

100 nF ceramic directly between the power pins 1 and 8 was one of the first things I tried (unsuccesfully). 

[2N3055]

Bipolar 555 needs good decoupling on Vcc. Very unstable otherwise. Try 100nF ceramic and 10uF elco..

100 nF ceramic directly between the power pins 1 and 8 was one of the first things I tried (unsuccesfully). 

I presumed you did, but try adding elco too.. Spikes switching inductive load are large. 555 itself have large switching spikes even without load on output..
Problem is that it has common ground for high current path (output) and logic. When you pull a current spike it's ground ref level starts "swimming" a bit.
Make sure you have low impedance grounding. 555 is sometimes finicky.
As for diodes, fair enough, I didn't see they were bistable relays. But you can still put diodes, except you need 4 of them. One diode from negative to 555 output and one from output to positive supply, on both 555. 555 has push-pull output, so one diode across each transistor. Basically what you would do on an H-bridge (which is what you have here).
Regards,

[Kleinstein]

The test setup for DA looks odd, with quite a few points that can cause trouble.
Unless one needs to test hundreds of capacitors there is no real need to automate the test. Just using manual switches should be good enough, at least for the slow DA that is of interest here.

Unless one has a really high impedance scope (e.g. more like GOhms input impedance), it is problematic to watch the recovery voltage with the scope at the time. It may still work for the large and high DA polyester cap though. The recovery is slow and the more suitable instrument would be a high impedance voltmeter.  Even then on could consider to disconnect it at time were no reading is needed.
It may be at least necessary to connect the meter only after the cap is shorted, so there would be no recovery from inside the meter.

The only point that may want some automation would be the discharge time, especially if shorter times are of interest too. So this would be a relay (no need to go bistable) to close some defined 1-10 seconds or so.

[Lesolee]

As for diodes, fair enough, I didn't see they were bistable relays. But you can still put diodes, except you need 4 of them. One diode from negative to 555 output and one from output to positive supply, on both 555. 555 has push-pull output, so one diode across each transistor.

The thing is, the push-pull output will absorb the flyback. I don't think the output exceeds the rails (but I will recheck that). In  this case the catch diodes won't be able to catch anything.

To be fair, I just knocked the circuit up yesterday, without too much care. It was supposed to be a quick test. Probably I should rewire SW1 (which is actually DPDT) so I can transfer the relay currents back to the individual driver LM555s. That will route the flyback currents more correctly.

[Lesolee]

The test setup for DA looks odd, with quite a few points that can cause trouble.
Unless one needs to test hundreds of capacitors there is no real need to automate the test. Just using manual switches should be good enough, at least for the slow DA that is of interest here.
...
The only point that may want some automation would be the discharge time, especially if shorter times are of interest too.

Not odd. Novel! It means anyone can easily measure DA without needing expensive electrometers. An improvement.

It also differentitaes between slow DA and VERY slow DA very easily, which is potentially useful (depending on the outcome of the testing).

I could not switch a switch for 1 second reliably. And now I have turned it down to 500 ms.

So this would be a relay (no need to go bistable) to close some defined 1-10 seconds or so.

The relays are bistable because I bought them for another project where the low thermals of a bistable will be critical. I am practicing using them, as I have never designed them in before. I agree they are not necessary here. LS1 could indeed be a mechanical switch, but I still need two monostables.

[Kleinstein]

The way the scope is connected is so that it would need the very high impedance of an electrometer to measure the slower DA. This is especially true for smaller caps.

With 10 M input impedance to the scope (with 10:1 probe) and 1 µF one has a time constant of 10 seconds only. So one would be limited to the rather fast part of the DA. If the scope is connected all the time, one could as well also measure the discharge phase or take the trigger from the start of discharge, which sets the start of the time scale. The lenght of the discharge is not that important, one just also captures more of the faster DA. Instead of defined discharge lenth one can as well take the voltage at the given time as the zero point. So a single captured curve includes DA from different time scales.
With just the scope, for longer times ( t >> R*C) the measured curve changes from a voltage reading to a current reading - still possible, but unusual. One may also run into a sensitivity problem, as scopes are not very good for small DC voltages. The intermediate time (e.g. 1 - 100 seconds) would be tricky to use.

If the meter / scope is permanently connected, one kind of needs a high input impedance. So the part of the circuit to build would be a high impedance buffer / amplifier, no so much automating the switches.

[Lesolee]

The results from the tests:









My scope is pretty down-market at 20 mV/div maximum sensitivity. A normal 2 mV/div scope would give a much more reasonable resolution on the measurement. It is disappointing that the best cap, the PPS, is only 4x better than the ordinary PET capacitor.

[Kleinstein]

Normally PP is supposed to be about 10 times better than PET. However not all the caps are the same and material purity can differ depending on brand and type. The PET capacitor looks relatively good. So the smaller factor of 4 could be due to good PET one, not necessary a poor PP and PPS example.
The difference gets even small for the longer time scales ! It is more like only a factor of 2 after 60 seconds.  Due to the limited time scale it is rather difficult to compare to the normal DS numbers.

[Lesolee]

If the scope is connected all the time, one could as well also measure the discharge phase or take the trigger from the start of discharge, which sets the start of the time scale.

I hope you are not suggesting that the scope measures the full 50 V and then somehow also measures the 8 mV recovery voltage. (?) That is unworkable. The scope wouldn't have the dynamic range, couldn't change range that quickly, or would overload so badly that it would take an indeterminate time to recover.

Probably I have misunderstood what you are proposing there.

The elegance of the present relay configuration is that the scope (or buffer) is never overloaded, so does not have a recovery problem to deal with. There is, however, a weakness with it, in that the impedance to the scope (or buffer) changes from 0R to 1M. That is an unnecessary error source.

... for example, I could put a 10M resistor in series with the scope input for normal use, then short that out when the measurement cycle starts.

[sorin]

To start I want to thank you for sharing your experiment with us, i appreciate that.
I want to suggest you a different method to measure dielectric absorption in a cap, connect a picoamp input OPAMP as unity gain buffer and measure the capacitor voltage drop over time (tens of seconds or even minutes and hours).
A cheap alternative scold be LMC662.
Don't forget to shield them.

or

https://web.archive.org/web/20180723192448/http://www.datasheetarchive.com/files/national/htm/nsc03883.htm

[Lesolee]

I want to suggest you a different method to measure dielectric absorption in a cap, connect a picoamp input OPAMP as unity gain buffer and measure the capacitor voltage drop over time (tens of seconds or even minutes and hours).

I’m definitely warming to the idea of a buffer. This data from the earlier 20 V test shows that the curves (when re-scaled) look pretty similar. The offset on the blue curve is simply the scope’s s/c to o/c offset.



I have (conceptually) fixed that in a revised design which I will be re-wiring to today. Both capacitors measured as being within 1% of each other on a Tenma LCR meter (72-6634), but then again the value of an X7R capacitor is dodgy anyway. I need to repeat that test with less noise (eg x10 buffer).

[Kleinstein]

For including the discharge phase it is enough to have the trigger time. So no need to measure the full voltage with the scope / amplifier.
The starting time for the DA processes is the start of discharge (more accurately about when the voltage dropped to 1/2 its value, but the discharge should be relatively fast anyway).

A buffer / amplifier would really help, especially with the relatively poor quality scope. It does not have to be a sub pA input current amplifier, for the slower processes it is also about low frequency noise and drift.  A constant input bias could be compensated by doing 2 tests with opposite sign of the voltage to start with and than take the difference of the two.
Already a relatively common amplifier like TLC272, TL072 or OP07 could be better than the scope, though far from ideal. The AZ OP from the µV meter could also work quite good.

[Lesolee]

The PET capacitor looks relatively good. So the smaller factor of 4 could be due to good PET one, not necessary a poor PP and PPS example.

I missed out a relevant result. The results for the capacitor identified as PE (old) were not mentioned as the capacitor is 35+ years old, and who knows where it came from. The PET recovered to 29 mV, whereas the PE recovered to 33 mV from the same 50 V initial voltage. Admittedly the PE is a 250 V version, whereas the PET is only 50 V.

[Lesolee]

A constant input bias could be compensated by doing 2 tests with opposite sign of the voltage to start with and than take the difference of the two.

Aaaahhhhhh. 

There are two distinct issues:
(1) choosing the best cap for a particular job
(2) measuring the best cap's performance

Obviously the best test method is to put the cap alternatives into the target circuit and seeing which is best. That's probably easier in a sample/hold than in this particular application where the 2µ2 is being charged/discharged via a 100K resistor.

I think this test setup is good for this intended application as the cap is being discharged. Possibly less good for an infeasibly long analog sample and hold (or integrator).

We shouldn’t get too hung up on the testing out of circuit! I need to see if I can find some better cap candidates.