PROJECT: Micro-Voltmeter Design

[sorin]

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.

You can try some Russian Teflon caps, I think that this are the best.

[Lesolee]

You can try some Russian Teflon caps, I think that this are the best.

At 2µ2 that would be seriously large. I doubt I could afford it. 

[Kleinstein]

For 2.2 µF the prime candidate is a PP film cap. Here not all brands may be the same. In my tests (https://www.eevblog.com/forum/metrology/diy-high-resolution-multi-slope-converter/msg2763326/#msg2763326) with 2.2 nF size and some 1-40 ms time scale however I did no see much difference. The best cap I found at this value was a TDK brand  NP0 cap, about 5 times lower DA than PP or PS.
However NP0 in 2.2 µF is a little tricky and large. The large ones may also behave different.

For just the 3-4 digit digital display a PP cap should be good enough. I am not sure one really needs 2.2 µF. The display module also does some averaging.

The brute force way would be to do only minimal analog filtering and than use a separate ADC, digital filtering and display. Digital filtering can be finite response type (like running average) and thus settle to 100% in a finite time - this may be an advantage over the exponential settling found for the analog filters, even with a perfect cap.

[sorin]

According to Bob Pease the next best thing is Polypropylene.

[MegaVolt]

That might actually be reasonable. They realistically expect the parts to be that good, but they can't test to that level routinely so they won't spec it. In that case you would hope they would test-screen a few diodes from each wafer, or each wafer batch, to ensure they are as expected -- but they don't mention that. It would give (me) more confidence if they did. But it does mean that not all BAV199s are created equal. The Diodes Inc data sheet shows a definitely worse diode.

This sounds absurd but you can use the ADA4530-1 as an expensive protection element with an input current in the fA area and a maximum protection current of 10mA. You must connect power to it and use only the input

[splin]

I don't know about the meter you used,  but these 5 digit meters use an 18bit ADC, MCP3421with differential input (the SOT-23-6 near the sense i/p pads) :

https://www.ebay.co.uk/itm/0-36-5Digit-DC0-33-000V-0-4-3000-33-000V-Precision-Digital-Voltmeter-Panel-Meter/202517747023?hash=item2f26ff954f:m:mUvj8klzZrGNsa3CWACgikw

You can cut the trace connecting the -ve input to the -ve supply and connect it to your local ground to eliminate the need to offset the input by 2V and the attendant drift and noise. There are two resistors dividing the input down to match the 2.048V ADC input range. The ADC has a PGA with x1, 2, 4 and 8 so the meter uses x8 for low voltages switching to x1 when the input voltage exceeds 4.3V (ish).  The divider totals 347k on my meters.

I have some of these meters and they are really good - see:

https://www.eevblog.com/forum/projects/$5-voltmeter-with-5-digit-(0-1mv)-resolution/msg591905/#msg591905

There are lots of sellers of these meters but they aren't all the same - check the photos of the PCB match this one.

[Kleinstein]

Even with an ADC that can operate with negative voltages this does not mean the µC to convert the number does actually show negative numbers as well. The LED display may not have a dedicated sign.  So it would be about making your own ADC + display part. Here the MCP3421 can be suitable relatively low cost ADC.  If one knows the µC used well (AFIAK STM8... in some examples) one could in theory hack die HW and adapt the software if needed.

The MCP3421 is at least capable to do essentially continuous sampling of the input. So there may not be much need for analog filtering in this case.

[splin]

Even with an ADC that can operate with negative voltages this does not mean the µC to convert the number does actually show negative numbers as well. The LED display may not have a dedicated sign.  So it would be about making your own ADC + display part. Here the MCP3421 can be suitable relatively low cost ADC.  If one knows the µC used well (AFIAK STM8... in some examples) one could in theory hack die HW and adapt the software if needed.

The MCP3421 is at least capable to do essentially continuous sampling of the input. So there may not be much need for analog filtering in this case.

True but usually you can connect the signal input to ensure a positive result, or you can include a reversing switch. The advantage is to avoid the drift and noise of the 2V offset required to the meter supply. If you decide to continue to use the 2V offset, driving the ADC -ve input directly allows you to separate the offset reference from the noisy 5 digit meter power supply rails and provides more flexibility for the 5 digit meter's supply rails so long as the inputs remain within the ADC's common mode limits.

[Lesolee]

This is the limit for the Mk 1 design. Starting on the buffered Mk 2 design tomorrow.

[Lesolee]

I don't know about the meter you used,  but these 5 digit meters use an 18bit ADC, MCP3421with differential input (the SOT-23-6 near the sense i/p pads) :
...

You can cut the trace connecting the -ve input to the -ve supply and connect it to your local ground to eliminate the need to offset the input by 2V and the attendant drift and noise.

The design was changed to offset the panel meter by 2 V and that is now pretty stable. But obviously if the offset could be removed it would make all voltages easier to read (compared to subtracting 2 mV from negative readings for example!)

Mine is not the same as yours.

[Lesolee]

The hugely anticipated integrator version of the dielectric tester (Mk2) is now working.

The LEDs to show relay state, although apparently trivial, make a huge difference in testing. When you have to wait 10 minutes and then find that the system wasn’t reset, or the scope wasn’t re-armed, or something else, that gets boring very quickly. 

The Mk2 construction may seem familiar to some. The Mk1 donated itself to the building effort. 

Clearly you get a lot more recovery voltage on the integrator version. How relevant that is, is debateable. If the capacitor is being used in a filter, or as a bandlimit, it will have a resistor around it, and that will shunt away the ‘weak charge’ hidden behind gigohms of series resistance.

For technical interest, I measured the ramp rate on the buffer output as 1.6 mV/minute. This equates to 6 pA leakage, which is more than adequate for the testing.

I have several more tests to run … 

[Kleinstein]

The curve looks a lot better now. The buffer / amplifier really helps.

To see how much is real leakage and how much is the very slow part of the DA.
To test the leakage one could check with a much smaller capacitor (e.g. 1 nF) range, preferably a low loss one like PS, PP or NP0.

[Lesolee]

Just as a sanity test, since it would have been easy to make a mistake, just one resistor change per test ...

[Lesolee]

To test the leakage one could check with a much smaller capacitor (e.g. 1 nF) range, preferably a low loss one like PS, PP or NP0.

Your command is my wish ...

You see that grey thing on which the tester board is sitting in the earlier post? It is a Sainsbury’s 100% cotton towel. I have been using it as a ESD build-up prevention mat. I couldn’t measure any conductivity across  it, even with a 1000 V hipot tester, but then I didn’t try that hard. So the last thing I should be doing is laying the veroboard tracks directly on top of this “conductive” mat. 

So lifting the board off the mat and supporting it just by the edges where nothing interesting is happening we get this result …



The charge injection spike is interesting. Obviously I couldn’t see that with 2200x more test capacitance. Presumably it is the relay coil or the LED contact causing it. (The Panasonic data sheet has no spec for capacitance.) 5 V swing to 6 mV means 833x smaller than 1000pF = 1.2 pF. I measured 2.0 pF using my Tenma 72-6795 capacitance meter, which all sort of ties up. I could correct for it if it was a problem, but it isn’t.

The ramp is roughly 2.5 mV/minute so that is a current of 1000E-12 x 2.5E-3 / 60 = 0.04 pA
(which is now in the opposite direction to previously) and is probably not going to stay put for very long!)

The capacitor measured as 1.003 nF which sort of confirms the meter (±0.5%) and the capacitor (±1%).

[sorin]

The charge injection spike is interesting. Obviously I couldn’t see that with 2200x more test capacitance. Presumably it is the relay coil or the LED contact causing it. (The Panasonic data sheet has no spec for capacitance.) 5 V swing to 6 mV means 833x smaller than 1000pF = 1.2 pF. I measured 2.0 pF using my Tenma 72-6795 capacitance meter, which all sort of ties up. I could correct for it if it was a problem, but it isn’t.

The ramp is roughly 2.5 mV/minute so that is a current of 1000E-12 x 2.5E-3 / 60 = 0.04 pA
(which is now in the opposite direction to previously) and is probably not going to stay put for very long!)

The capacitor measured as 1.003 nF which sort of confirms the meter (±0.5%) and the capacitor (±1%).

You understand this wrong, simply you can not use a relay for this type of measurements. It is not just about the relay leakage or charge injection. If you use a relay the mos significant error source in your case should be the voltage injection from the electromagnetic switching currents. I remember that I read a Application Note or a lab note about this topic some years ago. According to them, you are losing your time with this relay, you shold use solid state switch with Low leakage or jFET, something like this MAX329 or similar. In my opinion you can also try using Reed Relay.

[Lesolee]

You understand this wrong, simply you can not use a relay for this type of measurements. It is not just about the relay leakage or charge injection. If you use a relay the mos significant error source in your case should be the voltage injection from the electromagnetic switching currents.

It is interesting that you should make such an assertion. But it is just that.

Engineering needs to be more evidence based.

We have three possible coupling mechanisms:
(1) near field capacitive
(2) near field inductive
(3) far field electromagnetic

To say that you cannot use a relay, ever, for such a circuit is demonstrably not true. Any technique gives some error.

A solid state device with a leakage resistance above 1G ohm above 50 V is unknown to me.

There is certainly a magnetic loop, and a collapsing magnetic field. But a magnetic field only induces a voltage when it changes, so it can’t have a permanent effect on a capacitor (unless it is rectified).

Charge injection from a rising/falling driver edge is well known and established in the industry. Relays have been used for DVMs and oscilloscope switching for at least 40 years.

[Kleinstein]

I see no real problem using a relay. There can be some leakage, but still the leakage tends to be smaller than with JFETs or CMOS switches.
There can be some charge injection to, just from the coil voltage, but also from moving charged internal surfaces.
However this is mainly an effect for the initial jump and not a real problem.

Induction can cause transient effects from induction voltage, but this would be just transient for a few ms or so, and thus not a problem for the slow part.

A reed relay may be a slightly better choice (faster contact ringing) if one is also interested in the very short time scale part. For the slow part the normal relay is OK.

So the bias current seems to be really low in this case, which is a good thing. So  I don't think the drift seen with the 2.2 µF caps is from the input current, but probably still part of the really slow DA. For low leakage currents it is really difficult to separate DA and leakage. With the capacitors part of the slow DA is actually also though to be due to internal leakage current towards internal surfaces.

From my tests I found it useful to also look at the curve with a log time scale. The logical zero is the begin of the discharge phase.

[sorin]

Sorry if I'm not explain myself clearly, but please understand me, I'm very bad at writing English.
Don't forget that the capacitor that you are using is not a ideal capacitor. please see attached photo.


If you want to measure the voltage decay over a long period of time (more than 30sec) I maybe  the relay should be acceptable. You can also try doing the same measurement but replace the relay with a switch.
You can also try BF861C which have a leakage of only 0.1 pA

Sure the Relay are used all te time on multimeter and asciloscope but not in this configuration.

ps. not topic relevant but just for reference. https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-one/

[Lesolee]

Sorry if I'm not explain myself clearly, but please understand me, I'm very bad at writing English.

Not so bad, actually. Better than my Albanian

Don't forget that the capacitor that you are using is not a ideal capacitor. please see attached photo.

I wonder if that image is an actual representative model, or just a concept. Those models are often shown as concepts without values. I would be interested to see other people’s published model data to compare with my measurements.

Where did that specific one come from?

[Lesolee]

So the bias current seems to be really low in this case, which is a good thing. So  I don't think the drift seen with the 2.2 µF caps is from the input current, but probably still part of the really slow DA. For low leakage currents it is really difficult to separate DA and leakage.

There is of course the worry that the bias current might change with common-mode input signal. But the MCP6043 data sheet shows the (typ) bias current as a constant 1 pA for all common-mode input voltages between the negative and positive rails. Another proof is the linearity of the signal shape with charge voltage. And of course the linearity of the integration slope with the small capacitor.

The bottom line is that the plot is unambiguously all due to the capacitor and not the jig. There is no necessity to de-embed the drift from measured value.

[2N3055]

Not so bad, actually. Better than my Russian 

Not to nitpick but Albania has its own language. Closest country that uses Russian is Ukraine, three countries and some 650km away.. Not important, but just saying.

[sorin]

Not so bad, actually. Better than my Russian 

Not to nitpick but Albania has its own language. Closest country that uses Russian is Ukraine, three countries and some 650km away.. Not important, but just saying.

you are wrong It is 2058km away..
I think that the Geography is not his strong point
We are closer to London or Dublin than Moscow

Where did that specific one come from?

Please see my previews post

[Kleinstein]

The equivalent circuit for a capacitor with DA is a well known concept. The actual values to the resistors capacitors of cause depend on the capacitor. In addition there is no unique representation, especially as the measurement data are usually limited and the information contend is limited. There are many slightly different models that can give essentially equal results.  The main point is that there are DA effect with different time scales, ranging from very fast (e.g. ns) to very slow (e.g. months). Especially for the faster part one  may be able to identify processes behind it,  but in the slow range this is usually difficult.
The number and placement (choice of RC time constants) of the RC elements to represent the DA is not unique - more like use as many as needed to represent the observed curve. It is only for some of the identified (usually faster) processes that one has a defined dominating time constant and not just a sequence of conveniently spaced time constants (e.g. one per decade).

[2N3055]

Not so bad, actually. Better than my Russian 

Not to nitpick but Albania has its own language. Closest country that uses Russian is Ukraine, three countries and some 650km away.. Not important, but just saying.

you are wrong It is 2058km away..
I think that the Geography is not his strong point
We are closer to London or Dublin than Moscow

Where did that specific one come from?

Please see my previews post

An you cannot read :

Closest country that uses Russian is Ukraine, three countries and some 650km away

[Lesolee]

So we have a new lineup, with the X7R and PET discarded.



The board has been mounted so it doesn't rest directly on the mat, and it is altogether more mechanically stable.





Here are the details of the capacitors:



And last, but not least, are the results.



PPS was the best, but not any more.