PROJECT: Micro-Voltmeter Design

[Lesolee]

Here is my latest (tutorial) project, a voltmeter with ±1mV Full Range, 100nV resolution on the last digit, “hardly any” voltage noise, “hardly any” bias current, and all made on veroboard.

I will be quantifying the voltage noise and current noise over the next few days; be patient!



The DVM module can only handle positive voltages. Since I want both positive and negative input voltages I have biased the DVM module input up to 1V. This 1V then means 1mV referred to input since the amplifier chain is calibrated to x1000 (including any gain error of the DVM module).









So let’s look at some technical stuff. Given that I want “hardly any” bias current, the last thing I need is leakage to the + input pin of the ADA4522. The – input pin is on pin 2, right next to the + input pin (number 3) but that is ok because the input offset is 5µV. But pin 4 is at -2.5V so I would potentially have 2.5/5µ = 500,000 times more leakage to the power pin than to the – input pin!

Sadly the excellent ADA4522 chip is not available in a nice 8 pin DIP. I got a SOIC8 package. My solution was to lift pin 4 and link it to pin 5. The pin 4 pad is just floating.



Now the solder used is not a modern no-clean type. It has to be 20+ years old, 60/40 tin/lead solder so I washed the chip as best I could with 99.9% pure IPA. Even so, the best I could achieved was 300pA leakage. The measurement method was simply to put 1M in parallel with 100nF across the input terminals and observe the change in reading between the short-circuited input and the 1M input. (The overall gain was set earlier.) The worst case datasheet value is 150pA, so I am not million miles away, but even so, it was a bit disappointing. Needless to say, a few extra bits (R6,R7, R8) soon took that bias current down to the required “hardly any” level.

Now you might call this a “bootstrap design process” in the sense that I used the partially built micro-voltmeter to measure its own components. Next up were the protection diodes.

The input is designed to work up to ±1mV. At this level the leakage in the diodes is essentially independent of the bias direction (forward or reverse bias).



Since I am looking for low leakage at the best price I used the BC550B’s that I had in stock. (The J113 is a n-channel JFET, and using a JFET as a low-leakage diode is an old (>35 years old) trick of the trade.) I should mention that the 1mV was obtained by using a 1M resistor into a 100R resistor, and driving the series chain from 10V.

Next up I will have to do some measurements, unless there are any burning questions that have to be answered.

[EDIT: the circuit has a significant design error. U4 (MCP6043) has a not-CS pin which has to be tied low. I have not updated this circuit as it is obsolete, the Mk 2 version being superior in terms of noise, TC, battery life, stability, … Mk 2 starts at post #45.]

[Lesolee]

I have been switching it on and off, and unplugging the shorting plug at quite a rate, so I have never given it a chance to settle down fully. This time I left it for 2 hours before switching it on. No change of the input shorting plug.

Wow!



As a separate test I measured the noise after a 23 minute warm up. No change of offset pot or input connections. S/C input.

The displayed noise over 1 minute went between
1.0043 mV and
1.0039 mV

0.4 µV ptp

[EDIT: added new measurement of ptp noise over 1 minute, ... and then corrected the ptp noise value.]

[Andreas]

Hello,

I am wondering why you use the ADA4522 for this project with relative high input bias current.
I would have probably used something like a selected ADA4638 where I have measured < 22 pA input current.

with best regards

Andreas

[magic]

Given that I want “hardly any” bias current, the last thing I need is leakage to the + input pin of the ADA4522. The – input pin is on pin 2, right next to the + input pin (number 3) but that is ok because the input offset is 5µV. But pin 4 is at -2.5V so I would potentially have 2.5/5µ = 500,000 times more leakage to the power pin than to the – input pin!

I washed the chip as best I could with 99.9% pure IPA. Even so, the best I could achieved was 300pA leakage.

Look up the LMC662 picoammeter project. It achieves 0.002pA input current simply by lifting pin 3 and connecting it in the air. (And using a lower bias opamp, but your input current is much higher than the chip's spec).

[Lesolee]

I am wondering why you use the ADA4522 for this project with relative high input bias current.
I would have probably used something like a selected ADA4638 where I have measured < 22 pA input current.

That's easy, 0.1Hz to 10Hz ptp voltage noise is 1.2µV on the ADA4638 wheras it is 117nV on the ADA4522. That is a factor of 10x more voltage noise on the ADA4638.

[Lesolee]

Look up the LMC662 picoammeter project. It achieves 0.002pA input current simply by lifting pin 3 and connecting it in the air. (And using a lower bias opamp, but your input current is much higher than the chip's spec).

Probably you are not suggesting using the LMC662 itself, since it doesn't even quote 0.1Hz to 10Hz noise. It seems then that you are suggesting lifting pin 3 of a SM IC and wiring it directly to the input terminals, with the bias current compensation and clamp diodes all on the same wire. I was confident to lift pin 4 and wire it to pin 5 as there was a mechanical anchor nearby, and just one small wire as an inertial load.

Given the construction method (shown) I would have had to find a PTFE mounting point, and mounted it to the little SM to DIP converter board, which is pretty difficult.

[Lesolee]

Bias current: measured using 1M // 100nF across the input terminals. I turned off the bias current compensation (pot).

S/C   1.0029 mV
O/C   0.8390 mV      = 163 pA

I tried connecting pin 4 to pin 2 for extra guarding.

S/C   1.0032 mV
O/C   0.8490 mV      = 154 pA

(The offset voltage changed a little as I had the lid off and balanced the box up on its end, resting on the zero adjust pot, which probably moved it a little!).

The bias current was 300 pA last time I measured it, but that was within tens of minutes of flushing the board with IPA. Probably there was still some residue to evaporate over a period of a day or so. At least now the bias current is closer to the manufacturer’s spec.

Next test is bias current noise. 4 minute warm-up with the lid on for the zero
1.0029 mV
3 minute warmup for the 1M//100nF.
0.8515 mV      = 151 pA.

Then measure the max-min noise over 1 minute (by eye since this is not a bus controlled device).
7.9 µV ptp = 8 pA ptp

Adjust bias current.
4 minute warm up to zero
1.0033 mV
3 minute warmup to 1M // 100nF
1.0053         = 2 pA

Measure ptp noise over 1 minute: 5.8 µV   = 6 pA ptp

There is no way the bias current compensation has made the noise less, but it is demonstrably not significantly more. Clearly ptp readings have noise on them.

Try again at 100K // 100nF
0.9 µV ptp      = 9 pA ptp   seems reasonable.



[EDIT: fixed typo of a 100pF cap which should have been 100nF]
[EDIT: fixed factor of ten error in 100K bias current calculation! Also fixed typo for "minute". ]
[EDIT: The bias current noise measurement method is incorrect. You get too much Johnson noise in the available bandwidth. A bigger capacitor is needed. See the Mk 2 for better measurements.]

[Lesolee]

It's pretty boring, but I thought I should validate the protection network since using a transistor (BJT) in this way is not very standard.

To be clear, the transistors are in TO92 cases, and I just cut the collector leads off.

[Kleinstein]

The X7R type capacitor at the input can cause quite some surprise that can look line some input current. So I would replace it with a PP type capacitor. The class 2 ceramic can have very large dielectric absorption and thus release charge from the past.

For the AZ OPs one has to find a compromise between voltage noise and input bias. The ADA4522 is very low voltage noise, but also high current noise and high bias, kind of at the extreme. With only 0.1 µV resolution one could have chosen a slightly higher noise, but lower bias type, e.g. LTC2057 or AD8628 with about 2 or 4 times the noise but less input current.
The current is not so much about leakage, but part of the switching inside the OP. There is also scattering between individual units.

[Lesolee]

The X7R type capacitor at the input can cause quite some surprise that can look line some input current. So I would replace it with a PP type capacitor. The class 2 ceramic can have very large dielectric absorption and thus release charge from the past.

For the AZ OPs one has to find a compromise between voltage noise and input bias. The ADA4522 is very low voltage noise, but also high current noise and high bias, kind of at the extreme. With only 0.1 µV resolution one could have chosen a slightly higher noise, but lower bias type, e.g. LTC2057 or AD8628 with about 2 or 4 times the noise but less input current.
The current is not so much about leakage, but part of the switching inside the OP. There is also scattering between individual units.

The X7R comments are fair enough. To be honest I got stuck into the mindset of using a high voltage capacitor for low leakage. When the part actually arrived I wondered why a high voltage was sensible, given that the input can only take about 15V as a short-term overload. Then I thought I have it, so I might as well use it. I also wondered if the X7R would be microphonic. All good reasons to bin it, although I have seen no evidence on this unit of significant dielectric absorption.

The LTC2057 has 200pA bias current and double the noise, so it is worse on both counts. The AD8628 has 5 digits (0.5µV) ptp noise, which is horrific, and only 30% less current (worst case). I don’t think that is a better choice.

I agree that above 50K the current noise (which nobody specifies, but I have measured) is dominant.

[magic]

It seems then that you are suggesting lifting pin 3 of a SM IC and wiring it directly to the input terminals, with the bias current compensation and clamp diodes all on the same wire. I was confident to lift pin 4 and wire it to pin 5 as there was a mechanical anchor nearby, and just one small wire as an inertial load.

I think pin 3 would carry the weight of that input circuit just fine unless you put the device in a tumble dryer. Certainly DIP is robust enough. Not that it matters if you are satisfied with performance as-is.

By the way, shouldn't the connection from TP2 to R16 be as short and direct as possible? Not sure how it's routed here.

[dietert1]

As far as i remember a 1N4148 had about 300 MOhm at 0 V. You could use a second set of diodes and two resistors to generate two +/- 0,7 bias voltages to keep the protection diodes in reverse direction during normal operation. Then clipping occurs at +/- 1.4 V which is good enough and there won't be small forward currents in the protection devices.

The input of the ADA4522 should have a capacitor to ground, maybe another 1 or 10 nF. Chopper amplifiers work better like that.

There are very low noise LDO regulators ready to generate +/- 2.5 V, preferable to the shunt regulators you propose.

Regards, Dieter

[Lesolee]

By the way, shouldn't the connection from TP2 to R16 be as short and direct as possible? Not sure how it's routed here.

You're absolutely right. 

I had that in mind earlier, then it fell away later (senior moment)
and I routed it in some arbitrary non-optimum way.

[splin]

The LMP7721 is also worth considering when you need very low Ibias - 3fA typ, 20fA max at 25C.

LF noise is higher at 1.3uVpp 0.1 to 10Hz, but that is a lot lower than other CMOS amps as is the offset voltage of Voffs is 50uV typical; drift is 1.5uV/C .

The input current is so low that you could parallel four amps to halve the voltage noise or even use 16 to get down to 325nVpp!

Another one to consider is the ADA4625, although it is rather expensive. LF noise is only 150nVpp, Ibias 15pA typical, 75pA max at 25C. Voffset 15uV typ, .2uV/C.

So two in parallel will have the same LF voltage noise as an ADA4522 with similar or less Ibias. Big advantage is the much lower current noise at 4.5fA/rt(Hz) at 1kHz (compared to 800fA for the ADA4522). LF current noise isn't specified.

Horses for courses.

[Lesolee]

As far as i remember a 1N4148 had about 300 MOhm at 0 V. You could use a second set of diodes and two resistors to generate two +/- 0,7 bias voltages to keep the protection diodes in reverse direction during normal operation. Then clipping occurs at +/- 1.4 V which is good enough and there won't be small forward currents in the protection devices.

Not true. I did post the measurements on a 1N4148, which could have been easy to miss. With 1 mV forward OR reverse voltage I got 60 pA leakage. Leaky diodes just seem to act as if they have a shunt resistor across a good diode. I originally had a Schottky diode (BAT24) with a small bias so (as you say) the protection diodes would be reverse biased. It was found to be an unnecessary complexity. It was a good thought, but the reality of the physical component (as compared to a simple model) makes the idea non-ideal at these low working voltages. It might be different if the input needed to be linear up to 100 mV, but I have not tested for that level of input.

The input of the ADA4522 should have a capacitor to ground, maybe another 1 or 10 nF. Chopper amplifiers work better like that.

I have C1 at 1 nF. Or did you mean earth/ground. In that case I can't see why a battery powered device would need an external reference to earth.

There are very low noise LDO regulators ready to generate +/- 2.5 V, preferable to the shunt regulators you propose.

To be honest, this design sort of got iterated around a lot. I needed ±2.5 V for the MCP6043 because it can only take 6V (not the 8.2V directly from the batteries). I was going to power the DVM module from the batteries directly, but then I found that the input reference was its negative rail. Then I found it was taking nasty current spikes, so by powering from a shunt regulator I kept the overall current constant, which seemed nice. The current just gets diverted around a small path. I couldn’t use decoupling capacitors because the TL431 is only stable between something like 1 aF and 10,000 µF (I might be exaggerating that slightly).
I don’t think that the overall measured noise is excessive, and even if the TL431 is 10x noisier than an LDO, I am not sure that this would get through the opamp PSRR. The ADA4522 data sheet shows PSRR of 90dB at 100Hz. I make that 31,600x in voltage terms. 50nV ptp RTI would be 1.6mV ptp on the TL431. The TL431 data sheet shows an input noise voltage graph scaled to 10µV ptp over a 10 second interval. I think that gives me a x100 safety margin on power supply noise into the first stage.

The second stage has hardly any gain, so not enough to make the digits wobble via the resistors connected to the ±2.5V rails. The gain to the +2.5V rails (via R13) is x1, so 10µV compared to 100µV for the last digit referred to the DVM module input. The gain to the -2.5V rail (via R17) is even lower, so even less effect.

[splin]

LF noise is higher at 1.3uVpp 0.1 to 10Hz, but that is a lot lower than other CMOS amps as is the offset voltage of Voffs is 50uV typical; drift is 1.5uV/C .

The input current is so low that you could parallel four amps to halve the voltage noise or even use 16 to get down to 325nVpp!

1.3µV is 13 digits ptp. 

I can parallel 16 to give double the noise I currently have. 

You missed this bit:

The LMP7721 is also worth considering when you need very low Ibias - 3fA typ, 20fA max at 25C.

So for source impedance < 50k, stick with the ADA4522. If that covers all your millivolt measuring scenarios then fine, stick with it.

But there may be others considering building something similar but can't live with the relatively low source impedance limitation. Much above 50k then current noise dominates so you need a different opamp hence the LMP7721 and AD4625 suggestions.

Nothing stopping you have more than one front end in your meter, switch selectable depending on what you are measuring. That could include a very low noise bipolar - eg. LT1028 or even a discrete design for ultra low noise, low impedance measurements such as high current shunts.

[Lesolee]

You missed this bit:

The LMP7721 is also worth considering when you need very low Ibias - 3fA typ, 20fA max at 25C.

So for source impedance < 50k, stick with the ADA4522. If that covers all your millivolt measuring scenarios then fine, stick with it.

But there may be others considering building something similar but can't live with the relatively low source impedance limitation. Much above 50k then current noise dominates so you need a different opamp hence the LMP7721 and AD4625 suggestions.

I did indeed miss that part. Thank you for sticking with it 

Again I have had a very narrow focus on what I was then currently thinking. It's good to get a broader perpective, so  thanks.

[David Hess]

Sadly the excellent ADA4522 chip is not available in a nice 8 pin DIP. I got a SOIC8 package. My solution was to lift pin 4 and link it to pin 5. The pin 4 pad is just floating.

My solution with larger surface mount parts is to have an unplated hole drilled under the lead.

Look up the LMC662 picoammeter project. It achieves 0.002pA input current simply by lifting pin 3 and connecting it in the air. (And using a lower bias opamp, but your input current is much higher than the chip's spec).

My favorite was the LMC6081 tested for low input current but the manufacturer tested LMC6001 is available also.

But for this sort of application, low flicker noise is what matters because it is difficult to suppress and that is where chopper stabilized amplifiers shine assuming that a low AC impedance can be maintained to control current noise.  So a low flicker noise alternative like the LTC6240 with 550nVpp noise might be acceptable or better.

[Lesolee]

My solution with larger surface mount parts is to have an unplated hole drilled under the lead.

Interesting. I have been known to mill out the pcb as a slot between the pins. This is useful when you still need to make a connection to that pin.

[Kleinstein]

Having 60 pA at 1 mV for the 1N4148 is an effective resistance in the 160 M range - not that far of the 300 M number. This "resistance" can be highly temperature dependent so the difference can be just some 10 K higher temperature. For the ADA4522 the 1N4148 would not be such a big deal, as it is not that suitable for high Z sources anyway. However lower leakage (e.g. from transistors) is not expensive, so why not. The standard low cost, low leakage diode would be a BAV199 (SOT23 case with 2 diodes though).

For good filtering the current spikes from the AZ OP, a Pi type filter with 2 capacitors is more effective: the capacitor at the OP to convert the current spike from the OP to a limited voltage spike and than the RC / LC filter to reduce the amplitude. The capacitance directly at the OPs input can also help the OP as it sees a lower voltage spike.

In a voltmeter application reading the value from the display the relevant frequencies are lower than the standard 0.1-10 Hz band used for LF noise. Here the AZ OPs have essentially no 1/f noise, while the FET input OPs tend to have quite some 1/f noise. So even if the 0.1-10 Hz noise is comparable, in the more relevant 0.01-1 Hz band the AZ OP would have about 1/10 the noise, while with dominant 1/f noise the level would be about the same for the lower band.

The ADA4522 is good for a low impedance source, while other OPs are better with high impedance. So there is no amplifier good for all.
Those low bias CMOS ones are the other extreme - more like electrometer like.

The main weak point with the shunt regulators is the supply current, not noise. The other point is uneven load to both cells and charging them in series. This can cause trouble on the long run, though there is some equalization trough the resistors. So a single 5 V regulator and virtual ground may be the better solution.

[Rerouter]

You can have slots routed between soic pins, you just need to talk to your board provider, ones I have used in the past had no issue with a small number of 0.6mm unplated slots, but said if I over-did it I would have to pay more. and where happier with 1mm unplated slots.

For unused pins high end multi-meters also do the drill hole or slot path under the pin method so it is floating in free air without needing to deform the pin.

[dietert1]

So in your tests the 1N4148 had about 16.7 MOhm at 1 mV forward voltage. I remember a measurement of 300 MOhm around 0 V. Anyway the better solution is using a low leakage diode like BAV199 with a small reverse voltage. I guess millions of those circuits exist in ECG machines.

Sorry, you did not get my hint concerning the voltage regulators. It makes a difference whether the 1 nF cap is sitting on the chopper input or behind a 470 Ohm resistor. By the way: For good reasons most commercial voltmeters don't have a capacitive input but a resistive one, with protection resistors of some KOhms.

At 4 V battery voltage each shunt regulator takes almost 50 mA, correct? I was not talking about noise, but about quiescent current. Anyway, that is something easy to change.

Regards, Dieter

[Lesolee]

At 4 V battery voltage each shunt regulator takes almost 50 mA, correct? I was not talking about noise, but about quiescent current. Anyway, that is something easy to change.

Ok, I certainly didn't spot that as why you mentioned it. Yes they take a lot of current. I set it up to work on 3.7V. Of course a series regulator uses less battery power. In this case the batteries have 4200 mAh capacity, which is more than I know what to do with! With longer battery life and less power dissipation a series regulator would make a better choice. (Again I had TL431A regulators in stock so I used them!)

So in your tests the 1N4148 had about 16.7 MOhm at 1 mV forward voltage. I remember a measurement of 300 MOhm around 0 V. Anyway the better solution is using a low leakage diode like BAV199 with a small reverse voltage. I guess millions of those circuits exist in ECG machines.

I used the 1N4148 as an extreme example. I would never consider using a transparent glass bodies diode in a low leakage application. I have indeed used BAV199’s, and they are good diodes. In this case a SM part is non-ideal, physically. Is it "a better solution" if the difference is not measurable.

[Lesolee]

The other point is uneven load to both cells and charging them in series. This can cause trouble on the long run, though there is some equalization trough the resistors. So a single 5 V regulator and virtual ground may be the better solution.

Look carefully and you may spot that the individual 18650 cells are tapped at their midpoint, and the charger socket uses the centre tap as well. I can therefore externally monitor, and charge the cells individually, as necessary.

[Lesolee]

By the way, shouldn't the connection from TP2 to R16 be as short and direct as possible? Not sure how it's routed here.

Ok, let’s consider that from a tutorial point of view:

I measured a 7 cm length of the blue wire. At 2 A it gives 9 mV drop (measured using an ordinary hand-held DVM).

9 mV/2 A = 4.5 milliohms.

Put 4.5 millohms in series with the 100R gain resistor (R16) and the error is 45 ppm, which is irrelevant. It has a TC of 0.4%/°C (being copper) but the contribution to R16 is:

0.4% = 4000 ppm

4000 ppm x 0.0045/100 = 0.2 ppm

The TC is changed by 0.2 ppm/°C. But I am using 100 ppm/°C resistors anyway, so that is not a big deal.

BUT, there is a big gotcha. Is somebody else using that piece of copper for their own nefarious (and noise giving) purposes? The answer is YES!

It is useful (from a tutorial point of view) to show the text-book carelessness of my initial layout. Follow the blue wire down from R16 to the lower piece of veroboard. There is maybe 1 cm of track in which the noisy power supply current from the DVM module flows through the TL431 regulators. That doesn’t seem like much. But rather than guessing, I moved the blue wire from the lower veroboard straight to the negative input terminal.



Covers back on, and leave the box ON for 23 minutes to stabilise. Add a cover over the input terminals just as an extra precaution to stop air wafting over the terminals. Finally, stand away from the box whilst doing the measurement so my dog breath doesn’t waft over the terminals and cause slight thermal fluctuations.

Result: Measured this morning with a 23 minute warmup (but no protection over the terminals).
0.4 µV ptp over 1 minute.

Rewired R16 ground wire and more careful thermal management: 0.2µV ptp over 1 minute.

Yes I have arguably changed two things at once. But to get the lowest noise level you have to be careful. It would not have been repeatable to leave the terminals unprotected, and the good change of rewiring the 0V wire could have been masked by the poor overall measurement technique.

I am simultaneously sad that I wired it up so badly in the first place, 
and happy that I have now halved the ptp noise.