PROJECT: Micro-Voltmeter Design

[Lesolee]

I have been saying that the battery life was irrelevant because 4200 mAh / 50 mA is 84 hours.

Of course I naively assumed that when I bought 4200 mA batteries from Ebay, they would be as stated. Nope. The vendor claims they weigh 40-45g. They actually weigh 26 g. That is a red flag. (I have never handled 18650s before, but my first thought when they arrived was that they felt light!)

When flat they won’t even take 1 A (at 4.2 V) for an hour, tailing off during the hour to some smaller amount.

Obviously I have googled it and found that on Ebay fake batteries are just an industry in their own right. Maybe I should have been more suspicious when the battery text read “sheef life” rather than “shelf life”. Sadly I already gave feedback to the Ebay vendor before spotting the issue. 

Now I have another whole diversion into finding out just exactly how crap they actually are. 

[EDIT: The "4200 mA.h" batteries measured as 400 mA.h on a 50 mA discharge. They were sent back to the vendor as underweight and under-capacity.]

[Grandchuck]

The 18650 cells in old laptop batteries are often better than many 'discount' units sold today.  It is often easy to take the battery packs apart and test them and sort them.

[Kleinstein]

I just noted something interesting in the data-sheet of the LMP2021/2022 AZ OPs: There capacitance at the input seems to have an influence on the input bias, at least for that type and an unspecified competitor. If there is not much filtering to the input this could cause some surprise of the external capacitance can change the bias.

Ideally one may be able to reduce the bias current with a suitable capacitor value    .

[MadTux]

Diodes usually suck at low leakage, that's at least what I measured so far.
Selected transistor C-B junctions, with BE connected together usually make far better low leakage diodes than what normal diodes can do.

[magic]

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

Another possible gotcha is thermoelectric generation if connections to the positive and negative input terminal are done with different materials. Could be responsible for some of the warmup drift perhaps?

edit
That could be another reason to get rid of shunt regulators

[Lesolee]

Another possible gotcha is thermoelectric generation if connections to the positive and negative input terminal are done with different materials. Could be responsible for some of the warmup drift perhaps?

That could be another reason to get rid of shunt regulators

It has to be said that the regulators are not well isolated (thermally) from the amplifiers in my layout. But they do limit the spread of noise currents (artistically represented below by red wiggles).



We also know that power change is the enemy of stability. Suppose the display reads 1.1111 mV and then that changes to 0.8888 mV. Overall, the power consumed with the shunt regulators is constant. Not so with the series regulators. Provided we keep the series resistors (R1/R5) close to the shunt regulators (U1/U2), we should keep the temperature distribution more constant (although this should be a pretty small effect).

[Kleinstein]

The current consumption is not that constant with the shunt regulator: it will change quite a bit when the battery voltage changes.
Only a changing number in the display would not effect the current consumption.
The shunt regulators are not so bad if the resistors a chosen well, but I would not recommend that if one would build the circuit a 2nd time.

There is a positive side of the shunt regulator - it will show a drop out and thus unstable readings at a higher level - possibly useful to detect a failing battery, though this may happen a little early.

The diode leakage is not that critical in this case, as there is still the 1 G resistor in parallel from the current compensation. A 1 G input impedance is usually acceptable for a voltmeter.

[Lesolee]

The diode leakage is not that critical in this case, as there is still the 1 G resistor in parallel from the current compensation. A 1 G input impedance is usually acceptable for a voltmeter.

There is an interesting point about the 1G resistor. You and I know it is there. But what if we didn’t? Ordinarily the bias compensation is connected to the bootstrapped rails. That is important for a 10V swing. You evaluate the impedance as dV/di, in other words how much does the bias current change when you change the voltage.

For this meter the full range input is 1 mV. A bootstrap is irrelevant. Given the current noise, a 1 mV change in input has no measurable effect on the bias current. I guess it means the input impedance is uncertain to the level of 100M (10pA ptp current noise).

Sadly I tried putting a 10 nF cap directly across the chip inputs. It didn’t like that, giving 150 µV offset voltage. (No evidence of oscillation). The chip is now looking a bit fried. 
8 µV ptp where I used to have 0.2 µV. 

No more measurements for a while until I get a replacement. 

[Kleinstein]

To change to series regulator one could use a single +5 V regulator and create a virtual ground. This may be as a buffered 1 V to get the zero suppression.

[Andreas]

Fixed -2.5V regulators don't exist. 

really?
what about TPS72325
or with 2 resistors adjustable TPS72301 or LT1964.

Face the truth: The TO-92 package will be obsolete in near future.
As it is already with most J-FETs and discrete transistors.

with best regards

Andreas

[Lesolee]

Fixed -2.5V regulators don't exist. 

really?
what about TPS72325

The stupid distributor's search engine has LDOs separate from linear voltage regulators. 
I didn't notice that, so missed out on the LDOs.

Face the truth: The TO-92 package will be obsolete in near future.
As it is already with most J-FETs and discrete transistors.

It doesn't make it easy to build stuff on veroboard though, does it.

[Lesolee]

To change to series regulator one could use a single +5 V regulator and create a virtual ground.

I know that idea has been floated before on this thread (maybe by you?). It is a workable option.

[magic]

It doesn't make it easy to build stuff on veroboard though, does it.

Actually, SOT23 kinda fits onto three adjacent copper fields on a perfboard. If you must.

[Lesolee]

So the micro-voltmeter was working quite well, and all seemed fine. But taking a second look at it, the output buffer is, well, non-optimum at best. It is instructive to go through why in some detail.

The adjustment has been omitted for simplicity. I shall just assume that it magically has appropriate values to give exactly 1 volt on the display, corresponding to 1 V input relative to the -2.5 V rail.



Now what happens if the positive regulator changes by 100ppm due to a 1°C in its 100ppm/°C output rating?

100ppm  (0.01%) is 250 µV. This is scaled by a factor of -1 by R2/R1 giving 250 µV at the input the DVM module. That is 2.5 digits, which is ridiculously high. It needs to be 10x better than that at least. Then we have the tracking TC of R2 to R1 which is 200 ppm/°C (worst case), so that is bad as well.

Let’s try the same thing on the negative rail. Suppose the -2.5V rail rises by 250 µV. The output of falls by 250 µV x 100/240 = 104 µV. The overall change of input voltage (as seen by the DVM module) is 354 µV. The tracking TC of R2/R3 has the same scaling factor.

Whilst one could (theoretically) improve all the component TCs by a factor like 50x better, the circuit is fundamentally lousy.

I did consider using a -1V reference, hanging down from ground, and then powering the DVM module from a buffered version of this. But that means the DVM module has to run straight from the unregulated battery, and only works when the top battery is above 4 V. Not workable.

So the next version will need a x1 diff amp with 4 well matched resistors, and a 1 V regulator standing up from the -2.5 V rail.

[David Hess]

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.

Diode conductance at zero volts is 20 to 30 mS/mA * leakage which for a 1N4148 is consistent with your 300 Mohm measurement.  A true low leakage diode would be more like 300 Gohms.

[LaserEng]

Isn't there going to be quite a few digits (upto around a 100 counts) offset with a -1 mV input due to the DVM module and MCP6041 having the same -ve rail due to the the op amp only being able to get within 10mV  of the rails?

Edit.. Also just noticed the Linear Region output swing of the op-amp is to within +/- 100mV of the rails.

[Lesolee]

Isn't there going to be quite a few digits (upto around a 100 counts) offset with a -1 mV input due to the DVM module and MCP6041 having the same -ve rail due to the the op amp only being able to get within 10mV  of the rails?

The "zero" level is 1 mV referred to the overall µVM input. At this level there is 1 V into the DVM module (wrt -2.5 v). It is true that at -1 mV input to the overall µVM the input into the DVM module approaches 0 V with respect to its negative power supply. Probably it would be best to consider the overall input spec as +3 mV to -0.95 mV to avoid any non-linearities. I only checked the linearity up to -0.95 mV.

Alternatively I could change the offset to 2 mV referred to the input, to get more range. Obviously more offset means more noise and drift in the output buffer, but the new design might cope with that. 

[Lesolee]

Diode conductance at zero volts is 20 to 30 mS/mA * leakage which for a 1N4148 is consistent with your 300 Mohm measurement.  A true low leakage diode would be more like 300 Gohms.

It would be instructive for you to explain where this value has come from.

Certainly I would agree that a well biased base-emitter junction would give 26 ohm at 1 mA as an equivalent output resistance (to which a bulk resistance of say 3 ohms could be added). That would equate to 0.04 S/mA. But I would not be so bold as to expect that to work correctly at or near zero bias.

[EDIT: The formula is derived in post #67 below, and relates to the saturation current of the diode.]

[Kleinstein]

For the 1 V offset a difference amplifier is a good idea. As one has to adjust the 1 V level anyway, there is no need for super accurate resistors. Even if the resistors were 10% off this would still reduce the error from subtracting positive from negative reference / supply by 90%.

Alternatively use a single 5 V regulator and a 1 V ref. Level as virtual ground.

There are still few panel-meters with a +- 200 mV or similar range around, though often a little more expensive.

[Lesolee]

For the 1 V offset a difference amplifier is a good idea. As one has to adjust the 1 V level anyway, there is no need for super accurate resistors. ...

Alternatively use a single 5 V regulator and a 1 V ref. Level as virtual ground.

The diff amp still requires well matched TCs. I found some really nice Vishay matched pairs in (of all things) SOT23 packages. But with 2 ppm/°C tracking TCs they just need to be put in something!

But topologically speaking the floating battery with a single 5 V regulator is the better option. 
There is no matching required; it is all just 1:1.

I don't really like the idea of powering the opamps off -1 V, but I can trivially change my mind and use a 2 V offset, then generate +3 V and -2 V rails from the virtual ground.

New design in progress 

[Lesolee]

Here it is, the much anticipated new design ...



[EDIT: Updated to the latest circuit diagram.]

[iMo]

Is the R2=1k ok?

[Lesolee]

Is the R2=1k ok?

I'm not sure. Those values in the virtual ground circuit are almost placeholders at the moment. I can't quite see how much current will flow in R2. I am thinking it is almost none (just enough for R8). It's difficult for me to get my head around. R8 is there so there is a definite direction for the current, so U3 output stage is not hunting around zero.

If you have a different view please let us know.

[EDIT: Actually there is 3V/22K for R7 and 3V/100K for U4 = < 200 µA]
[EDIT: R8 is worse than useless! ]
[EDIT: R2 at 1K is useless when you reduce C7 to look at fast edge response. The -2V rail bounces and you get overshoot of 3%.  Fast current flows through C7 via R16 and is LARGE! This transient current causes problems if the virtual ground impedance is high.]

[Kleinstein]

1 K for R2 may still work. The more usual values would be more like 100 Ohms, so just enough to isolate the OP from a capacitive load.
R8 has essentially no effect and no need for it.

[Lesolee]

I just encountered a gotcha in the process of building up the Mk2 design. I was about to wrap an AVX Skycap 1 nF 100 V capacitor between the terminals of the MicroVoltmeter, leaving the ends up in the air for connections to other parts. It is difficult to solder onto the copper terminal post, so it is convenient to have thin wires to connect to.

I noticed that the wires were attracted to my pliers! I checked the data sheet and it just says the leads are 100% tin. Except tin is paramagnetic. And these leads are STRONGLY attracted to magnets.  I would think they are tin-plated mild steel or iron. In any case ferromagnetic and low thermals are incompatible.



Ordinarily not a problem. In this application, total disaster!