Resolve sub-millivolts with a PIC?

Resolve sub-millivolts with a PIC?

This is a one-off project.

I want to measure the output voltage of a peltier device. I have access to both wires. Voltage is under 10mV. I'd like 7 bits or so of precision. I said "precision". I don't care nearly as much about the accuracy, I can calibrate that out. Needs to have short term stability at fixed temperature long enough to get from calibration cycle to measurement cycle. I want to cobble this onto an existing project that uses a PIC16F627 to send data out the serial port. It has no A/D converter. I really don't want to add instrumentation amps to boost the signal anyway.

I've been looking into ways to generate a voltage with the PWM, stack the peltier device onto that, stuff it into the comparator...and "fix it in software". I fear that I'll never get the system noise down to the point where this is practical, but never hurts to ask.

Yes, I know that a couple of thermistors would be easier to manage electrically, but much more difficult mechanically. The Peltier device is just the right mechanical configruation for what I want. And I have it in my hand, as do I the pic16F627 system.

I'm not much interested in advice on using different devices or technologies. I can think of lots of 'em. I can get what I want with an IR thermometer and a calculator, but I'm bored with that. I'm interested in using what I already have to get me where I want to go.

A technique to measure small voltages would have general applicability to other situations.

So, any clever ways to resolve sub-millivolt signals with a PIC (without internal A/D) and not much else?

Thanks, mike

Reply to
mike
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You meant "resolution"? Precision is usually something absolute :-)

Have you looked into dual slope AD conversion? Essentially your PIC (hoping it has at least a comparator ...), plus a couple resistors and a cap. I think Microchip even has an app note about that. The resistors set the scale factor.

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Regards, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

Sorry, no.

And peltiers pretty much measure delta-T, not temperature... essentially they measure heat flow. Both ends would have to be pretty thermally stiff, and you'd have to know one temperature, for the peltier to measure the other temp meaningfully.

John

Reply to
John Larkin

I think I was fine with his usage. He used the term to assure the rest of us that he, himself, wasn't confused about the idea of accuracy vs precision. Which was wise.

I don't like your phrasing, but I won't bicker. Others can come to their own conclusions. But since you took issue, I'll use that as a foil to just put out a few words and let folks comment on their own understanding of each of them:

repeatability resolution precision accuracy correctness

Three of those can be considered independent concepts that can stand alone (the other two are dependent entirely upon developed relationships between particular pairs of those three.)

Jon

Reply to
Jon Kirwan

If you do want to measure a small temperature difference, Peltier junctions can be nice - the Johnson noise level is low, and you don't get into trouble with self-heating.

IIRR some calorimetry projects go further and use a feedback system to inject enough current into a second Peltier junction to compensate for the heat flowing the through sensing (zero current)Peltier junction at the measured temperature difference, as well as through the compensating Peltier junction.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

Sounds like you have been trying to invent a delta sigma converter. If your input is stable you can average out noise. The comparator input offset stability is probably the limiting factor.

Reply to
nospam

Not particularly clever, but that part has some rather crappy comparators built in (10mV Vos maximum).

You could make a slope converter using a resistor as an almost constant current source, a capacitor, and time transitions. Probably a few resistors, a cap, a 4053 or even a 2N7002 and that's about it.

I can't be bothered to think too much more about this, since it's such an artificial constraint-- almost like a classroom exercise.

Best regards, Spehro Pefhany

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"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

There is an application note about delta sigma AD conversion:

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I wonder if dual slope woul be possible with passive components (besides the PIC and the internal comperator), only. You do need at least one switch for switching the input voltage you want to measure?

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Frank Buss, http://www.frank-buss.de
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Reply to
Frank Buss

** see below

if you demand the Bible is perfect (or the equivalent)...

Reply to
Robert Baer

I mentioned temperature only in the context of the voltage measurement system stability.

Heat flow is exactly what I want to measure.

Both ends would have to be pretty

I believe the surfaces represent the proper measurement conditions. and you'd have to know one temperature, for the

Again, it's heat flow. I don't care about either temperature.

I've been considering using the peltier device to drive its own voltage to zero. The heat flow through the device should be a function of the energy input required to achieve zero open-circuit voltage...if I can successfully factor out the efficiency/self heating of the device. Thermal time constants are long enough that I should be able to multiplex the stimulus and measurement.

Reply to
mike

Thanks for the link, but I was hoping for something more clever. For example, I should be able to return the peltier device to the reference instead of ground. That way, the whole counter resolution can be spread over the maximum input voltage plus the comparator offset plus the loading effects on the reference. Maybe parallel two more outputs across the peltier device so I can PWM some energy into it to drive its voltage to zero. I haven't figured it out yet, but my gut tells me something interesting can be done with the 10-bits of the pwm/ccp module.

Reply to
mike

I knew that would be an issue. I agonized over it before changing the subject to "resolve...". I think "precision" works in this context as one could include the noise issues that degrade the precision from the number you'd expect to get from the resolution alone.

Reply to
mike

That is a very good one.

Can't find the Microchip example back, I think it was call PIC multimeter or something like that. Very simple, just the uC and some discretes.

--
Regards, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

PWM is the way to go, but your VCC for the uC must be super precise if you want to do that without any external parts. If it ain't then I'd consider a TLV431 or LMV431.

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Regards, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

That would work if you have a cal factor for heat flow versus current: alternately apply current and measure device voltage, servo the current until the voltage is zero. At that point, the delta-T across the peltier is zero, and you knew how much current it took to get there.

One problem is that the transient recovery from heat-pump mode to thermocouple mode will be slow and complex, so this would only work is the thermal masses are large and you're in no hurry.

But you'll still need an opamp or something to amplify the small thermoelectric voltage from the peltier. You should characterize that first, and decide what sensitivity you need to meet your accuracy target. We'd need numbers here.

If all you do is sense the sign of the peltier voltage, you'll have a slow bang-bang feedback loop. It might take hours or days to make an accurate measurement.

Digitizing would sure be better. The simplest thing would be to not drive the peltier, just digitize the peltier voltage and convert to heat flow. Use one of those cheap DVMs that has a serial interface?

John

Reply to
John Larkin

Maybe this one?

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They don't mention "dual slope", but of course it is the dual slope concept. But you can't measure an input voltage with it, only resistance, so something for converting small voltages to a resistance is required, maybe a FET?

--
Frank Buss, http://www.frank-buss.de
piano and more: http://www.youtube.com/user/frankbuss
Reply to
Frank Buss

It almost looks like you're looking for a kelvin bridge input?

Reply to
Jamie

You can also do it for voltage measurements, without needing an external mux. I can't find the Microchip note back but here is the one from NXP:

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--
Regards, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

Ok, but you'll need an internal mux :-) But this is possible with the PIC16C62X, too.

--
Frank Buss, http://www.frank-buss.de
piano and more: http://www.youtube.com/user/frankbuss
Reply to
Frank Buss

Ok, but so far the uCs I had to deal with had a mux in front of the comparator. A uC without one wouldn't be all that useful to an analog guy like me :-)

Essentially that's just about the only way to zero out the comparator input offset which is usually quite horrid in uC. Well, back to my new toy now, this one just arrived:

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So far (except for a rather serious amplitude setting glitch) it's been quite impressive.

--
Regards, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

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