favorite temp sensors

everything else for dead at +/-0.2C (or better, if you are prepared to pay more).

digital number, which is going to be a non-linear function of temperature

then you have to cope with the temperature dependence of the reactive element.

information. You've just claimed to know a "better" way (probably incorrectly),

Better way posted. Your way is asinine.

gratuitous personal abuse. Definitely an exhibition of psychopathology, >though you've skipped the criminal aspect on this particular occasion. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Seiko has some cheap ones, too.

Is this accurate to +/- 0.2'C (without calibration)?

--

John Devereux

s:

     ...Jim Thompson

Not in my experience. (but my experience is fiarly limited... a few transitors tested.)

I always got a number that was a bit off ~0.3%, so about 1 degree at room temp. I always assumed the error was due to the transistor beta... Since the current is Ic and Ib. (I think I got a temperature that was always a bit high, but I'd have to check my notebook.) You could add some beta 'fudge factor'.... but then beta changes with temperature too.

It also depended a bit on the collector current. (1 uA to 10uA were 'nice' currents)

George H.

So midst the noise I've seen the following mentioned:

LM71 DS18B20 LM75 AD7414

Of those, only a few seem to come in non-surface mount packages and I'm not sure how we'd couple surface mount devices to the copper cooling lines.

Given that we just want to measure the coolent temperature on both sides of the liquid-cooled heatsink, and not reinvent anything [1]; it looks like the DS18B20 may do the trick.

It's 2-4X the price of LM35's but still affordable.

Thanks for the suggestions.

1] Or rather "we are inventing enough on the product; we don't need to do so on the test instrumentation too..."

--
A host is a host from coast to coast.................wb8foz@nrk.com 
& no one will talk to a host that's close........[v].(301) 56-LINUX 
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is busy, hung or dead....................................20915-1433

Here it is done at the device-level on one of my chips...

Of course you can do it with off-the-shelf OpAmps and analog switches, which is what the devices are all about. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

If you use a large device it should be. For those of you in the discrete world keeping the current ratio firmly at 10:1 (or whatever ratio you choose) will be your biggest problem.

I think Jim Williams wrote this up a long time ago except he used resistors instead of current mirrors. I have two of his books, but didn't find it. Maybe an EDN article? Or maybe it was Bob Pease?? ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

[about thermometry]

It's an absolute-temperature measurement, so +/- 0.2 C implies better than a part-per-thousand error band. Voltage references and resistor values for the current pulse would have to be better than 0.1%, so probably the answer is NO. The only way to get components with that accuracy is with trimming, and that means ALL qualified sensors are dependent on calibration or trimming. You can choose which, but you must pay, either way.

Calibration after soldering is better than trimming before soldering.

On Friday, April 12, 2013 9:27:51 AM UTC-7, David Lesher wrote: [about temperature measurement, with mention of 0.2C accuracy]

If you want the difference to be accurately measured, that's actually an ideal place to put in a thermocouple, or several thermocouples in series. If you use a single batch of thermocouple wire, it'll have a good zero-temp-difference characteristic guaranteed by design. Input pipe is hot junction, output pipe is cold junction, and an autozero op amp can condition the signal easily.

Which section of your specification was that in? ;-)

A published source on this is found here:

Starting on top of page 7.

...

I take George's recent experiences using these seriously, though. I'll add a short note to his writing on the topic.

Jon

Thanks! So I did remember correctly that it was a Jim Williams' scheme! And stored it mentally, then redid it with current mirrors instead of analog switches and resistors...

in a custom chip...

...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Cool, if you can't afford a FLIR.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation

Hi, George. I posted up a link to Linear's AN45 elsewhere under this topic. See page 7 there. But I take your experiences here seriously and wanted to think about this, not at the 'charged gas' theory level but at the higher (and more usual for an EE) device modeling level.

The two-current pulse method, using say 1X and 10X currents, depends upon the following:

dV = (k/q) * ln( 1+Ic/Is ) dT

Although k and q are known, the entire factor that includes the ln( 1+Ic/Is ) part isn't knowable in advance. But if one assumes that the +1 term is negligible then the two pulsed currents results in:

dV1/dT = (k/q) * ln( Ic1 ) - (k/q) * ln( Is ) dV2/dT = (k/q) * ln( Ic2 ) - (k/q) * ln( Is )

Subtracting dV1/dT from dV2/dT yields:

dV1 - dV1 = (k/q) * ln( Ic2/Ic1 ) ) dT

And if the ratio of Ic2/Ic1 is known a priori then the entire factor, (k/q) * ln( Ic2/Ic1 ) ), is also known. And as a consequence could be used to measure temperature without having to calibrate the system. Or so it seems at first blush.

But it's also the case that the value of the saturation current, Is, is itself a rather complex function of T:

Is(T) = Is(Tn) * (T/Tn)^3 * e^( -(q*Eg/k) * (1/T-1/Tn) )

Where Tn is some chosen T(nominal).

In fact, this particular component is what overwhelms the first equation (which is positive vs temperature) and yields the usually quoted -2mV/K figure (very approximately.) So, in fact, Is(T) is the dominant factor in Vbe change over T and in no possible way is it a simple function of T!

(Even the above Is(T) equation itself is a simplification. The power ((T/Tn)^3) for example is an approximation and not strictly true in practice. Same with Eg, which itself is also taken as a single approximation value.)

Just as a guess, the idea of ln( Is ) being cancelled entirely out of the equation by ratiometry, even assuming that the die is at thermal equilibrium, would make me worry a little. (I accept that pulsing the 1X/10X current change fast enough or that using low enough currents, like the 1uA and

10uA you mention, would yield a near-equilibrium state.) I'm just not sure that at this level of modeling, that _Is_ remains dead stable as a modeling parameter when facing a 10X current change. There is a lot of linearity over orders of magnitude change, as a broad statement. But exactly how linear is it when provided a two point ratio a decade apart, vs device variation?

I wonder that some temperature error is swept under this Is(T) rug and hidden from the analysis, so to speak. Even assuming thermal equilibrium. Because it may really be that Is(V,T), not Is(T), as both the power (^3) and Eg are taken as simple constants for simplification when they aren't, in fact, invariant at this level of modeling.

You mention base currents as a possible error. I've ignored that so far. The equation:

dV = (k/q) * ln( 1+Ic/Is ) dT

in the diode connected case refers to Ic. The currents through it, on whole, are (beta+1)/beta times as much. If you cobble up precision current sources at exactly 1X and 10X, the ratio of Ic2/Ic2 would still be 10, even though you are driving Ie, I think. However, beta itself changes vs Ic. So there is that to account for, if you were only using a beta level model. But the:

dV1 - dV1 = (k/q) * ln( Ic2/Ic1 ) ) dT

method doesn't use or rely upon beta. So I'm not imagining a problem there because (1) the ratio is still 10X and (2) beta isn't used in the analysis method.

Interesting problem getting past a certain level of accuracy, though. There must be several papers that go beyond the AN45 app note I'd posted up earlier. I haven't seen one, yet.

Jon

Did you see my post that I bought the one you pointed out? It is due here this afternoon sometime.

John S

But accurate, and precise. Your idea of "better" needs work.

--
Bill Sloman, Sydney

Whenever is a thermistor "precise" ?? ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Thermally conductive adhesive? The AD7414 data sheet mentions this on part

2. formatting link

If the copper inlet and outlet pipes run close together - or can be made to run close together, you might mount two AD7414 parts on a flexible printed circuit, and glue the back of one to the inlet pipe and the back of teh ot her to the outlet pipe. You'd separate the two AD7414 parts on the flexy by more than the worst-case separation of the two bits of pipe to which you w ere going to glue each one (with thermally conductive epoxy) so a to leave a bow in the flexy to accommodate any movement between the two points.

And you'd have to do a zero-point calibration when the heat-sink wasn't dis sipating any power. The temperature error data on page 15 of the data sheet is a bit depressing. Interchangable thermistors can be quite a bit more ac curate - put two (one on each pipe) in a bridge, and AC-drive with a bifila r wound centre-tapped transformer to set up a Blumlein bridge.

umlein-bridge.html

--
Bill Sloman, Sydney

Both LM35 (analog) and LM71 (digital) types could have problems at the ends of long cables. LM35s love to oscillate if they see much load capacitance, and any induced noise can make them do weird things. SPI would of cource have to be handled carefully at the end of a cable.

RTDs and thermocouples are better for field temperature measurements.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation