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I don't think I need 24 bits. I will be returning to work tomorrow and need to call the person that needed this device to get more details.

This is going to be a hand held device that needs to generate a clean and high res output analog signal from a laptop's USB port. The laptop is going to define the generated data, also the unit needs 2 outputs and

This will be a QC calibration tool so we must make sure we are using a good DAC. I think I'll be using a ladder dac, I see issues using DS types.

Jamie

Hmm, if emission coefficients go as high as four then it is somehow different from the non-ideality factor which is between 1 and 2.

Grin, sure I'm all about models being close to the real physics.

For the single point calibration, I measured three diodes, Vf (at

George H.

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Yeah I've been using them near room temp. Diode laser and the permenant magnet for an NMR. The magnet has a nice trick, you leave the thermal loop open til you start to use it, and then you just ask the loop to keep the T whereever it is. And you adjust the nmr frequency to match the magnet. It harldy needs calibrated thermistors.

George H.

.highlandtechnology.com  jlarkin at highlandtechnology dot com

They go as high as needed for diodes. For BJTs, they are almost always very close to 1, at least with small signal types I'm more familiar with, testing out against hand calculuations. I think they are the same thing, as I've seen both terms used interchangeably and I don't know of another meaning for the term you are using.

I think you are just talking about this:

Ic = Is*(e^(Vbe/[n*k*T/q]) - 1)

With 'n' being your ideality thing. Elsewhere, that's just called the emission coefficient. You can see it called 'ideality' here, under the "Shockley diode equation" subheading:

But if you read that paragraph, you will also see it called what I call it, as well. So I really do think we are talking about the exact same thing. If not, you need to show me the equation you are talking about and the factor in it.

The emission coefficient isn't just about recombination, by the way. It hides several things under a single umbrella, including carrier generation.

And as if that wasn't enough, things are dramatically different at different currents, as well. At low currents, you have recombination of carriers AT THE SURFACE, which must be accounted for. You have recombination of carriers in the emitter base space charge layer. And you have the formation of emitter base surface channels. Each of these (only ideally, though again) do vary with Vbe and with different tau's and added to the other shockley equation. At higher currents the injection of minority carriers into the base region starts to become significant, relative to the majority carrier concentration. Since space charge neutrality is maintained, the total majority carrier concentration increases by the same amount. The non-ideality factor for this is usually taken to be 2 and is incorporated into the EM3 model (by Webster, I believe.) This becomes another one of those exponentials sitting as a divisor on Is.

As mentioned, Rube Goldberg would be jealous. The underlying physics is NOT modeled, but instead just higher level exponential math models applied until the behavior starts to look "mostly okay." Behavior in the corners is modeled very badly, for example, because the various exponentials don't reflect the actual shape there very well. It's good enough for horseshoes, though, and engineers. Not for physicists.

Well, the spice stuff isn't very physical. If you look at the differential equations you find that there are multiple exponentials. The -1 term in the Shockley equation just mentioned is there so that you calculate out a nice Ic=0 when Vbe=0. The exponentials are only approximations of a reality that includes MANY taus, not just one. And as you know well, there is no single exponential that can model the sum of multiple ones. And there are some non-exponentials that result from the integral equations that basically no one in the non-FAB land bothers to use. I got myself deep into this some time ago when attempting to model the behavior of a Hamamatsu diode against actual observation over differing die temperatures (from -70C to about 80C) and over light flux varying from femptoamps to microamps. The ONLY way I got results that were close was to go back to applying dopant concentrations, gradients, and a vague approximation (luckily not too hard with photodiode I can examine) of the physical dimensions. I had thought that a 1D model would be good enough. It wasn't.

I can see why spice doesn't go there.

Okay. So you had a calibrated diode to work with. I guess my question came from the fact that while the 1X/10X method provides some basic removal of the Is parameter for a given device, it does not (so far as my poor awareness permits me) provide an absolute position which probably has to come from somewhere external. Apparently, you had that. But if you had a method to develop an exact calibration down to absolute zero without any calibration, then I'd love to have learned of it. (So would a lot of people with expensive freeze points, I suppose.)

Jon

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I've thought of trying audio parts like this (for non-audio) but the specs always look weird, as does the interface, and I end up with more expensive "industrial" parts. Usually one is interested in what happens at DC which audio does not care about of course.

Got it Jon, Thanks for all that! Re: absolute T calibration. The only way I know to do that is by the Johnson noise. There is some group at NIST that is trying to do that. (OK I'm sure there are a slew of other fundamental ways to get T... but the noise method can get you ~1% with out too much sweat.)

George H.

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No matter what they claim, you're not going to get anywhere close to

It seems you've just scratched the surface of the real problem.

That's pretty fundamental. And it applies to the kTC noise in capacitors. I suspect that Johnson's success at applying Boltzmann's theory here may have given the mental analog to Shannon for his excellent paper (which is how I actually felt, finally, a better understanding of Boltzmann's work -- which wasn't nearly so easy to follow in my opinion.) I haven't read, but probably should, Johnson's work. I'd probably benefit from it. Shot noise (say, across a PN junction for example) would be another method, I suppose.

There is an interesting page from NIST on Johnson noise thermometry. But sadly, it's more for those already familiar with it and wanting to know the more recent advances, than those wanting to know good Johnson noise thermometry itself can be:

They claim 40 microKelvin per Kelvin combined uncertainty using AC Josephson quantized voltage noise sources and using a Johnson noise thermometer, at 693 K, or about 28mK at that point.

But I think their aim isn't to measure temperature, but to measure k to good accuracy.

How long do you take in using that method? I would guess from basic statistics guesswork on my part that it would "take a long time" in order that the standard deviation of the resulting noise measurements reaches a sufficient level that the measurement is well known. Uncertainty goes as sqrt(N). I imagine a good measurement takes a while. Not to mention the difficulty of separating out actual Johnson noise from other systematic noise sources in the rest of the measurement chain. And not to mention over what temperatures you might get values you can "see" in the end with much accuracy.

I found these with a quick google search, too:

There is an interesting paper called "Continuous Resistance Temperature Detector Calibration using Johnson Noise Thermometry." It points out some reasons why Johnson noise thermometry isn't widely used. (1) long cables have capacitance that alters the transmitted noise and the need to periodically measure the cable transfer function, (2) the thermal signal is small and easily contaminated by EM and microphonics, (3) a need for DSP to reject band limited noise (the shape of the Johnson noise is known a priori), (4) stability requirements for high gain, wide bandwidth.

But I'm not familiar with the technique, never having been around anyone doing it. So any experience you have here would be very interesting to read about.

Jon

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I don't think a modular plug and jack is going to cut it for even 16 bits for very long. For one they have rather low mating cycle ratings, like in the 100s. 3.5 mm "stereo" jacks used as differential pair with shield per channel would be better, but not a whole lot; still not long life. I really just don't know of a small, good life connector with the density you want.

types.

Well, back to the original problem, I looked up the chip I am using with a 100-ohm platinum RTD. It's a Maxim MAX1403, which has an 18-bit delta-sigma converter. It's plenty good enough for my application.

Joe Gwinn

The "24-bit" parts aren't actually 24-bit - usually closer to 20-bit - but they tend to deliver 24-bit data with the last few bits essentially random noise.

You can trade off fewer bits for higher conversion speeds, but for temperature measurement you rarely need speed,

Sigma-Delta DACs rely on a particularly sophisticated form of pulse-width modulation. Their advantage is that it's a lot easier to set up really accurate time intervals than it is to make really accurate resistor arrays.

The disadvantage is that you've got to filter the PWM modulated output to get rid of the ripple.

Hmm the PN diode is a bit of a pain to do shot noise with. It's doable, but as you forward bias it, it's resistance changes and that throws a bit of a monkey wrench into the measurment.

Doing shot noise with a photodiode is a lot easier. But you don't get the temperature from it. (In^2 = 2*e*Iave*bandwidth)

OK I know absolutely nothing about the QVNS part of that.

But nist is doing ~10^6 (or 10^5) type of measurements. (there is this paper too, arxiv.org/pdf/1101.0312)

So I'm guessing the QVNS gives them a way to calibrate the gain/ bandwidth of their signal chain.

idity/johnson-noise...

OK, the Johnson noise part is pretty easy to understand.

Vn^2 = 4*k*T*R*bandwidth. So you gain up a noise signal from a resistor. (with opamps and 0.1% resistors.) You can get the square of the voltage either by measuring a voltage time record and doing the multiplication in software, or with an analog multiplier. You then need to measure (or constrain) the bandwidth. Measuring the resistor value is pretty easy. And then you need the amplifier noise, which I do by reducing the input resistor to a few ohms.

Getting the bandwidth is the squishiest part. Our 'brute force' approach is to make the bandwidth flat out to ~1 MHz, and then use a two pole low pass at 100kHz and lower frequencies. This throws away ~90% of the noise signal.

(I think I read somewhere that the nist group has an 11 pole low pass that they somehow calibrate)

Re: the time to make a measurment. If the badnwidth is 100kHz, you can think of this as a new noise measurement every 10us. So in one second, you've made 10^5 measurments with an uncertainty of

So getting to 1% is not that hard, but that's 3 degree's K at room temp.

So the four points you listed are all relevant. The cable capacitance is perhaps the biggest constraint. Our original prototype had an active shield (bootstrap) that could reduce the cable C by ~90% or so*... but it was deemed 'a step too far' for the students to understand and was axed. The other thing I haven't mentioned is amplifier current noise... which we ignore at the start by using FET opamps.

I just got done testing on of these,

T = 295.93K expected noise density 1.634E-16 V^2/Hz

bandwidth measured density (Hz) (xE-16 V^2/Hz)

(errors from a 3 second measurment time.)

Not any better than the transistor connected diode and 10x current games.

George H. (* you could gain up the active shield bootstrap a bit to get a 'better' looking spectrum, but then the bootstrap gain knob could be subject to misuse... "how much noise do you want"?)

things

That's a pretty good price. What's the temperature accuracy?

We pay about $1 for an LM35 in TO-220, which is what I use on heat sinks. Output is linear, 10 millivolts per degree C. There are a couple of gotchas regarding oscillation and latchup.

For thermocouple reference junctions, I use a 1206 size 1K thinfilm platinum RTD, from Enercorp, about $2 in maybe 50 piece buys. They are very accurate.

Thanks, George. I'm keeping this for later, to think over.

The one question remaining to me is your statement, "Not any better than the transistor connected diode and 10x current games." We were discussing absolute T calibration, I thought. But the 1x/10x games are about knowing, a priori, the step size. Different things. I must remain confused, I guess.

Anything to enlighten that would help.

Thanks again, Jon

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Oh nothing mysterious, both are based on some physics equation that contains kT. With some electronics you can use either to measure kT to ~1%. I guess if you want to do better you can use the triple point of water as a reference... electronics and water never seem to mix to well though. (Hmmm.. I could make an ice bath in my dewar and try it, could I stick a resistor/diode in a deionized water ice bath and not have them 'wig out'?)

OK what other 'fundamental' ways are there to measure T? I'll start with; Thermocouples, resistance of metals, (and Pt in particular) speed of sound (in air) of an acoustic resonantor, ...others?

George H.

I read somewhere that well-stirred water and crushed ice, using most any tap water, gets within 15 mK of 0C.

I think the serious triple-point cells close a loop on electrical conductivity.

Most of the authorities talk about a "well-stirred" ice bath. If you've got a propellor-on-a-shaft type stirrer, you can stir the contents of your Dew ar flask easily enough, but most places I've been rely on Teflon coated mag nets spun by an external magnetic field, and those are hard to get spinning in a Dewar flask.

The latent heat of fusion of ice is quite high, and an ice-bath in a open b eaker lasts for quite a while.

propellor-on-a-shaft type stirrer, you can stir the contents of your Dewar flask easily enough, but most places I've been rely on Teflon coated magnets spun by an external magnetic field, and those are hard to get spinning in a Dewar flask.

beaker lasts for quite a while.

"well-stirred" refers to Martini's. I've just read a treatise comparing 30 second, two minute and three minute Martini stirs >:-}

I prefer "30 second", but then I like the flavor of Gin, Bombay Sapphire being my favorite. Longer stirs kill off the "ornamentals"

...Jim Thompson

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Oh I was more worried about the electrical conductivity (or something else?) of the water. The last time I tried sticking resistors and diodes in tap water it didn't work, I'm not sure why.

George H.