Experiments with Vref Ovens

[enut11]

There are lots of hits on the Forum if I search for 'ovens' or 'voltage Refs' or both and I will be spending some time catching up on the latest info.
Meanwhile I would like to post my recent experiments on a relatively cheap approach that I am using to improve my own Vrefs.

A friend once said, referring to the study of Voltage References, “temperature is everything”. Granted it is not that simple and other things can come into play but the thought has stayed with me ever since.
A couple of years ago, when the Aussie Cal Club was active, I built several LM399 based voltage references and left them powered up to condition the components. They did soak for over a year until my office got flooded from a leaking water pipe. All my gear had to be removed and the whole mess took over 3 months to repair. That was more than 12 months ago.

I recently dug out all my references from storage and re-powered them. Then, inspired to improve the outcome, I decided to experiment with a controlled temperature oven. The oven controller/heater design, also from that same friend, was a simple bridge circuit monitored by an opamp. One of the bridge legs is a thermistor. The opamp drives a power transistor to control the heating element, all standard stuff.

The controller pcb and Ref pcb are mounted inside a small metal box which in turn sits inside a larger insulated foam box.
The idea is to keep the set temperature as stable as possible to isolate the Ref from ambient variations.

This was most important for this project as my Ref pcb used garden variety components to reduce costs. The thought being that low tempco components would not be necessary if the temperature does not vary!
More to come…
enut11

[enut11]

The setup for measuring Ref oven temperature vs ambient consists of a Laptop PC connected to a HP 3456A DMM via USB-GPIB.
Ambient and oven temperature data is fed through a home made 2-input scanner to the 3456A in 2W/Th mode.

Because XL wants to 'join-the-dots', the temperature plot comes out as a green band.
The upper part of the green band is internal oven and the lower part is ambient temperature.

[enut11]

The first oven controller was built on veroboard and installed, along with the Ref pcb, in a 100x100x60mm diecast metal box. The diecast box was insulated from ambient inside a small polystyrene ‘broccoli box’ sourced from the local vege market.  I have since used several of these foam boxes as they are very easy to work with and cost nothing.

First 24hrs data from this prototype showed a nice flat profile for the oven temperature along with variable ambient so things were looking good.

[enut11]

I was curious about the stability of the oven so took a closer look.

Direct data over a shorter period of time showed an interesting oscillating temperature pattern.
The variation of around 0.15C peak to peak was thought to be caused by poor thermal coupling between the controller sensing thermistor and the heating element.
Persistent overshoot and undershoot of the set point was not desirable and had to be investigated.

[enut11]

Oven C was not suitable for the mods I had in mind so I built another oven in a smaller diecast box, about 1/3 the size, just big enough to hold the controller (now on a proper pcb) and the Vref.
In this build I located the controller sensing thermistor closer to the heating element.
A 24 hour plot of oven temperature vs ambient temperature again shows a nice flat upper area with a variable lower part. Even at this stage it looked better than the first run.
The gap in the plot was due to loss of power to the scanner for a short time.

[enut11]

A closer look at Oven A temperature over a shorter period showed no signs of oscillations. Indeed, the oven temperature profile was virtually flat with max variation of 0.01C.
Now that is not to say that it will stay that close to the set point over 24hrs but much better than the first oven.

[TimFox]

I'm unfamiliar with the term "broccoli box":  could you give the rough dimensions?

[enut11]

Good question @TimFox. I was about to show some more pix of the oven materials. The foam boxes measure 38cm long x 28cm wide and height can be 15-20cm. Wall thickness is 1.5cm.
It is easy to cut out holes for gauges, etc using a sharp razor knife.

For additional insulation I have used polystyrene, foam rubber, packing foam and roof/ceiling insulation bats. They all work. The only difference that I have noticed is the oven idle current varies depending on the effectiveness of the insulation.

For smaller projects, I have cut the boxes down and glued the parts together with white glue or epoxy glue. Some glues will melt foam so beware.

[enut11]

For the oven, I use the smallest diecast metal box that fits my Vref and controller boards. This box measures 11cm x 6cm x 3cm.
I don't use those bulky banana sockets inside the oven as they take up too much space. Instead I wire directly from the PCBs to the front panel sockets mounted outside the foam box.

The Ref pcb is 5x3cm and the controller 6x4cm and both fit comfortably inside the metal box along with a string of twelve 2W 100ohm resistors configured as a 33ohm heater plus some insulation to prevent circulating air currents.

[enut11]

One of the things I had to decide on was the minimum temperature to run the oven at.

I mounted a powered voltage reference inside a foam box with insulation and monitored the temperature rise using a thermistor inside the metal box.
On a warm day here (27C inside my office) the metal box heated up to 39C just from heat generated by the voltage reference!
Hence, I now set the oven controller temperature to 40-41C. Not a big margin but it has been OK so far.

I monitor the controller heater current using an ammeter and can see if the oven shuts off by itself when ambient temperature rises.

[enut11]

I now prefer an analog meter to monitor heater current - easier to read at a glance. This pic shows the oven has settled at about 30mA (100mA FSD) with an ambient temperature of 20C.

The oven controller was set for 41C but the digital meter shows a slightly lower temperature because the sensor is outside the metal box in the oven.

Other front panel sockets give access to an internal 5K thermistor for data feed, regulated 15v input for Vref, Vz out and 10v out.

15v power supplies for vRef, oven controller and digital thermometer are all external so as not to interfere with the inner oven environment.

As an aside, I found the digital thermometer will also run happily from a usb 5v supply.

[enut11]

24hr oven internal temp vs ambient for a third Ref oven (Oven B) has just completed. Once again the oven temp is very flat compared to the significant changes in ambient temperature. Very happy with the end result so will continue to condition the Vref boards inside the 3 ovens.

The biggest challenge now is how to quantify the benefits of a stable Vref temperature environment but that is another story.
enut11

[dietert1]

When you have two ovens each one with its own voltage reference, it's more fun. Then you can measure the voltage difference (to 100 nV in the lowest 3456A range) as a function of the oven temperature while keeping the other temperature constant. You will need to use Low Thermal EMF connections to get that precision. Then you can determine TC of each reference as a function of temperature, find a good operating temperature where the curve is near flat, determine reference noise etc.

Regards, Dieter

[exe]

What thermistor do you use?

Do "typical" NTC thermistors drift over time?

[enut11]

When you have two ovens each one with its own voltage reference, it's more fun. Then you can measure the voltage difference (to 100 nV in the lowest 3456A range) as a function of the oven temperature while keeping the other temperature constant. You will need to use Low Thermal EMF connections to get that precision. Then you can determine TC of each reference as a function of temperature, find a good operating temperature where the curve is near flat, determine reference noise etc.

Regards, Dieter

@dietert1, this does sound interesting! More info please. This will be a new learning for me.

I am lucky enough to have 3 good meters (3456A, 34401A, 34461A) but they are all based on LM399 internal reference.
In effect, measuring an LM399 with an LM399 does not sound practical?
enut11

[TimFox]

If the two references are close to the same voltage, then measuring the difference between the two voltages (to the resolution of the lowest full-scale range) will have more resolution than measuring the two voltages individually.  The internal reference of the voltmeter will have almost no effect on the result, since it will proportionally affect the (small) difference voltage.

[enut11]

What thermistor do you use?

Do "typical" NTC thermistors drift over time?

@exe, you did not specify which thermistor so I assume the feedback in the controller bridge circuit.
This is a thin film 10k NTC available on eBay
Search for "NTC10K Film Thermistor Ultra Thin Temperature Sensor ". They are used in 3D printers.
I purchased the one with the orange film cover.

Do "typical" NTC thermistors drift over time?
I don't know. Maybe others on the Forum can advise on this.
enut11

[exe]

Search for "NTC10K Film Thermistor Ultra Thin Temperature Sensor ". They are used in 3D printers.
I purchased the one with the orange film cover.

That's what I meant

[enut11]

I use a squashed down lug connector. Insert the thermistor tip and set with thermal paste.

I have also tried bead thermistors but they are not as quick to respond to temperature changes.

[enut11]

When you have two ovens each one with its own voltage reference, it's more fun. Then you can measure the voltage difference (to 100 nV in the lowest 3456A range) as a function of the oven temperature while keeping the other temperature constant. You will need to use Low Thermal EMF connections to get that precision. Then you can determine TC of each reference as a function of temperature, find a good operating temperature where the curve is near flat, determine reference noise etc.

Regards, Dieter

@dietert1, this does sound interesting! More info please. This will be a new learning for me.

I am lucky enough to have 3 good meters (3456A, 34401A, 34461A) but they are all based on LM399 internal reference.
In effect, measuring an LM399 with an LM399 does not sound practical?
enut11

I forgot to mention that I also have a CROPICO 1.00000v ESC1 Standard Cell so this may be useful in some way to verify my ovenised 399 references.
However, all my gear is un-calibrated.

[doktor pyta]

these thermistors are most likely manufactured by this company:
https://tewa-sensors.com/products/

[antintedo]

Do "typical" NTC thermistors drift over time?
I don't know. Maybe others on the Forum can advise on this.

Yes, that's why glass encapsulated thermistors exist. Manufacturers rarely publish this information, especially for cheap stuff.
Some info I found while selecting sensors for a TEC oven:
http://www.farnell.com/datasheets/169207.pdf
https://ieeexplore.ieee.org/document/7281460
https://www.eevblog.com/forum/metrology/long-term-stability-of-temperature-sensors/
Ysi 55031 glass beads are available from Farnell - that's what I eventually got.

these thermistors are most likely manufactured by this company:
https://tewa-sensors.com/products/

Unlikely since it was purchased on ebay from Chinese seller. Plenty of local companies produce identical looking sensors.

[essele]

I've been experimenting with ovens for a few months now, this is my latest incarnation...



It uses a PTC (pair) and an ADS1247 ADC and then generates an analogue heater control voltage using two of the current DAC's in the PSoC which then drive 6 different transistor/resistor heaters around the board.

It's intended to support a daughter board with a large number of parallel REF102's (as per the paper that's floating around that I can never find when I need it), so the rest of the components are power supplies and some options for buffering and filtering to experiment with.

It's all in a small(ish) die-cast enclosure, but I've had to increase the size of it to cope with the (future) power dissipation of the REF102's. I had some good results with air. I tried adding a circulation fan, but that made things incredibly noisy (electrically), and now I'm doing some experiments with mineral oil.

Without crazy tuning I'm typically seeing temperatures within about 20mK (of 44C) measured on an external DMM and the thermistor you can see in the picture, this is with overnight ambient variations of 5 to 6 degrees. However I am starting to doubt the accuracy of that as a validation mechanism -- I'm fairly sure my DMM isn't great with temperature changes, and I'll definitely have some thermal emf in the test setup. I'm able to keep within four or five mK if you believe the ADC results.

I gave up on PID as a control method quite some time ago, I now have a focus on the first and second derivatives of temperature and am getting much better results. At the moment I'm specifically working on trying to improve recovery from upsets ... which can be quite slow in oil!

Lots of things still to do:

- I've built a TMP117 module for use as an external sensor to pre-warn of ambient changes to get a head start before the ADC detects it.
- The ADC is noisy and I have no hardware filtering, so I need to do something about that. (Software filtering has helped a lot though)
- My test rig isn't particularly well sealed because of the cables, I'll probably add another layer of insulation.
- The heaters presumably heat the PCB and the PTC's are directly on the PCB so I think that's skewing results, ideally I'll put the PTC's on the daughterboard, but that needs a bigger redesign. May just add some more slots for the next revision.
- There are a few other board tidies to do (as you can see I missed the Vref cap and connections completely on the ADS1247!)

Very keen to get any input ... and happy to share schematics/code if anyone is interested (neither of which are tidy at the moment!)

EDIT: I've added a photo of the daughterboard if anyone is interested. It's double sided and can take 120 x REF102's. It needs a redesign and I need to be comfortable everything is looking good before I shell out hundreds of pounds on REF102's!

[enut11]

@essele, quite an impressive setup and much more sophisticated than mine. Where is the oven heater?
Also, is the pcb with a large number of REF102s designed to condition the chips or for another purpose?
enut11

[essele]

Hi enut11,

The heaters are Q1 to Q6 and the pairs of resistors near them. I went for 6 distributed around the board, it meant they didn't need to be too chunky and I assumed being distributed would help even things about a bit.

The REF102 board is intended as a massively parallel set of REF102's to reduce noise and improve stability. I was inspired by this...

https://www.researchgate.net/publication/305800682_Eleven_years_of_monitoring_an_ultra-stable_10_V_zener-based_voltage_standard

... and I know it's not really a cost effective thing to do (120 REF102's don't come cheap), but they did seem to get quite impressive results and it felt look a good mad hobby project ;-)

I've added another layer of insulation now and it's actually reduced the heater power needed by quite a bit, a good test for my algorithm, but actually a problem as I'm not going to have much headroom over the heat generation of the REF102's!

[Andreas]

EDIT: I've added a photo of the daughterboard if anyone is interested. It's double sided and can take 120 x REF102's.

I fear you will have to increase the setpoint temperature when the 120 REF102 are heating (about 2W additional power).
I have ~30 references in my ageing box and temperature without "heater on" is 30 - 40 deg C depending on environment temperature.

with best regards

Andreas.

[essele]

Hi Andreas,

Yes, I think you are right. At the moment, with two layers of insulation and an ambient of around 22.8C, I'm only needing 1.4W to sustain 44C so I don't think I'm realistically going to be able to put more than 60 or so in at the current set point.

The other option would be to reduce the insulation again and use the external temp sensor for better control ... at the moment I'm just using the internal ADC results, I'm trying to get that as good as I can before I introduce additional inputs.

Lee.

[dietert1]

Thanks for interesting link. Several things come to mind.
a) Operate the array of plastic references in a controlled humidity environment.
b) For the physical arrangement there should be a massive heat conductor in touch with each reference that keeps the temperature of all references similar. Otherwise you will have ever changing thermal EMF in the system and that is difficult to fix later. Maybe an aluminum carrier PCB.
c) There should be redundant temperature monitoring in case something fails. When you read that report, the biggest shift happened when the reference had to be repaired. The repair took nine months, strange. I also had that problem with a reference to reproduce the previous temperature precisely.
d) The heat problem can be solved using a TEC to keep the alu box cool and at a constant temperature.
e) Statistics roughly says that averaging 100 references you get a factor 10 better than the base unit. So when building such a beast i would include a MUX. Not only for groups as in the linked report, but for each single device, similar to what Andreas did. Then you can make something like a Maxwell demon to sort things out a little. Depends on what you want to do with your reference.
f) Get access to a national reference lab...

Regards, Dieter

[essele]

Thanks Dieter ... I have a plan for some of these (which may of course be misguided!)

(a) That was the theory behind the oil ... I'm assuming that if everything is immersed in oil then there's very little chance of humidity changes affecting things ... I am completely guessing though, so keen to understand more.
(b) Again I was hoping the oil would help to do this.
(c) I have the capability to add I2C sensors pretty easily, however I'm not sure what failure scenario wouldn't be detectable with the ADC. Any major failure should cut power to the heaters anyway. But I will give this one some more thought.
(d) I have thought about this, but was trying to avoid it.
(e) I have worried about detecting misbehaving references, but I don't think muxing them individually is practical. I'm going to test each one prior to soldering and then just hope. I am also thinking about making it a bit more modular, which will help a little.
(f) Yes, that would be great! Do you know anyone who'd let me into one here in the UK? 

Lee.

[essele]

Validating your oven temp can be a difficult challenge ...

I've been seeing quite some swings on my thermistor measurements (this is just for validation, not part of the control loop) in the order of several tens of mK, at the same time my ADC measurements have typically been within +/- 4mK ... so I've just put in a four wire PT1000 as an additional measurement.

During the same 30 minute period I saw the following results...

This is the PT1000 (measured on a DMM7510) in four wire mode...


This is a 5k thermistor (measured on a 34470A)...


Quite mad ... there's presumably some level of equilibrium going on (with the PT1000, which was new into the device) but the reality is that the temperature should have reduced (because I removed the lid to put the PT1000 in) and then should have been increasing again one the lid was back on.

So I believe the PT1000 at this point, but I'm going to continue watching both of them for a while!

EDIT: Err ... user error. Both devices weren't particularly well fixed and the oil level was slightly low so I think I was getting a bit of a hybrid oil/air temp. Now that they are better fixed they actually match each other reasonably well ... I'll still keen an eye on them for a while.

[dietert1]

Sorry i did not read about the oil before.
Adding the PT1000 is exactly what i meant by redundant temperature measurement. In case one of your sensors fails, you will be able to fix the device without loosing calibration. And it's a good idea to measure the sensors with calibrated instruments, i mean outside the temperature control circuit.
In my failed Geller reference (AD587) i only have one NTC within an analog control loop and i am not sure whether i will ever get the same temperature again. Don't know yet what failed...

Regards, Dieter

[essele]

So I've been logging data for part of the day, unfortunately (or maybe fortunately) I'm also doing my day job so the door has been open and closed and the room heating and cooling a little bit.

There are three graphs in the attached image:

Top: ktemp -- this is a 4wire PTC connected to an external DMM, it's in the oil very close to the onboard PTC's.
Middle: ambient -- this is the room ambient temperature, very near the oven (I also have humidity and pressure data but it doesn't seem that relevant to this)
Bottom: bbtemp -- the temperature as calculated by the ADC using the baseboard PTC's and controlled by the heaters.

I've been battling with why I'm seeing several 10's of mK variation in my validation temps whereas the onboard ADC seems to be within a few mK. In the attached graphs you can see that ambient swings roughly 2 degrees (from 25.5 to 27.5), at the same time the external validation swings about 50mK (~ 44.66 to 44.71) but the ADC measurements only move by about 5mK (43.999 to 44.004)

The graphs now make me think that the 50mK variations are actually caused by the negative temperature coefficient of the external DMM (a new (to me) DMM7510 in this case), the specs say 0.003 degrees/degree TC (but when when outside of +/-5 from Tcal, which I am outside of in this sauna!) Interestingly it doesn't quote an accuracy, only a "resolution" which is 0.01 degrees ... not sure I understand what to take from that!

So my thinking at this point is that my ADC seems to be the most accurate measurement (especially since it is itself in the oven) and the system is working really quite well ... so therefore I can believe my ADC numbers and go back to focusing on the control algorithm, and perhaps adding some hardware filtering in the next board revision.

Does this theory sound reasonable? Or am I missing something?

Thanks,

Lee.

[dietert1]

50 mK * 0,39 %/K of the PT sensor would correspond to 200 ppm/K. Difficult to believe a DM7510 has such a problem.

I observed something similar with my LTFLU ovens. In the bottom, were the flat cable enters, i have an NTC inside and the TEC outside of the alu box to get a fast response for the TEC controller. With the Arroyo TEC controller i can keep that NTC to some mK.

But then a second temperature measurement from the LTFLU reference chip itself (TC of transistor Ube) yields a temperature offset and temperature variations correlated with ambient. This is feed-through from the other sides of the alu box. I can use that signal for a second control loop and fine-tune the reference chip temperature variations down to better than +/- 0.2 mK typical. That works with a GPIB DAC pulling the temperature measurement of the TEC controller by very small amounts. That second loop is slower than the first one since there is an air gap between the alu box and the reference. At the same time that air gap makes a low pass filter to keep rapid ambient temperature variations away from the reference.

Regards, Dieter

[essele]

Wow ... 0.2mK .. very impressive.

I had originally thought my issue was caused by some kind of feed-through, however it's negative TC is the thing that doesn't make sense to me! The negative TC wasn't obvious until I graphed it against ambient.

I don't think it's an "issue" with the meter because I had (what I think was) exactly the same behaviour with a 34470A.

Perhaps I'll rerun the logging using 4W resistance mode and then post-process the data, that way the spec should be a bit clearer.

It's quite frustrating ... I'm building the oven so I have something at a constant temperature so I can better understand TC in the meters and eliminate it from the reference, but I don't have anything that's at a constant temp so I can check the oven is ok! I'm jealous of the man with two clocks!

[IRFP]

Hello,
this is what I use for my measurements 

https://www.eevblog.com/forum/metrology/manufacture-an-oven-for-measurements-in-a-temperature-controlled-environment/

[enut11]

Hi @IRFP. Interesting design and able to condition much larger projects.

Do you have any temperature/time charts of it working?
enut11

[IRFP]

Hello,
The project is recent, it is in the evaluation and adjustment phase.
Cordially

[enut11]

When you have two ovens each one with its own voltage reference, it's more fun. Then you can measure the voltage difference (to 100 nV in the lowest 3456A range) as a function of the oven temperature while keeping the other temperature constant. You will need to use Low Thermal EMF connections to get that precision. Then you can determine TC of each reference as a function of temperature, find a good operating temperature where the curve is near flat, determine reference noise etc.

Regards, Dieter

Is it also possible to determine Vref noise when two 10v refs are connected opposed and measuring the difference on a 6.5 DVM? If so, what noise component is it?

[MegaVolt]

Is it also possible to determine Vref noise when two 10v refs are connected opposed and measuring the difference on a 6.5 DVM? If so, what noise component is it?

If we assume that the noise is Gaussian and the same for both devices, then the measured noise will be equal to the noise of each multiplied by the root of 2.

[dietert1]

A standard method with voltage references seems to be noise measurement in a 0.1 Hz to 10 Hz frequency range using a large capacitor instead of a second reference, and with a special low noise amplifier. That method yields numbers that can be compared. You can find several threads here on how to do it. Like one paralleling 16x OPA140 OpAmp channels.
What you want in the end is something predictable for months or years, not seconds. And that can't be checked with a capacitor, unless that capacitor is very special, ovenized... Better use a second reference to compare to.
For me, a DVM difference measurement resembles the intended application most. Starting with sampling intervals of 10 seconds and averaging at different time scales. Of course, everybody is tempted to look for a separation between drift and noise. Yet even after months you won't know whether an observed change was permanent or temporary (e.g. seasonal changes).

Regards, Dieter

[enut11]

Is it also possible to determine Vref noise when two 10v refs are connected opposed and measuring the difference on a 6.5 DVM? If so, what noise component is it?

If we assume that the noise is Gaussian and the same for both devices, then the measured noise will be equal to the noise of each multiplied by the root of 2.

Is that the sum of the 2 noise components times sqr root of 2?

ie (RMSnoiseRef#1 + RMSnoiseRef#2) x 1.414

[trobbins]

If any control/power/sensing cables are likely to provide significant thermal conduction through any insulation barrier and to a location that is being temperature controlled, then using the minimum practical gauge wire, and/or adding some cable loop length inside the insulated domain, may be worth checking.  As per standard thermocouple mounting practise.

[MiDi]

Is it also possible to determine Vref noise when two 10v refs are connected opposed and measuring the difference on a 6.5 DVM? If so, what noise component is it?

If we assume that the noise is Gaussian and the same for both devices, then the measured noise will be equal to the noise of each multiplied by the root of 2.

Is that the sum of the 2 noise components times sqr root of 2?

ie (RMSnoiseRef#1 + RMSnoiseRef#2) x 1.414

No, it is RMSnoiseRef#1 = RMSnoiseRef#2 = RMSnoise|| × sqrt(2).
This only applies if both have same noise figure, otherwise it will give false results.

[MegaVolt]

,ie (RMSnoiseRef#1 + RMSnoiseRef#2) x 1.414

Summ = Sqrt((RMSnoiseRef#1)^2+(RMSnoiseRef#2)^2);

if RMSnoiseRef#1=RMSnoiseRef#2

Summ = Sqrt((RMSnoiseRef#1)^2+(RMSnoiseRef#1)^2)=Sqrt(2*(RMSnoiseRef#1)^2)=RMSnoiseRef#1*sqrt(2);

This is one of the typical noise estimation methods of different standards. For example, for crystal oscillators, two generators are multiplied. For voltage sources, noise is observed between two identical ones.

Assuming that the sources are the same, we can assume that the noise that they produce is the same. Therefore, the formula is correct.

[dietert1]

Just finished some noise test with an Arroyo Instruments TecPak 585 and a VREF oven, maybe the post fits here.

Regards, Dieter

[MK]

What often causes problems for measuring and maintaining a very stable temperature is the self heating of the thermistor.
One product I worked on many years ago maintained the temperature of a water bath to 0.002 degrees variation over any week of interest. The glass encapsulated ones are very stable.

Another product I worked on oscillated when I inherited it, but dropping the bridge supply rails cured that as the self heating issue went away.

[dietert1]

Thanks for the proposal to use a bridge circuit to improve this measurement.

Meanwhile more experiments have shown that the HP 3456A used in the above tests has a TC of 0.54 ppm/K in its +/- 12 KOhm range. This is the TC of its reference resistor. This was determined by replacing the thermistor by SFernice foil resistors (10K2 + 1K3) inside the oven and waiting for some days of initial drift. When measuring the oven temperature using that HP 3456A with a 10K thermistor, the DVMs own TC contributes a temperature dependent error of 12,4 uK oven/K ambient (factor 11500 Ohm/500 Ohm more). When using the DVM for an outer control loop as described in my little report this effect remains hidden, but must not be forgotten. It results in much larger fluctuations than the 1.6 uK rms noise limit observed.

Next i will try to determine the own TC of the Arroyo TecPak 585. Will probably put it into an incubator and use another TecPak to run the oven.

Regards, Dieter

[dietert1]

Meanwhile there is an observation of an Arroyo TecPak 585, regarding the resistance measurement by that unit. It turned out to be worse than expected. The measurement of a very stable ovenized 11K5 resistor exhibited drift of about 140 ppm within some weeks. Would be interesting to find out which one is the reference resistor inside the TecPak and maybe replace it by something better.

There has been more info on reference ovens in the LTFLU thread, e.g. here: https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg2637174/#msg2637174 and recently calculations of oven power requirements: https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg3429710/#msg3429710.

Regards, Dieter

[iMo]

..
There has been more info on reference ovens in the LTFLU thread, e.g. here: https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg2637174/#msg2637174 and recently calculations of oven power requirements: https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg3429710/#msg3429710.

Regards, Dieter

Added the xls file there for the heat transfer estimation calculation..

[dietert1]

Yesterday i tested the experimental TEC oven driver shown in the image.
It has a Nucleo STM32L432 module, an ADS1256 ADC module, a linear power supply for the output stage with 4x IRF540 and a 0.235 Ohm shunt. The SIC473 buck converter adapts the supply voltage according to TEC output current, so the lower mosfets of the bridge stay cool and work in linear mode. The design TEC current is +/- 2 A.
The output stage is controlled by a DG4053 MUX for heat/cool switching and an OPA140. Hidden below the ADC module is another MUX for voltage reversal of the thermistor and its reference resistor (blue, 10K2, 5 ppm/K). Also there is a DAC8551 for TEC current control.
The thermometer with voltage reversal gives one measurement per second at a noise level of 10 uK (standard deviation). Noise of the resistance measurement is 0.4 ppm.
So i connected a little oven and implemented configurable PID control. Gains were adjusted by hand. They are pretty high and the first diagram shows the damped oscillation (47 s period). At steady state the effectve gain is less due to thermometer noise and TEC current digitization noise. The diagrams show temperature and TEC current logs at two different time scales. Oven temperature in °C in red and TEC current in A in blue. The oven and its heatsink are protected from air movement. During the last hour the oven temperature log exhibits a 11 uK standard deviation.
The thermometer circuit has unused channels, so one can also measure ambient or TEC heatsink temperature and/or use a second thermistor mounted on the oven lid.
Last image shows a similar four channel TEC output stage that i made for a LTFLU reference array. It has an OPA4140 and a DAC8554. There is only one buck converter as all four ovens run at similar temperatures.

Regards, Dieter

[thermistor-guy]

What thermistor do you use?

Do "typical" NTC thermistors drift over time?

As others have noted, class-encapsulated thermistors are more stable than the epoxy versions. BIPM recommends
glass encapsulation:
    https://www.bipm.org/documents/20126/41773843/Guide-SecTh-Thermistor-Thermometry.pdf

NTC thermistors do drift a little when operated at 100 deg. C or so, and accelerates at temperatures above that. The drift
tends to be "thermometric", where the whole resistance/temperature curve shifts.

At operating temperatures below 75 deg. C, I'd expect a glass device to be stable within 1 or 2 mK over 1 year, with a good device
having less than that. From BIPM:

The most stable thermistors are bead thermistors encapsulated in
glass. Within the range –20 °C to 60 °C, selected and pre-aged thermistors may be
stable to better than a few tenths of a millikelvin per year.

I only use glass devices, typically in a DO-35 package. You might use an epoxy device in a cost-sensitive application.

[dietert1]

Yes, i am using glass thermistors with good results. They are pretty expensive now.
In my oven setups the thermistor serves to control the oven heater, providing very low temperature noise.
There is another thermometer for the critical device inside the oven, usually a pn junction (-2.1 mV/K) or a Pt resistor. An outer control loop fine-tunes the oven set temperature to bring the DUT to the desired temperature. Need to take a closer look at the temperature drift data.
The outer oven i made for our ADR1399 evaluation board also has a glass thermistor and it includes a SHT35 temperature/humidity sensor. After nearly a year the SHT35 temperature may have drifted +10 mK but that isn't a measurement, more like an upper limit. That oven isn't really hermetic and there has been a large drift of humidity. When talking about sensor drift, you want both constant temperature and humidity.

Regards, Dieter

[dietert1]

Meanwhile i learned about the TI ADS1235 that is a similar delta-sigma ADC as the ADS1256 but includes a bridge voltage reversal MUX. For the time being i will continue with the separate MUX as it gives more flexibility. The ADS1256 operates at 20 msec cycle time plus some msec pauses for switching inputs and voltage reversal MUX. With three thermistors i can use 16 measurements for each thermistor in one second: 4x thermistor, 4x reference resistor, 4x thermistor with voltage reversed and 4x reference resistor with voltage reversed. This gives 3 x 16 x 20 msec = 960 msec, so 40 msec remain for switching.

Until now i wired two thermistors, the second one on the oven lid. I calibrated the two thermistors against each other and their temperature difference was only 0.042 K. These are TE CONNECTIVITY GAG10K3976A1. When running the oven at 37 °C and 21 °C ambient, the temperature difference between bottom thermistor (near TEC) and top thermistor (on lid) is about 0.35 K - more than expected. The PID controller can steer the average of those two temperatures as well, with about 16 uK standard deviation. Next is a third thermistor on the TEC heatsink.

The DAC8551 with its 16 bit resolution was replaced by a PWM running at 80 MHz / 64 000 = 1250 Hz pulse frequency, plenty enough for < 0.1 Hz bandwidth. When combined with some sigma-delta type modulator implemented in firmware it gives another 10 bits resolution in 1 second. Accuracy doesn't really matter in this case, but resolution is important, as the TEC usually runs at low currents. Another solution would be two TEC current ranges, one for power-up and a second one for steady state operation.

Also i added a TFT for 8 minutes of real time log display which helps a lot for tuning the PID. I also implemented kind of a demodulator that determines period of (damped) oscillation and damping factor. This also helps with the PID configuration. So there is no autotune but it became rather easy to configure the PID correctly.

Regards, Dieter