Author Topic: Ultra Precision Reference LTZ1000  (Read 1333397 times)

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Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1900 on: October 19, 2017, 11:30:57 pm »
It's very sensitive and digital more likely to inject more noise into the system, than not :)
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Offline Vtile

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Re: Ultra Precision Reference LTZ1000
« Reply #1901 on: October 19, 2017, 11:38:09 pm »
It's very sensitive and digital more likely to inject more noise into the system, than not :)
So digital chip designers still failing to deliver?  >:D ;)



PS. If someone is digital chip designer, take above as friendly teasing.  :)
« Last Edit: October 19, 2017, 11:47:12 pm by Vtile »
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1902 on: October 20, 2017, 04:42:19 am »
Interesting to know, for some reason Rhopoint has changed the specification for their 8G16D resistors from ±3ppm/°C to ±5ppm/°C.

-branadic-
Hello,

I have seen that on the web-page. (from one week to another).
But if you look at the data sheet the 5ppm/K were always the "standard" tempco.

If you look at a very old data sheet of 8E16 (not the 8G16) they have stated
5 ppm max for -55...125 deg C
3 ppm typ for 0..85 deg C

with best regards

Andreas
 

Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1903 on: October 20, 2017, 07:42:46 am »
Since the heater/oven seems to be one of the achilles (as lazy reader of these electron splitting threads, which are really interesting. :) ) of the LTZ1000, why everybody is doing analog controller of the oven. Put it digital and run it through MPC or even digital PID....?  :o
The temperature regulation in the LTZ1000 chip is still on a time scale that is easy to handle in an analog way. The analog PI regulator circuit is actually working very well. The A version still uses the same regulator setup and they do not compensate for the nonlinear heater function. So if really needed there would be still some room for even better regulation in the analog domain. However the current system seems to be well good enough and very fast regulation is not needed.  It would be demanding to get the same performance with a digital regulator: the on chip thermal loop is rather fast (up to the kHz range) and really low noise.

A digital regulator is attractive with large thermal systems (e.g. for the whole circuit or larger), because this will include longer time constants in the minutes range, that can get difficult in the analog domain. For such slow systems one also tends to aim for relatively fast reaction and thus a closely tuned regulator that a digital system can provide. For the fast LTZ1000 heater nobody cares if thermal stabilization needs 20 seconds instead of a possible maybe 1 second.
 
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Offline kj7e

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Re: Ultra Precision Reference LTZ1000
« Reply #1904 on: October 20, 2017, 10:53:01 am »
Someone remind me to post my experiences with the LTZ1000 in about 10 month. I had a design, few hundred of these were built into some equipments, and my NDA will be over by then.

Reminder, spill the beans.
 
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Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1905 on: October 20, 2017, 07:33:44 pm »
Since the heater/oven seems to be one of the achilles (as lazy reader of these electron splitting threads, which are really interesting. :) ) of the LTZ1000, why everybody is doing analog controller of the oven. Put it digital and run it through MPC or even digital PID....?  :o
Mhm,

you are aware that the 0.05 ppm/K correspond to 2uV/K change at the temperature sensor?
Would be a very interesting digital controller design to get the readings fast enough with a much lower resolution/noise floor.

And thermal time constant of the A-Version is damned fast.
When I measure the Zener voltage with a temperature setpoint step with 10NPLC
it is in steady state within 2 readings. (less than 800ms).

with best regards

Andreas

 

Offline kj7e

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Re: Ultra Precision Reference LTZ1000
« Reply #1906 on: October 20, 2017, 10:22:18 pm »

... you are aware that the 0.05 ppm/K correspond to 2uV/K change at the temperature sensor?


Andreas,

I think you forgot a decimal. 1ppm of 7.15v = 5.1uV, so 0.05ppm would be 0.255uV/K
 

Offline Vtile

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Re: Ultra Precision Reference LTZ1000
« Reply #1907 on: October 20, 2017, 11:09:26 pm »
Well isn't the 0.05 ppm same than 50 ppb so for 7.15 Volts the 0.05 ppm would be equal to 50 ppb and as: 7.15 V * 10^-9 * 50 = 357.5 nV (/ K)
 
My throw did get out of hand yesterday it seems.

Edit. 7.5 V corrected to 7.15V
« Last Edit: October 20, 2017, 11:11:22 pm by Vtile »
 

Offline kj7e

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Re: Ultra Precision Reference LTZ1000
« Reply #1908 on: October 20, 2017, 11:16:39 pm »
^ Need to scale 7.15 to 10 for a true ppm comparison.  7.15/10 = 0.715.  357.5nV*0.715 = 255nV for 0.05ppm based of 10.
« Last Edit: October 20, 2017, 11:30:33 pm by kj7e »
 

Offline Dr. Frank

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Re: Ultra Precision Reference LTZ1000
« Reply #1909 on: October 20, 2017, 11:21:43 pm »

... you are aware that the 0.05 ppm/K correspond to 2uV/K change at the temperature sensor?


Andreas,

I think you forgot a decimal. 1ppm of 7.15v = 5.1uV, so 0.05ppm would be 0.255uV/K

You have  to calculate differently.
The RefAmp w/o the oven has a T.C. of about 50ppm/K.
A 0.05ppm change in the reference voltage is therefore equivalent to 1mK change in the oven temperature.
The sensing element, i.e. the base- emitter diode has about -2.1mV/K sensitivity, so 1mK is equivalent to a change of 2.1uV.

B.t.w.: Has anybody fully calculated, how the oven works, especially,  on which transistor fundamentals it's stability is based on? (Like Early voltage, temperature voltage Ut, and so on)
We have some theoretics-only guys here in this forum, participating also in this thread,  so that would be great, as such a model would give an idea about the dependency of the oven set point, and the stability parameters of the oven circuit, drift over time , and regulation stability.

Frank
« Last Edit: October 20, 2017, 11:24:56 pm by Dr. Frank »
 
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Offline Vtile

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Re: Ultra Precision Reference LTZ1000
« Reply #1910 on: October 20, 2017, 11:24:29 pm »
^ Need to scale 7.15 to 10 for a true ppm comparison.  7.15/10 = 0.715.  357.5nV*0.175 = 255nV for 0.05ppm based of 10.
Ah, there were invisible 10. :) I were a bit confused a moment why 255nV "error".

Edit. After Dr.Franks post.
I have not calculated anything else than above. I'm only armchair theorist here at these subjects, so bear with me. :P

While I haven't done any calculations and I will not do so in the future. I'm pretty confident that control system is not linear in microscopic level it is kept and that the control values would have to be dynamically changed for different parts of control window to get optimal stability and control over the system. Analog propably not, digital maybe, especially since you could use all the computer software to analyse the control problem in hand and parametrice the controller from there.
« Last Edit: October 20, 2017, 11:39:04 pm by Vtile »
 

Offline Awesome14

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Re: Ultra Precision Reference LTZ1000
« Reply #1911 on: October 21, 2017, 09:27:57 am »
When I wanted to build a LTZ1000 ref, I just called LT and asked how to do it.
Anything truly new begins as a thought.
 

Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1912 on: October 21, 2017, 10:13:40 am »
For the temperature sensing transistor it is V_BE at a given current around 100 µA that matters. The early voltage can have a slight effect on the gain of the transistor and thus the voltage gain the OP sees. For the stability of the temperature this would be a second order effect.  V_BE changes by some 2.1 mV/K, but the transistor with the 70 K already gives gain and thus the collector voltage already has a TC of around 500 mV/K.

So getting a good temperature reading is not that difficult, though it likely would need more than a µC internal ADC. The more tricky part for an digital regulator would be the output: The heater is good for something like 100 K temperature rise and should be adjusted fine to the 0.1 mK level. That would be something like a 20 bit DAC.  Not really attractive if half an LT1013 can do it as well (likely lower noise, but a little slower) and with much less EMI problems.
 

Offline Dr. Frank

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Re: Ultra Precision Reference LTZ1000
« Reply #1913 on: October 21, 2017, 11:27:40 am »
For the temperature sensing transistor it is V_BE at a given current around 100 µA that matters. The early voltage can have a slight effect on the gain of the transistor and thus the voltage gain the OP sees. For the stability of the temperature this would be a second order effect.  V_BE changes by some 2.1 mV/K, but the transistor with the 70 K already gives gain and thus the collector voltage already has a TC of around 500 mV/K.

So getting a good temperature reading is not that difficult, though it likely would need more than a µC internal ADC. The more tricky part for an digital regulator would be the output: The heater is good for something like 100 K temperature rise and should be adjusted fine to the 0.1 mK level. That would be something like a 20 bit DAC.  Not really attractive if half an LT1013 can do it as well (likely lower noise, but a little slower) and with much less EMI problems.

Kleinstein,

when I mentioned these 'theorists-only guy' in this thread, I exactly meant you.. maybe you were able to calculate this circuit correctly and fully... by means of the Ebers-Moll model, for example.

The oven circuit is more tricky.
Your estimate is probably correct, in that the T.C. of the collector voltage is much higher than the Ube usually would be (-2.1mV/K), but this Ube of the oven sensor does not change in this circuit, as you describe.

It is nearly constant, determined by the reference voltage Uref, like so: Ube = Uref(20°C) * (T-20°C) * alpha * R5/(R4+R5), with alpha being the T.C. of the RefAmp, about +50ppm/K.

So, how did you calculate these 500mV/K, and can you provide a complete calculation for this whole circuit, please?

Frank
« Last Edit: October 21, 2017, 11:29:55 am by Dr. Frank »
 

Offline Cerebus

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Re: Ultra Precision Reference LTZ1000
« Reply #1914 on: October 21, 2017, 01:51:15 pm »
It is nearly constant, determined by the reference voltage Uref, like so: Ube = Uref(20°C) * (T-20°C) * alpha * R5/(R4+R5), with alpha being the T.C. of the RefAmp, about +50ppm/K.

Kleinstein is describing the open loop behaviour. Obviously you don't see this change when the circuit is closed loop .

Quote
So, how did you calculate these 500mV/K, and can you provide a complete calculation for this whole circuit, please?

Frank

The -500mV/K is just VBE change with temperature -2.1mV/K times the transistor's gain as a common emitter amplifier RC/Re where RC = 70k and Re is 26.1mV*/IC = 261R, so gain = 268. And 268 * -2.1mV = -563 mV/K.

* That figure is for 25C, the proper figure can be had from kT/q, with T for the operating temperature, k = Boltzmann's constant and q = fundamental electron charge.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1915 on: October 21, 2017, 01:53:14 pm »
V_BE changes by some 2.1 mV/K, but the transistor with the 70 K already gives gain and thus the collector voltage already has a TC of around 500 mV/K.

You want to cheat. That what you want to describe is not the digital way.

If you do it the "true" digital way you will diode connect the transistor (shorting collector and basis) with a pull up resistor.
So you have only the temperature of -2mV/K.

with best regards

Andreas
 

Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1916 on: October 21, 2017, 03:09:37 pm »
Andreas is correct in that a true digital way would be measuring the V_BE directly and not the amplified signal relative to the set-point from the divider. However I am afraid this is kind of difficult, as there are not many high res. ADCs that can directly use the 7.0x V reference voltage.  This might give a way to get rid of the 2 more critical resistors for the temperature set point. Except maybe for that divider drift I see no need to improve much on the temperature regulator. There is no real need to have a faster reaction as there are usually no large temperature fluctuations on the very fast scale.

While a digital temperature control it not really useful, there might be a chance that the set point divider could be checked with a high quality ADC, especially if such an ADC is present in the system anyway. This would be a way to detect and maybe digitally correct for long term drift of the divider. So maybe of you really want to cut costs on the two resistors in a low cost 7 digit meter.

In principle chances are good that the analog regulator could be tuned to a faster reaction if needed. The standard version uses the same circuit for the normal and A version and it does not correct for the nonlinear heater - so there is definitely some room for a faster control. However I doubt this would make a big difference, as the critical part is more like drift over months and not a small contribution to noise in the 1-1000 Hz range.  If at all it would be likely the low power region that could profit and allow better performance when just at the edge before the temperature regulator drops out of regulation. For normal use my feeling it that it is not worth the effort - maybe for the few cases when the whole circuit is in an oven and thus can / should constantly run at a low heater power. In this case the change can also be rather simple: higher gain and a rather low limit to the heater power.
 

Offline zhtoor

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Re: Ultra Precision Reference LTZ1000
« Reply #1917 on: October 21, 2017, 03:17:12 pm »
or maybe deriving the heater power from the already available 7v in the main zener reference
and implement a pwm heater controller using this derived voltage to drive the heater, pwm control being
affected by digital circuitry with set point coming in from the sensor transistor in the chip in the form of
a target pulse width.

regards.
 

Offline martinr33

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Re: Ultra Precision Reference LTZ1000
« Reply #1918 on: October 21, 2017, 06:53:45 pm »
Turning things around a bit, according to the datasheet, a 7 Ohm change in R3 (the 70k heater setpoint resistor) causes a 0.2ppm shift in output, or about 2uV. Therefore, with a small series resistor, I could adjust the output of the LTZ1000 with some precision. For example, to move 100uV, I would add 350 ohms. Seems like this might be an easy way to trim the reference to have fewer digits (i.e. get more trailing zeros).

Also, I am not so sure that heater stability is a problem. I'm more concerned about the unheated parts.
 

Offline tszaboo

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Re: Ultra Precision Reference LTZ1000
« Reply #1919 on: October 24, 2017, 12:11:59 pm »
Someone remind me to post my experiences with the LTZ1000 in about 10 month. I had a design, few hundred of these were built into some equipments, and my NDA will be over by then.

Reminder, spill the beans.
All right, but no specifics. Dont even ask for them, I consider them the IP of my previous employer.

I give you a story instead.
So I was designing high end battery testing equipment. The largest electric car manufacturer, (back in the day, not anymore) was using our system for testing the chemistry and they wanted much better voltage accuracy. Few PPM region. You need this, because open circuit state of charge calculation is very voltage dependent, and errors accumulate etc,etc. So they asked for an improved precision of our system.
We are talking about a big noisy system, which can supply 50A into each battery, and gets hot, even though it is water cooled. Not really the ideal, quiet, nice environment for voltage references. We narrowed it down to the LTZ1000 and the LM399 for the voltage reference, or integrating something into our system (management said no to this before you could finish a sentence). The LM399 was obsolete at that time, even from Linear, I think they were changing to the lead free process, but dont quote me on that.

So  I gathered all the circuit information, reverse engineering and pictures that I could, and designed a circuit. And immediately realized how stupid I am because Pin Configuration: BOTTOM VIEW. DOH! Funny enough, if you solder the LTZ1000 on the bottom side it works.
The final circuit is the one from the datasheet. It is on a small board with SMD components (except the opamp and the LTZ), carefully selected 5 PPM/K (max) thin film resistors (1206), SMD substitutes for the diode and transistor. Onboard voltage regulator, is a 7815, nothing fancy. Just place it to the other side of the PCB. And I didnt use any fancy swastika milling or anything like that. There was some to separate the 2N3904 and the VREG, and the resistors, but that's it. Maybe total of 40mm milling, on a credit card sized board. There was also an EEPROM onboard, to store the calibrated values. It went into a small plastic box (since there are big fans in the device) and it attaches to the base board with two nice gold plated board to board .1 inch header. I also placed the 4 critical resistors in close proximity. Thin film, no magic routing, no magic opamp upgrade, no magic low noise voltage regulator.
It worked out beautifully. We couldnt measure reliably the tempco, it was some 0.3ppm for the assembly. After a year we recorded some 2 ppm drift. (Both measurements were done with freshly calibrated 3458A). Inside the system it also worked as it should, making measurements, that are comparable to the 3458A, only faster. Bear in mind that we had to work in a single range, around 4 volt, since we were measuring Li-ion batteries, and nothing more.
Subsequently it was designed into a product, that was selling in larger volumes. Production ramped up. We had a lengthy burn-in process, I think it was at least 2-3 weeks. So I made a PCB holding and powering 5 of these. The output went into a switch matrix that went into the 3458A. Thinking about using a generic purpose relay card to do this? Well, forget it. Normal relays heat up when on, and the contacts will act like thermocouples. So let's design our own switch card.
Calibration setup looked like this: there was a stack of 30 LTZ1000 boards sitting on the base board. All went into another stack of relay cards. That went into the 3458a. Computer controls everything, python. 1000 NPLC voltage reading from the DMM. That is 20 seconds for one reading. One measurement is not measurement. I think we did some 15-30ish measurement of each card, throw away highs and lows, and average the rest. Confidence had to be high. All in the office, done by an engineer. Just making the test setup is expensive. Custom boards designed, firmware written, only to speed up production.
And then the calibration procedure started with an unskippable 30 minutes wait. Before that, there was an earlier setup, where each board was individually connected by hand. So that was about 16 board/day throughput for the calibration phase. Expensive? Well, 16 of these went into machines with "you cannot afford it" price tags, so no. We had a lot more assembly and building to do on each tester.

And that is my story. It has been a great ride! Maybe once I will go back to design precision analog, I definitely miss it sometimes.
 
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Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1920 on: October 24, 2017, 01:24:20 pm »
Thanks for the story  :-+. So essentially you needed low-noise stable reference, and stability was just to get away without frequent (expensive) calibrations. Guess nothing wrong in using LTZ in that perspective, since customer pays.
Measuring tempco to ~0.1ppm or wee bit better is not a problem if one to use large (20C+) temperature delta (gracefully).
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Offline tszaboo

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Re: Ultra Precision Reference LTZ1000
« Reply #1921 on: October 24, 2017, 02:19:38 pm »
Yes, stability was very important. And also the tempco had to be better than the reference we used. The 0.5PPM/K would have been enough. I think the final specification called for 30ppm full error (tempco+drift of Vref and AFE total) in a single voltage range, for a year, but the second system was a lot better than spec. The LTZ was used to correct the errors of the Vref driving the ADC, sort of background calibration.

You know, I would've liked to measure every last parameter of the circuit. But, you know how it is. I did a set of tests to prove that it is better than the required specification. This, when they are waiting to launch production, maybe in overtime, not having the right tools in the lab. I've used cooling packs to verify the tempco, on an overnight measurement, because during the day everything was too noisy. Once it was done, it is hard to convince management to spend more resources to say: OK, but how much better than spec is it? No, they are not interested in bragging rights.
 

Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1922 on: October 24, 2017, 02:27:04 pm »
Yea, getting job done and getting paid is little bit different than getting voltnut hobby fed. I'd be curious what ADC was used , LTC 240x series? We see many members here will LTZ refs, but only few like Andreas to have next step actually using REF to do measurements.
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Offline tszaboo

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Re: Ultra Precision Reference LTZ1000
« Reply #1923 on: October 24, 2017, 02:47:06 pm »
The old system was using a 24 bit multichannel delta sigma from TI. ADS12xx. In normal operation, the system took samples much faster than these few SPS measurements, and it uses digital magic, oversampling, filtering and continuous calibration to achieve the good samples.

The AFE I designed was using a 18 bit SAR from LT. We got 20-21 effective bits from that part after all the digital filtering (typical INL of 0.5ppm). Again, the system was running a 0.1ms control loop on these values, to make the main feature of the device (battery tester). But the real magic usually is between the DUT and the ADC.
 
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Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1924 on: October 24, 2017, 03:31:49 pm »
OK, thanks :) I've got the idea.
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