Author Topic: Ultra Precision Reference LTZ1000  (Read 832478 times)

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Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1450 on: July 22, 2016, 02:59:32 am »
The impedance at the transistors collector depends on wether you look at the closed loop case, or the open loop case. In close loop the point is rather low impedance (simulation shows something like 300 Ohms, using a 2N3904 model for the transistor). A signal due to added current appears nearly same size at the output. Voltage noise and drift an noise of the OP at low frequencies (e.g. less than about 1 KHz) are attenuated by a factor of about 200. So to compare current and voltage noise one has to reduce the voltage noise by this factor or multiply the current noise or impedance with it. So the effective impedance for the current noise is about 60 kOhms. Not by accident this is the same as the impedance of the node when looking at it in the open circuit noise. So for the choice of the OP one should consider an source impedance of about 60 K (slightly less than the 70 K resistor, due to the finite output admittance of the transistor).

For this high impedance the LT1013 and LTC2057 are good choices, but the LTC1677 or an similar LT1007 are not. So low and stable bias and low current noise are important.

Looking at the simulation of the current loop, there is a rather low phase margin in the 1-10 kHz range. This could become a problem with an AZ OP, as AZ OPs tend to have extra phase shifts in this frequency range. To be on the safe side one might want to modify the circuit to add extra phase margin if an AZ OP is used.
 

Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1451 on: August 08, 2016, 05:12:57 am »
I know some guys here going to hate me for this, but let's do some myth-busting. There are lot of talk about long/short legs, sockets and all that around LTZ-designs.
So here it comes. Built one more module with leftover parts.



Design configuration:
* xDevs.com KX LTZ1000 PCB, Rev.B01 board
* LTZ1000CH made in 1991, legs are NOT trimmed
* AUGAT gold-plated socket for LTZ
* Linear LTC2057 opamps for both drive and thermostat
* VPG VHP 1K000 0.2ppm/K + 10K+2K wirewound for temp setpoint
* Wirewound 120R. 10K is Z202 VPG
* Paralleled Fluke 3ppm/K 100K+250K resistors as 71.5K for bias
* Film capacitors for all signal path locations except 22nF (Which is C0G 1210)
* 392K for LTZ1000CH option installed



Board is covered in box and shielded from airflow now.

Closeup on parts:



Had to bodge some copper wiring to fit these giant resistors..



Board is powered from K2400 at +15V and measured via 3458A@NPLC100. Also turned on pair of my previous LTZ modules, hooked to K2002's for comparison.



Live data:



I have 3 more LTZ1000CH from same batch, and dodgy (1ppm shot noise) desoldered LTZ1000ACH from ebay 3458A's PCBA. So I'll have each chip run for some time and then proceed with swapping. Then rotate in opposite direction, to see if any voltage shifts due to socket connections.

If have any ideas - feel free to ask.
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Online Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1452 on: August 08, 2016, 07:24:19 am »
I know some guys here going to hate me for this, but let's do some myth-busting. There are lot of talk about long/short legs, sockets and all that around LTZ-designs.

Hello Illya,

we all can only learn from any experiment.
For me the procedure is clear (at least for the LTZ1000A with 12K5 setpoint resistor).

Step 1:
let the legs long and do not populate R9.
Measure T.C. of reference (with LTZ properly thermal shielded).

Step2:
Shorten legs if T.C. is < 0 T.C. will increase around +40ppb/K for half shortened and around +130ppb/K for full shortened legs.
I think its better to have at least 10 mils distance between PCB and the bottom of the LTZ to avoid stress to the chip.
Measure T.C. of reference again (with LTZ properly thermal shielded).

Step 3:
If T.C. is still <0 populate R9. 400K will compensate around 80 ppb/K. 2 Meg around 20ppb/K.
But this compensation is non-linear.
Measure T.C. of reference again (with LTZ properly thermal shielded) and eventually repeat Step 3.

For the layout:
keep all non constant heat sources away from the LTZ.
The heater transistor is not necessarily a problem if the supply voltage is constant.
But the voltage regulator from changeing battery voltage is critical. (here the slots will help).

With best regards

Andreas
 

Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1453 on: August 08, 2016, 02:29:09 pm »
But experiment results and execution often vary. Hence the discussion.

I still have difficulties measuring TC in sub-ppm zone. I had shown TC test with ramp +24C to +44C with stock HP 3458A A9 PCBA, which did not allow to calculate any TC. Perhaps method should be changed to test two different temperatures for very long time, instead of ramping temperature (speed was +1K each 2 hours) ?

There are no extra heat sources on LTZ module (except 5mA LED perhaps?). Also I did not see any measurable supply voltage dependency on output when tested before. Output was same either with 10V input or 16V. I usually use 15V or 12V from battery.
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Online Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1454 on: August 09, 2016, 05:14:33 am »

I still have difficulties measuring TC in sub-ppm zone. I had shown TC test with ramp +24C to +44C with stock HP 3458A A9 PCBA, which did not allow to calculate any TC. Perhaps method should be changed to test two different temperatures for very long time, instead of ramping temperature (speed was +1K each 2 hours) ?


Hello Illya,

you will have to find out where your measurement problem is.
I have a typical repeatability/noise of around 1uV for a 7V reference.
So over 20 deg C you should be able to detect at least 0.05ppm/K.

I measure either with (good warmed up, my K2000 needs longer than the 34401A) 6.5 digit instruments in 100mV range
as difference measurement against my "best" 7V reference,
or with one of my 24Bit temperature compensated ADCs (if the room temperature gradient is not too high).
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg907048/#msg907048

I average all measurement values within 1 minute. (This is possible since I have checked for a gaussian distribution).
And for the ADC I sometimes have to build a sliding average over 20 minutes.
I use ramp speeds typical of 0.1 deg C / minute. (not below 0.05 deg C/minute and not above 0.3 deg C/minute).
I measure ramp up and ramp down to average out temperature measurement delays.
Without a closed loop I would not be able to detect drift/hysteresis of my measurement setup.
And without continous ramping I would not be able to detect non linear response of the LTZ.
(e.g. large drift when the temperature regulation loop is out of control).

So I think its not a good idea to do only two temperature points.
Of course a constant temperature might be the starting point to look how to improve the stability of the setup.

I use also battery supply for my references and for my ADCs to avoid ground loops and mains noise.
For critical measurements I try to use a grounded metal plate where I put all my devices and wiring on.
(Where is the side wall of my desktop?).

With best regards

Andreas


« Last Edit: August 09, 2016, 05:18:16 am by Andreas »
 

Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1455 on: August 09, 2016, 02:37:19 pm »
Thank you for settings, that's helpful. I think part of problem that I was measuring direct reading, not the delta like you. Even though I use 3458, it's own TC much larger than source, and my environment is very far form desired +-0.1K and still air conditions... I'll have to build thermal chamber for 3458A to solve this...

Last night results with LTZ chip 8 (dark-blue) vs HP A9 PCBA (cyan, TEC cooled +23.40°C) and my old LTZ module (orange) : https://xdevs.com/dcv_kltz8/
That's with 1 minute average (10 samples). Raw samples are displayed by green dots.
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Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1456 on: August 10, 2016, 12:26:33 am »
There is 3458, but following this idea Keithley 182 or Keithley 2002 + EM A10 preamp on 20uV range will work much better.
Currently I have least 2 pairs of LTZ modules which are within <10uV from each other. Thanks for idea, I'll try this.
I have suitable relays for this job.
« Last Edit: August 10, 2016, 12:29:48 am by TiN »
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Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1457 on: August 10, 2016, 12:51:23 am »
And many many boxes of thermal and air insulation around everything.
Ok, added one more item into lloooong ToDo list. I have few more urgent projects to finish next weeks, but after that I'll be fully equipped to start on this challenge.
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Online Dr. Frank

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Re: Ultra Precision Reference LTZ1000
« Reply #1458 on: August 10, 2016, 01:28:11 am »
Ken and TiN,

sorry to disturb, but I think, nV- measurements are over-engineered, and not really solving the problem.

I agree, to measure the difference between an environmental- temperature - stabilized LTZ1000 and a environmental -temperature-changing DUT, if the 3458A itself cannot be used in a constant environment.

Anyhow, if you want to determine a 0.01ppm/K dependency, as worst case, that would be about 71.5nV/K  for a typical 7.15V output.
Measuring, let's say over a 10°C range, that would be about 715nV.
Both orders of magnitude can be well measured with a 3458A, and using statistics would give a reasonable T.C. = dU/dT calculation.

The problem at that point is of course the extreme sensitivity of such a measurement, which a nV meter would probably not improve in principle.

I mean, what do you really measure, the T.C. of the LTZ1000 circuitry, or any thermocouple (asymmetry) inside the measurement loop?

Even if you reverse the output of the LTZ1000, you will still have thermocouples left between the relays and the output directly at the LTZ1000 pins, which you cannot distinguish from the LTZ-circuitry -T.C.

Therefore, this LTZ-T.C. measurement, and all this trimming to lowest possible T.C.s (below 0.1ppm/K level) is mostly esoteric... or do I miss something ?

Frank
« Last Edit: August 10, 2016, 01:35:45 am by Dr. Frank »
 

Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1459 on: August 10, 2016, 02:53:59 am »
Dr.Frank

For me it's not so much for nV-measurements themselves, but rather to change method of measuring TC to see if results correlate.
As with just direct measurement LTZ's output over environmentally unstable 3458A does not provide any clear result, leading to erroneous results or inability to see relation between LTZREF temperature shifts and output voltage variations. I don't feel comfortable just applying average(256) filter and analyzing likely meaningless data.

Here just today's example with raw data.
* HP 3458A A9 PCBA in TEC-thermostat controlled at +23.4°C with +/-0.04°C, from 10pm to 11pm.
* Cyan line, moving average 10 samples at NPLC100
* Variation Vout = 1.4uV(pk-pk) which is double of your theoretical 715nV over 10K.

Or I interpret result incorrectly, and we dealing here with furry carpet stairs?
Also old initial results on all three modules show much higher TC (0.5 to 1.5ppm/K), not talking about <0.1ppm/K TC's.
So still need clear answer for myself if it was design problem in module/components or measurement error.

Alternative to that is building thermal chamber for DMM to stabilize its conditions, which technically is easier, but would occupy lot of space in the homelab.
« Last Edit: August 10, 2016, 02:56:01 am by TiN »
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Online Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1460 on: August 10, 2016, 05:10:10 am »

Therefore, this LTZ-T.C. measurement, and all this trimming to lowest possible T.C.s (below 0.1ppm/K level) is mostly esoteric... or do I miss something ?

Hello Frank,

Perhaps its not so essential if one has a temperature stable home lab in the basement.
But with my lab under the roof I have typical 6 deg C (24-30 deg C) variation over the day in summer.
Extreme values may be up to 10 deg C during one measurement.

So my references have to be a factor 10 better than someone elses with only 1 deg variation.

Besides that I had more problems with EMI-noise (from PC + USB-cables) than with thermocouple voltages.
If I can keep air drafts away from the setup by cloth and use thermally coupled connectors (D-SUB) the later should be avoided.
The EMI can be detected by putting the hand on/over the power supply batteries or the wiring.
I had up to 38uV output voltage difference (depending on sample) between unbuffered and buffered LTZ output before soldering the 100nF capacitor across the output.
(capacitive load isolation cirquit is necessary to avoid oscillation of the buffer).
After the change the difference between unbuffered and buffered output (LTC2057 offset) were near my noise limit (1-2uV).

https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg846835/#msg846835

With best regards

Andreas
 

Online Dr. Frank

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Re: Ultra Precision Reference LTZ1000
« Reply #1461 on: August 10, 2016, 06:49:53 am »

That *is* why they all us "Volt Nuts", no?


What, ME telling?  O0 O0 O0

I really think, at this point, there is a natural, physical limit, where it's not useful anymore, trying to measure something, which is buried too deeply in other effects, like e.m.f.s, zener and circuit noise, etc. If you were able to have a "chopper" principle available, (like Lock-In), to cancel all e.m.f.s, and other drifts, then you would be done... but that's not the case here, as  these effects are inside the circuitry.

Also, a nV meter, i.e.  < 10nV noise, would dig into the 0.001ppm/K region, which makes absolutely no sense anymore.. simply look at this order of magnitude..

That has also nothing to do, that we as "amateurs" would be limited in means, compared to standards labs. I think that we are pretty close to them.
And they also are limited by the rules of physics. I really doubt, that they would give much better StD or T.C. figures for that circuitry.



I like to take the chance to present my latest measurements on a newly assembled LTZ1000 circuit, PCB and schematic by Andreas (many thanks again!!).
It's running on 45°C, the PWW resistors are selected, to give a theoretical T.C. of about +0.045ppm/K, 430k resistor for "T.C. compensation" assembled.
Measurement is done in constant environment, max. 0.8°C RT change, absolute voltage measured at NPLC 50 with the 3458A, which also was stable to 0.2 / 0.5°C, internally.

The first diagram shows the datasheet assembly, i.e. only these 3 cap's, and the LTZ in a socket. No electrical or thermal shielding, besides a styrofoam cup on top of the LTZ.
1h-noise is about 750nV, or about 0.1ppm. Not the best performer of my five new LTZs. 



Second diagram shows the same LTZ, soldered fix into the PCB w/o cutting its legs. I avoided to heat the chip during soldering.
The circuit is equipped with all capacitors, as defined by Andreas, except C14, C15 (fear capacitors), and C13, which I think would spoil the ovens feedback constant of C1, R7. The metal shield is also in place, but no thermal enclosure on PCB bottom yet.




1h noise has gone down to about 270nV, or 0.04ppm. These 0.5ppm dips, and glitches were gone, at least these are much, much smaller.
So, all these capacitors, which Andreas has introduced into his circuit really improve the performance.

What's left is a random medium term noise, or drift, of about 0.05 .. 0.1 ppm, see trend line (200 points average).
That may further be improved by completing the thermal shield, and putting the whole assembly into the aluminium box.

The 16h stability, that is the combination of the 3458A and the LTZ1000 circuit, is on the order of about +/- 0.1ppm.

First conclusion, this is a quite stable measurement, mainly due to stable temperature.. it really can be done that way..

Second conclusion, despite the stable output voltage, you still see a lot of medium term noise, or variation, on the order of several 100nV.
Maybe this is caused by thermal draught, but probably also by zener instability, i.e. random walk, popcorn noise, or whatever you would like to classify this.
And I really cannot imagine, how to  principally improve this, so to be able to extract changes on the order of 0.01ppm, caused by temperature variation.

Third conclusion, although there is RT change, and 3458A temperature variations, there is no correlation visible between T and U. In other measurements also, I never have seen a correlation.

Illya wonders, why he also does not see a correlation in his measurements.

Maybe, that's because there is no effect, or the effect is buried under other effects.

Well, of course, I will also try to measure the T.C., when my box will be finished.
But I will definitely stop at this 0.05 ppm/K (~100nV) perception frontier..


Andreas, I really think that 0.05ppm/K is realistic, and that all your LTZ circuits are already having that T.C. or better, by design. No need to worry further. 
At a 10°C change, that would be below a 1ppm output change, which would be extraordinarily fine.

I agree, that shielding and EMI is a superior problem over the T.C. .. I also see changes on the order of ppm, depending on the configuration of the shield (outer case to ground, or using the inner shield as a guard).

Frank
« Last Edit: August 10, 2016, 07:00:36 am by Dr. Frank »
 
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Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1462 on: August 10, 2016, 02:17:29 pm »
Andreas,

Is the schematics posted on page 58 latest? I'd like to try few capacitors you used on my module to see how it affects results.
If you have updated one, please post in PDF format for reference. Thanks.
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Online Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1463 on: August 10, 2016, 04:20:42 pm »
Hello Illya,

If you mean this one:
https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=189655

yes it is the latest schematic of my current PCB. Only the 100 nF capacitor at the output of the buffer is missing.
And I do not populate C16-C19. (no filtering against the inner shield).
Edit: C14,C15 cannot be populated together with fast OP-Amps like LTC2057. C13 is only necessary if C14 is populated (see Frank).

If you have the datasheet cirquit not all capacitors are possible without cirquit modification.
E.g. C9 will require R19 and C8 to prevent oscillation.
R21, R22 and C22 prevent the output buffer from oscillation with capacitive loads.

So from datasheet cirquit I would start with C11 and C12.
And if possible add C8,C9 and R19.
See also revision history.

With best regards

Andreas



« Last Edit: August 10, 2016, 04:44:21 pm by Andreas »
 

Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1464 on: August 11, 2016, 09:13:22 am »
I've added C8,C9,C12 (film SMD caps) and R19 (10K Fluke wirewound). Had also to R13 (50K Fluke WW) or circuit would not start.
Output shifted +0.96ppm. Set to datalog, will see how it goes in some hours. Wiring/settings not changed.



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Online Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1465 on: August 11, 2016, 02:45:09 pm »
Hello Illya,

I am missing C11.
For R19 and R13 simple SMD or metal film resistors are sufficient.
A PWW resistor for R19 might pick up more magnetic noise.

C11 and C12 are directly at the LTZ pins.
See also my very first PCB LTZ1000A where I added some components manually.
C11 + C12 at the LTZ circle.
R19 as metal film below the LTZ.

By the way.
I have difficulties to interpret your diagrams.
For me there is too much information on them.
I would prefer a diagram with not more than 3 lines on it.
(1 LTZ unfiltered+filtered + 1 temperature sensor).

With best regards

Andreas

 

Offline zlymex

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Re: Ultra Precision Reference LTZ1000
« Reply #1466 on: August 12, 2016, 12:27:47 am »
In my opinion, the measurement of T.C. of a voltage reference should be done by comparison instead of direct measurement. This is mainly because the T.C. and hourly stability of an 8.5 digit meter is usually not as good as the reference to be measured. Some multimeters such as 3458A, they have very good noise and short term(<=10 min) specifications but not very good hourly or daily, the period often required when measure T.C. of voltage references.

The reference should be better(ideally much better) than the voltage to be compared, should be noiseless, good short-term stable and most of all low T.C. If the low T.C.  cannot be guaranteed, use constant temperature chamber, temperature lag, or temperature correction/compensation, or some/all.

By means of comparison, back-to-back(as mentioned above by DiligentMinds.com) is a very simple and effective. However, it only works for two references of equal nominal voltage, we cannot back-to-back an 7V with an 10V for instance. A switch(preferably an automatic one) can be used here together with a low-noise, high linearity meter such as 3458A to compare references of different voltages such as 7V and 10V. The switch can be made of an latching relay with 2-Z contacts(or even 1-Z). Automatic switches have the advantage of multi-readings and averaged later to give more confident result.

I don't think the thermal EMF matters very much, because the reference now days are very large in value(>=7V). Plus, constant thermal EMF has no effect on T.C. measurement result.

Below is a comparison measurement of mine(with my 4910-AV) for 16 voltage references for 30 hours, purple line is the ambient temperature in degree C.


All the curves are very thin indicating a low noise system.
Many curves are straight(not vary with temperature much) indicating low T.C.
Some curves show clear variation with temperature, and can be further processed to obtain the actual T.C.
The curve marked with grn4 is the worst, indicating there are problems with it( https://www.eevblog.com/forum/metrology/repair-datronwavetek-4910-voltage-standard/msg885426/#msg885426
Andreas probably excuse me for violating his saying of 'not more than 3 lines' because my lines are much more straight.
 

Online Dr. Frank

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Re: Ultra Precision Reference LTZ1000
« Reply #1467 on: August 12, 2016, 12:58:22 am »
In my opinion, the measurement of T.C. of a voltage reference should be done by comparison instead of direct measurement. This is mainly because the T.C. and hourly stability of an 8.5 digit meter is usually not as good as the reference to be measured. Some multimeters such as 3458A, they have very good noise and short term(<=10 min) specifications but not very good hourly or daily, the period often required when measure T.C. of voltage references.

The reference should be better(ideally much better) than the voltage to be compared, should be noiseless, good short-term stable and most of all low T.C. If the low T.C.  cannot be guaranteed, use constant temperature chamber, temperature lag, or temperature correction/compensation, or some/all.

By means of comparison, back-to-back(as mentioned above by DiligentMinds.com) is a very simple and effective. However, it only works for two references of equal nominal voltage, we cannot back-to-back an 7V with an 10V for instance. A switch(preferably an automatic one) can be used here together with a low-noise, high linearity meter such as 3458A to compare references of different voltages such as 7V and 10V. The switch can be made of an latching relay with 2-Z contacts(or even 1-Z). Automatic switches have the advantage of multi-readings and averaged later to give more confident result.

I don't think the thermal EMF matters very much, because the reference now days are very large in value(>=7V). Plus, constant thermal EMF has no effect on T.C. measurement result.


All the curves are very thin indicating a low noise system.
Many curves are straight(not vary with temperature much) indicating low T.C.
Some curves show clear variation with temperature, and can be further processed to obtain the actual T.C.
The curve marked with grn4 is the worst, indicating there are problems with it( https://www.eevblog.com/forum/metrology/repair-datronwavetek-4910-voltage-standard/msg885426/#msg885426
Andreas probably excuse me for violating his saying of 'not more than 3 lines' because my lines are much more straight.

Hello, Zlymex,

principally you are right about the 3458A stability, on the other side, its LTZ reference is as good as the external LTZs, we want to measure.

The back-to-back method is better only, if the backing reference is better or equally stable as the DUT, otherwise, the absolute method is fully equivalent, as far as the A/D and amplifier stage are stable, too..
This is the case also for the 3458A in the 10V range.


In the volt-nuts thread, somebody measured several solder junctions; all had about 3µV/K, which is 0.5ppm/K related to 7 V.
Therefore, this is relevant if you resolve fluctuations to 0.1ppm or smaller.


Your diagram is 15ppm wide, in contrast to mine, which in comparison is zoomed 15  times, ie. 1ppm wide, resolving 0.01ppm or better.
So yours only looks better at the moment..

Would you mind providing a similar resolution, so to better see, how stable these 732B references are, in comparison to LTZs, and in comparison between differential vs. absolute measurement?

Thank you

Frank

« Last Edit: August 12, 2016, 01:21:39 am by Dr. Frank »
 

Offline zlymex

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Re: Ultra Precision Reference LTZ1000
« Reply #1468 on: August 12, 2016, 01:17:20 am »
......Would you mind providing a similar resolution, so to better see, how stable these 732B references are, in comparison to LTZs, and in comparison between differential vs.
Sure. But now it's getting very late here, I'll do it tomorrow.

..soldered junctions; all had about 3µV/K, .....
This per K is for temperature difference between two thermal junctions.
When we change the temperature of a voltage reference for 10 degree K(say) for testing T.C. we don't get 10 degree K difference for thermal EMF junctions. Usually very much less if properly handled.
 

Online Dr. Frank

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Re: Ultra Precision Reference LTZ1000
« Reply #1469 on: August 12, 2016, 01:24:53 am »
This per K is for temperature difference between two thermal junctions.
When we change the temperature of a voltage reference for 10 degree K(say) for testing T.C. we don't get 10 degree K difference for thermal EMF junctions. Usually very much less if properly handled.


.. provided if you balance all junctions in pairs.. if you have an unpaired junction, that might be different, isn't it?

But you're right, practically, such effects mostly cancel out.

Frank
 

Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1470 on: August 12, 2016, 03:19:21 am »
Andreas, Dr.Frank
Sorry, I just got used to overdo my logs with all data possible and often include multiple instruments logs, as I have only one GPIB dongle to run experiments.
I did excel graph with three datasets over same sample time.

This is RAW samples, no average at all. NPLC100, AZERO ON. Only math is in excel to remove offset, so graphs match same point.

.

Based on this, I see no difference in result, and all three measurements resemble Dr.Frank's second graph closely, both in span <0.2ppm and spike behavior.

Baseline - is my original circuit with PWW's, which I shown on previous page with plenty photos. TEMP? was lower, room was airconed.
Trimmed - same stuff, but cut legs so LTZ chip sits within 1mm of socket. TEMP? around +44, no aircon
Andreas - same as trimmed, but with caps/resistors mod. TEMP? around same, no aircon

« Last Edit: August 12, 2016, 03:20:59 am by TiN »
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Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1471 on: August 12, 2016, 04:06:22 am »
Solder junctions are only a problem if there are temperature differences inside the solder junction itself.  If you want to compensate, it is important to have the same thermal conditions on both junctions. If the thermal conditions can't be matched well it is likely better not to add an extra junction, but better make sure the temperature gradient at the junction is small. 

With the joints at the LTZ itself there is essentially no way to compensate. With the typical LTZ1000 circuit there are only the connectors, where you have extra connections. Here the junctions are usually balanced / symmetric anyway. But it is still important to keep temperature gradients small in these areas and get the gradient of the thermal chamber in the pure, preferably unbend wire.
 

Online Dr. Frank

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Re: Ultra Precision Reference LTZ1000
« Reply #1472 on: August 12, 2016, 06:25:34 am »
Andreas, Dr.Frank
Sorry, I just got used to overdo my logs with all data possible and often include multiple instruments logs, as I have only one GPIB dongle to run experiments.
I did excel graph with three datasets over same sample time.

This is RAW samples, no average at all. NPLC100, AZERO ON. Only math is in excel to remove offset, so graphs match same point.

.

Based on this, I see no difference in result, and all three measurements resemble Dr.Frank's second graph closely, both in span <0.2ppm and spike behavior.

Baseline - is my original circuit with PWW's, which I shown on previous page with plenty photos. TEMP? was lower, room was airconed.
Trimmed - same stuff, but cut legs so LTZ chip sits within 1mm of socket. TEMP? around +44, no aircon
Andreas - same as trimmed, but with caps/resistors mod. TEMP? around same, no aircon

Yes, seems to be similar. In the end, they have to be..
Though, the indicators are incorrect, the lines are 0.1 ppm apart, so the vertical arrows are 0.2ppm and 2ppm.

Frank
 

Offline zlymex

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Re: Ultra Precision Reference LTZ1000
« Reply #1473 on: August 12, 2016, 11:18:41 pm »
.......
Your diagram is 15ppm wide, in contrast to mine, which in comparison is zoomed 15  times, ie. 1ppm wide, resolving 0.01ppm or better.
So yours only looks better at the moment..

Would you mind providing a similar resolution, so to better see, how stable these 732B references are, in comparison to LTZs, and in comparison between differential vs. absolute measurement?

Thank you

Frank
Here is the same chart but 1ppm wide, 0.1ppm per line for voltage, for 18 hours. I measure these references by 3458A, NPLC=50, AZ=on like always.
The green line is raw 732B data except offset.
The orange line is 732B compared to 4910-AV, eliminated most of the influence from 3458A, but of course added influence from 4910AV.
The other two lines are channel 1&2 of 4910 compared to 4910-AV.
As can be seen from these 4 lines that comparison is better than measurement.
 
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Online Dr. Frank

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Re: Ultra Precision Reference LTZ1000
« Reply #1474 on: August 13, 2016, 11:25:23 pm »
Here is the same chart but 1ppm wide, 0.1ppm per line for voltage, for 18 hours. I measure these references by 3458A, NPLC=50, AZ=on like always.
...
As can be seen from these 4 lines that comparison is better than measurement.

Thank you very much, zlymex!

Your statement depends on, what you are after.

If you compare both methods concerning pure noise, see left frame, 4h - 6h, in your diagram, then I would guess, that both methods give equivalent results.
Would be nice, if you could do an StD analysis on this time frame for both methods, and I estimate both give about 100..200nVrms of noise.



If you take the right frame, 10h - 12h, there is an additional thermal drift visible, caused by the T.C. of the 3458A, as its internal temperature rises proportionally to the RT.
This is the parameter w/o ACAL, i.e. <=0.5ppm/K, which compares to <=0.15ppm/K for backing of each of two references, as per their individual specifications.

From your measurement, I would estimate the 3458As T.C. as having about 0.2ppm/2K = 0.1ppm/K, and the backing method about 0.05ppm/2K = 0.025ppm/K.

Therefore, the references "beat" the 3458A in the category of their T.C.s only, but not necessarily concerning noise.

The absolute method may be a bit more noisy, due to the different noise figures (at NPLC100) of the 3458A, for the 10V ( => 100nVrms), and the 100mV (=> 20nVrms) range.  Your StD analysis may tell.

Here's again my latest measurement, absolute method, Andreas optimized circuit, 2.5x zoom, with stability noise figures for 1h. Let's see, how this compares to the 732B.


Frank
« Last Edit: August 13, 2016, 11:41:57 pm by Dr. Frank »
 


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