Author Topic: Ultra Precision Reference LTZ1000  (Read 990644 times)

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Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1400 on: March 29, 2016, 09:30:10 pm »
The wire resistance has an effect mainly with pins 3 and 4, as here the highest current flows.
With pin 3 and 4 mA zener current, a 2.7 mOhms change in resistance for a 20 K increase in temperature gives 10-12 µV of extra voltage. However the Kovar wire will

At least for pin 4 the behaviour is not linear.
A 100 ppm change on R1 gives 0.14 ppm change of the zener voltage.
So for Pin 4 its only 0.22 uV (and not 5-6) over 20 deg C.

With best regards

Andreas
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1401 on: March 29, 2016, 09:38:15 pm »
Hello,

Measurement on LTZ#4 with short leads. LT1013A again as OP-Amp.
And R9 = 402K to compensate the remaining -0.1ppm/K.

For R9 = 1 Meg I get a T.C. increasement of around +0.04ppm/K
100K give around +0.4ppm/K over 20 deg C.

While the K2000 had some warm-up delay for the first hour,
both the HP34401A in 100mV range against LTZ#5 and ADC16 (with 2:1 divider)
show a nearly zero T.C. (less than 2uV change) between 20 and 32 deg C environment temperature range.

With best regards

Andreas

 

Online Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1402 on: March 30, 2016, 06:05:50 pm »
The extra resistance of the longer leads at pin 3 can explain rather well the drop in voltage from shortening the leads. However one would extect an more negative TC with the shorter leads, but the observed change is in the other direction  - so the change in TC must have another explanation. The wires at pin 4 should have even less influence, so they cant be the cause either.

The with the compensation with R9, the TC gets smaller, but the curve also get quite nonlinear. Here I don't think the effect is due to nonlinear influence at pin4, but due to the nonlinear relation ship with heater voltage. So the heater voltage might not be such a good choice for compensation. As the relationship heat voltage to power gets more linear at higher power / higher set temperature, the curvature could well be less critical at higher temperature setting.
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1403 on: April 22, 2016, 09:17:42 pm »
Hello,

I did some changes to LTZ#3 which had a too high amount of T.C. (-80ppb/K).

see also:
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg881955/#msg881955

now it is nearly in final state:
- slot between regulator and LTZ
- TEKO shield soldered
- and: I tried to reduce the T.C. by "half shortening the legs"

from LTZ#4 I know that mounting the reference directly on the pcb makes a difference of around +0.1ppm/K against long legs.
So I did not want to "overcompensate" the T.C. and tried a shortening of around 5 mm on the legs.

The result:

The T.C. regression curve has changed from -80ppb/K to around -40ppb/K.
Which is now "within spec". (50ppm/K of datasheet)

Now the question is: shall I do further shortening the legs leaving a rest of 2 mm to obtain "zero T.C." or not?

With best regards

Andreas

 

Offline zlymex

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Re: Ultra Precision Reference LTZ1000
« Reply #1404 on: April 23, 2016, 12:13:32 am »
Now the question is: shall I do further shortening the legs leaving a rest of 2 mm to obtain "zero T.C." or not?
That's entirely up to to your requirement.
My suggestion is to leave it untouched, and compensate this little T.C. in later stages such as 7V to 10V step-up since there will be additional TC introduced which will void the zero T.C. you may have achieved.
Also, I don't recommend adding a critical component to solve a none critical problems since the stability is the number one priority than T.C. in most cases.
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1405 on: April 23, 2016, 10:14:55 am »
I forgot where I read it [maybe in the LTZ's datasheet?], but Linear-Tech recommends 1mm spacing between the bottom of the TO99 package and the PC board. 


Hello,

I could not find this in the LTZ datasheet.
Has anyone a hint where this info can be found?

But if I think about it then I did a error when shortening the pins of LTZ#4 to zero.
On the other side the T.C. on #4 was very widely off without this measure.

This cathode resistor should be very stable [Edwin's PWW with stabilization or VPG foil with PMO].

That is not my way of adjusting the T.C. You need a additional very stable component. (and my PCB layout is not prepared for this).
And finally the copper traces on the layout determine a large amount of the T.C. of that additional component.
In a small range I can adjust the T.C. by populating R9 (which is already in Layout). And here I can use a standard 1% metal film resistor.
For a working point of 50-55 deg C the compensation is around +0.04 ppm/K for each 1 Meg resistor.

That's entirely up to to your requirement.

I think it was more or less a rhetorical question. As a volt nut I have at least to try it.

The 10 V output is still far away.
I think I will have to use a DAC and a NTC to compensate for the temperature changes
in order not to worse the 0.05 ppm/K spec of the reference.

With best regards

Andreas

 

Offline plesa

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Re: Ultra Precision Reference LTZ1000
« Reply #1406 on: April 23, 2016, 11:50:53 am »
I forgot where I read it [maybe in the LTZ's datasheet?], but Linear-Tech recommends 1mm spacing between the bottom of the TO99 package and the PC board. 
I could not find this in the LTZ datasheet.
Has anyone a hint where this info can be found?

There is info about the lead diameter only, same nor similar to LM399.
LEAD DIAMETER IS UNCONTROLLED BETWEEN THE REFERENCE PLANE
AND THE SEATING PLANE (0.254mm – 1.143mm)

One of my LTZ was almost impossible to insert close than 1mm due to the leads angles.
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1407 on: April 27, 2016, 09:26:50 pm »
Hello,

further shortening of leads on LTZ#3
See also:
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg924793/#msg924793

now measuring 7.5 mm from top LTZ to PCB.
(should have been 0.5-1.0 mm shorter but its difficult to keep exact height when the TEKO shield is already soldered).

Result: reduction from -42 ppb/K to -22 ppb/K for the regression curve.
Still not "zero T.C." but a value I can live with.

Further I changed LTZ#6 with R9 = 1M8.
Regression curve changed from -28ppb/K to -15ppb/K.

Had expected to be closer to Zero T.C. with the formula +40ppb/K for each 1 Meg resistor (or +22ppb/K for 1M8),
 but obviously this is not always exact.

With best regards

Andreas
« Last Edit: April 27, 2016, 09:30:13 pm by Andreas »
 

Online Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1408 on: April 28, 2016, 03:49:18 pm »
The compensation via R9 is nonlinear, as the heater voltage is nonlinear depending on the temperature, so the compensation might only work for a limited temperature range and is likely also influenced by the thermal setup (e.g. isolation cap, cutouts around the LTZ1000) and the length of the wires: Shorter wires need more heating and thus R9 has more effect.  With a relatively small value for R9 one might even have to make sure that R9 itself is stable enough.
 

Offline doktor pyta

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Re: Ultra Precision Reference LTZ1000
« Reply #1409 on: May 03, 2016, 03:14:25 pm »
Progress:
-inox front and rear panel done (photos). As You can see it is designed to be stand-alone voltage reference or to be an OEM module to be built into some instrument.

-top and bottom LTZ1000 covering cup made of SUNDPLAST SP163.

-4 units assembled. Banana terminals are Pomona low thermal EMF type.

-TC measured (I don't use 400k resistors for temperature compensation):
Unit#1 with Vishay Z201 resistors T.C.=+0,03ppm/C
Unit#2 with precision wirewound resistors T.C.=-0,14ppm/C
Unit#3 with precision wirewound resistors T.C.=-0,08ppm/C
Unit#4 with precision wirewound resistors T.C.=-0,11ppm/C

By the way, I've bought some G.R. 8G16D wirewound resistors and some of them have T.C. as high as 7...8ppm/C instead of declared max. +/-5ppm/C. Does anyone else notice similar problem ?


To do:
-laser engraving of text on front and rear panels

-making better temperature chamber with PID controller

-redesign PCB to allow testing of CERN or PREMA solution with a JFET. My goal is to develop an EVAL board that allows testing different configurations.

-perform LF noise testing

P.S. Is someone of colleagues interested in putting one of my units into T.C. testing?
I could borrow one to compare the results (please send a PM).
« Last Edit: May 04, 2016, 05:22:21 am by doktor pyta »
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1410 on: May 03, 2016, 08:44:54 pm »
Hello,

Frank had a issue with one value (120R) of the econistors:
https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg624931/#msg624931

For the T.C.: is it measured on the direct zener output or after some buffer/divider?
(I am still missing for your schematics).
Over which temperature range did you test. (ramp or fixed points with how long rest time at the temperature?)

I would try to compensate the T.C. by R9 = 400K on Unit#2+#4 and perhaps around 500-680K on unit #3.

With best regards

Andreas

 

Offline doktor pyta

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Re: Ultra Precision Reference LTZ1000
« Reply #1411 on: May 03, 2016, 10:56:53 pm »
Hello Andreas,

Quote
Frank had a issue with one value (120R) of the econistors:
https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg624931/#msg624931
Thanks, I haven't seen that before. This agrees with my observations. I measured T.C. close to 8ppm/C for 120 ohm econistors.
So this explains exactly the meaning of this "eco" prefix. You get what You pay for. All in all they are good for the 70k collector resisitors :)

Quote
For the T.C.: is it measured on the direct zener output or after some buffer/divider?
I use typical  LT application LTZ1000ACH + LT1013 and additionally a follower made of OPA2277.
I'm measuring the voltage at the buffer's output.

Quote
(I am still missing for your schematics).
I will show them before the next iteration of PCBs.

Quote
Over which temperature range did you test. (ramp or fixed points with how long rest time at the temperature?)
I tested the reference in three points: 15'C ; 22'C ; 30'C because my setup allowed only that points. I gave 4 hours (before each measurement) to thermically stabilize. Now I'm working on better PID controlled chamber.
« Last Edit: May 03, 2016, 10:59:06 pm by doktor pyta »
 

Online Dr. Frank

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Re: Ultra Precision Reference LTZ1000
« Reply #1412 on: May 04, 2016, 09:17:55 am »

Quote
Frank had a issue with one value (120R) of the econistors:
https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg624931/#msg624931
Thanks, I haven't seen that before. This agrees with my observations. I measured T.C. close to 8ppm/C for 120 ohm econistors.
So this explains exactly the meaning of this "eco" prefix. You get what You pay for. All in all they are good for the 70k collector resisitors :)


I cannot confirm, that these econistors are really a bargain.

I confirm, that certain values have higher T.C.s than expected.
I got replacement for these 120 Ohm types, which were in spec, i.e. around +3..5ppm/K
Keep in mind, that the 120 Ohm is less critical, like both 70k resistors.

The 12k all were very good, around +1ppm/K or less.
1k were between +3..5ppm/K.

Therefore, these latter resistors numerically give around 0.04 ppm/K in the completed LTZ circuit. Shall I say 'only'?

I got the feeling, that the overall T.C. in the end simply has to be measured, and other T.C.  compensation been added (length of leads, 200k .. 1M of "T.C. compensation resistor, and so on).

I think, that it's not useful to strive for the best possible (towards zero T.C.) parts, as even the best selected resistors may give much higher T.C.s, than expected.

Frank
 

Offline Mickle T.

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Re: Ultra Precision Reference LTZ1000
« Reply #1413 on: May 05, 2016, 11:25:08 am »
How about Datron LTZ1000 TC corrections? Datron 4000, 4700, 1271 e.t.c. have one.
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1414 on: May 06, 2016, 09:52:00 pm »
How about Datron LTZ1000 TC corrections? Datron 4000, 4700, 1271 e.t.c. have one.

Hello,

Could be a interesting method for compensation of the 7 -> 10V Divider.

Attached I have a overview of the 4 LTZ1000A references #3-#6 (7V) with all tweaking and measures done.

T.C. is trimmed from up to 230 ppb/K down below 25 ppb/K on all devices. Either per trimming lead length or by adjusting with suitable R9.

PSRR is reduced from up to 0.15ppm/V down to 0.05-0.06 ppm/V by slots between the voltage regulator and the LTZ.
The soldering of the teko inner housing has also contributed a part.

The slots and TEKO also help against voltage shifts by tilting the reference. (now less than 2uV or 0.15ppm).

R9 and shortening the legs have no visible impact on 1/f noise of the zener.
The popcorn noise on LTZ#5 is still there.
I decided not to exchange the zener by another LTZ1000A because the tempco of this device is near zero without shortening the legs / using R9.

Next steps:
- record ageing drift of the devices (against my pre-aged references #1 + #2 with around -1 and -2 ppm/year drift).
  here I hope to see wether the (popcorn-) noise has a correlation to ageing.

With best regards

Andreas

 

Offline Mickle T.

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Re: Ultra Precision Reference LTZ1000
« Reply #1415 on: May 07, 2016, 05:58:28 am »
A classic Datron schematics in the small form-factor (another DMM modding).
TC 20-40 ppb/K without any adjustment or compensation (R9=0).
 
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Offline acbern

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Re: Ultra Precision Reference LTZ1000
« Reply #1416 on: May 07, 2016, 07:06:30 am »
Looks very interesting! Can you elaborate a little bit about it (parts used, circuitry details..).
 

Offline Mickle T.

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Re: Ultra Precision Reference LTZ1000
« Reply #1417 on: May 07, 2016, 07:27:40 am »
It's a simplificated Datron circuit without guarding and Vishay custom resistors network.
R2, R3 - hermetic foil type S5-61, R1 - hermetic foil type R2-67, R4-R5 - foil Alpha Electronics FLC.
 

Offline Mickle T.

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Re: Ultra Precision Reference LTZ1000
« Reply #1418 on: May 07, 2016, 09:43:13 am »
A copper chamber with the all sides heating. Was taken from the metrological level standard cells group H488/1.
 

Offline acbern

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Re: Ultra Precision Reference LTZ1000
« Reply #1419 on: May 07, 2016, 12:59:23 pm »
It's a simplificated Datron circuit without guarding and Vishay custom resistors network.
R2, R3 - hermetic foil type S5-61, R1 - hermetic foil type R2-67, R4-R5 - foil Alpha Electronics FLC.

What values for R16 and R18 did you use? R9=0ohms, right. R7 and R15 are 160 ohms?
I would think one can also modify this to work on non-symmetric voltages (12V), with all signals referenced to 0V being AC-coupled.
« Last Edit: May 07, 2016, 01:01:48 pm by acbern »
 

Offline Mickle T.

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Re: Ultra Precision Reference LTZ1000
« Reply #1420 on: May 07, 2016, 01:10:02 pm »
R16+R18 is needed for DMM mod only. The exact value (~3.4k) is individually adjusted for each ref module.
 

Offline doktor pyta

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Re: Ultra Precision Reference LTZ1000
« Reply #1421 on: May 12, 2016, 01:01:12 pm »
Hi, Mickle T.
What are the advantages of the temperature control section proposed by DATRON?
Is Linear approach somehow worse ?
What are Your observations ?

Online Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1422 on: May 12, 2016, 04:19:54 pm »
The Datron temperature regulation uses a more simple compensation (one less series resistor) and a different output stage. Both changes are rather independent.
The current controlling output stage does not look good to me, as it has less gain at low currents / power and thus makes the nonlinear heater curve even more nonlinear. So I don't see this as an advantage, but more a not so good decision.

The removed resistor in series to the feedback cap reduces the gain in an intermediate frequency range. It depends on the thermal properties on how important this is for regulation. AFAIK LT uses the same for both the LTZ1000 and LTZ1000a despite of different thermal characteristics. Also the heater curve is a square law, so loop gain is lower at high environmental temperature or lower set-point. So the circuit is not really tuned for a very fast response, which is not needed nor best for low noise. To really judge on the regulation one would need to measure the properties of the thermal system.

It depends on the environment if more gain is better or worse. If there is not much external variations (e.g. well shielded, not much  uncontrolled turbulence) the lower gain can be a small advantage. In a more turbulent environment higher gain would be better as in this case noise of the temperature sensor is lower than thermal variations.  Anyway the difference will be small as the thermal variations of the sensor itself should not contribute significant to reference noise in the relevant frequency range (e.g. 0.1 .. 10 Hz). So it may be just economics to use smaller caps, omit one resistor and save a few cents.
 
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Offline doktor pyta

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Re: Ultra Precision Reference LTZ1000
« Reply #1423 on: May 15, 2016, 02:37:25 pm »
I'm placing final photos and the schematic diagram of my version.
The temperature controller is taken from the LT app note.
The Zener section combines Datron and LT schematics, so You can test both configurations.
Future revision C will have minor changes, but I'm satisfied with current revision B.
Using Z201 resistors in the temperature setpoint divider and Zener current sense + Econistors in the rest of the circuit I'm obtaining T.C. in +/-0,06...+/-0,03ppm/K range without additional compensation (initial tests). That's fine for me and shows that there are no big errors in the (thermal) layout.
Still waiting to finish better temperature chamber. Then I will publish my measurements.
« Last Edit: May 15, 2016, 02:41:58 pm by doktor pyta »
 
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Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1424 on: May 15, 2016, 04:33:41 pm »
Very nice, one of best looking assembled LTZ units (if not the best) :) Looking forward for testing and performance results.
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