Electronics > Metrology

New LTZ1000 10V Reference build (WAS: Agilent Keysight 03458-66509...)

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Rax:
I decided to radically modify this first post and organize in it the information collected during this pursuit. I'm hopeful this will help others put together a functional and accurate LTZ1000/A 10V standard. Of course there are other designs out there (and here), but the designer of this solution was very kind and helpful, and without that there would be no project.

Below are some of the more significant posts, links and other information necessary to build this.


* Dr. Frank's kit link: https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg3886166/#msg3886166
* OSHPark PCB (design by "pepaslabs") can be ordered here: https://oshpark.com/projects/uwqf5gCo/view_design[/url
* Bulleted post with directions #1: https://www.eevblog.com/forum/metrology/agilent-keysight-03458-66509-dcv-reference-pca-for-3458a/msg5211921/#msg5211921
* Bulleted post with directions #2:  https://www.eevblog.com/forum/metrology/agilent-keysight-03458-66509-dcv-reference-pca-for-3458a/msg5258568/#msg5258568
* Attenuation factors for the critical resistors (TC-wise) in the circuit: https://www.eevblog.com/forum/metrology/agilent-keysight-03458-66509-dcv-reference-pca-for-3458a/msg5212140/#msg5212140
* Critical TC resistors can be ordered (in the US) at https://webdirect.texascomponents.com/SearchResults.asp?Cat=1857. No minimum quantity, excellent performance, reasonably priced (YMMV), ready in a few days in fully custom values and specifications. Frankly, it's pretty mind blowing this option is possible. I recommend the .25% Z201s for all these critical positions. You money should go to superior TC, not tolerance. Another options would be PRC (long term drift specified: https://www.eevblog.com/forum/metrology/agilent-keysight-03458-66509-dcv-reference-pca-for-3458a/msg5214996/#msg5214996), but I found this path difficult to source and therefore not recommended.

ORIGINAL POST BELOW:
In trying to put together more references in my lab, I've also been looking at these.

How much stability is to be expected from these? I am assuming one positive aspect is those available on the marketplaces out there are probably well aged.

That said, the PCB design is very vanilla, with no cutouts, etc.

In case these are worthy to chase, there's then the issue of cleanly supplying them, putting them in an optimal case, etc. Has anyone taken on a project like this before?

Thank you.

alm:

--- Quote from: Rax on November 29, 2023, 02:42:04 pm ---How much stability is to be expected from these? I am assuming one positive aspect is those available on the marketplaces out there are probably well aged.

--- End quote ---
They might be well aged if they were continuously powered. If they were laying on a shelf for a long time, that might well start a new aging cycle. See the long term drift part of 3458A service note 18. If they have been powered for a long time, then I'd expect them to have settled below 4 uV/V/year in drift.


--- Quote from: Rax on November 29, 2023, 02:42:04 pm ---That said, the PCB design is very vanilla, with no cutouts, etc.

--- End quote ---
No voodoo slots of unclear function? I think quite a lot has been written about them in the long LTZ1000 thread.


--- Quote from: Rax on November 29, 2023, 02:42:04 pm ---In case these are worthy to chase, there's then the issue of cleanly supplying them, putting them in an optimal case, etc. Has anyone taken on a project like this before?

--- End quote ---
There was a supply of relatively cheap (~$100) 3458A reference boards a number of years ago. For example see this volt-nuts thread. If you search for posts from that time on volt-nuts or eevblog you might find people who repurposed them as stand-alone reference. Note back then there were some reports of having popcorn noise on some of those boards suggesting they were factory rejects, but I don't know if that applies to the currently available boards.

I would be concerned about the high temperature set point accelerating aging for a voltage reference.

Rax:

--- Quote from: alm on November 29, 2023, 03:08:11 pm ---No voodoo slots of unclear function? I think quite a lot has been written about them in the long LTZ1000 thread.

--- End quote ---

Well, good to learn an informed opinion on the value of that. As far as I gathered, its purpose is releasing mechanical tensions/vibrations/whatnot such that the reference would be pampered for immaculate Vref out.

But as much as I tried - multiple times - I've been unable to go through that entire LTZ1000 thread. I don't expect to live quite as long as that requires!... Not with my current lifestyle.


--- Quote from: alm on November 29, 2023, 03:08:11 pm ---I would be concerned about the high temperature set point accrelating aging for a voltage reference.

--- End quote ---

Not sure at all what you mean with this. Is "accrelate" a GRE word?...

Dr. Frank:

--- Quote from: Rax on November 29, 2023, 02:42:04 pm ---In trying to put together more references in my lab, I've also been looking at these.

How much stability is to be expected from these? I am assuming one positive aspect is those available on the marketplaces out there are probably well aged.

That said, the PCB design is very vanilla, with no cutouts, etc.

In case these are worthy to chase, there's then the issue of cleanly supplying them, putting them in an optimal case, etc. Has anyone taken on a project like this before?

Thank you.

--- End quote ---

Hello Rax,
to my knowledge, nobody has used these expensive HP3458A boards as an external, standalone reference.
That makes no sense at all, as these have many disadvantages, and even design flaws.

First, the LTZ1000A is misused by hp engineers, as it's running on about 90..95°C oven temperature. That's necessary because the 3458A is intended as an industry  environment DMM, not as a metrology instrument, i.e. it is specified to run up to 55°C.
As a proper reference, the oven temperature must not be higher than 60°C; the Datron / Wavetek / Fluke 7001 (designed by John R. Pickering) uses 45..50°C instead, like all other oven based references from Fluke, i.e. the 732's. I also have designed my references like this, i.e. @ 45 ... 55°C.
That gives a drift rate from scratch (w/o elaborate pre-aging) of < 1ppm/year, typically -0.5ppm/y.
References boards from hp commonly age -4 .. 8ppm/y, see also the threads about the 34470A and its reference board.
Only the high stability option is tested to -2ppm/y. Waste of money, to my opinion.

As a stand-alone reference, you urgently need a buffer circuit, to protect the LTZ1000 reference from short circuits, which would shift the reference voltage.
As well you can use this buffer to create a reasonably stable 10V output.

HP designed several other errors into the circuit, especially they still assemble this 200k resistor , which should NOT be used on the A version of the LTZ1000, as it creates an unwanted T.C. of about 0.3ppm/K. W/o that resistor, it probably would have near zero T.C.
The whole board is not covered, so the delicate solder joints might create thermal noise from the continuous air draught. In the LT datasheet it is strictly recommended to protect ALL solder joints of the PCB from air draughts.   

There are several other improvements to this standard circuit, like the adding of several blocking capacitors by Andreas, which definitely gives better suppression of external e.m.c., and to my opinion better noise performance.

I have designed the most simple PCB, single sided, non -A version, no voodoo slits, no fancy 4 layers with mystical, squiggled tracks.
Simply thermally balanced tracks, where needed, and least expensive components, i.e. no ultra stable Vishay hermetically sealed, oil filled VHP something resistors.

I have put it in a double shielded enclosure, i.e. tuner box with thermal insulation, and an outer aluminum case, and used a simple 12V supply.
Andreas and branadic have added a low noise LDO on their boards, so that's the best solution, I guess, especially when you add a battery backup.

These circuits simply run ultra stable over time, and are nearly free of noise and "popcorn" disturbances.

So I recommend to build one or two of those reference boards:

https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg1336573/#msg1336573

https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg3886166/#msg3886166

Frank         

Rax:
This is great, Frank, thank you very much for guiding me through the weeds of the gianormous LTZ1000 thread and pointing a couple of posts with very substantial input.

This gives me a lot to go on. Thanks!!

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