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

[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]

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.

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.

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

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

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?

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]

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

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.

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

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

[Dr. Frank]

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.

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!!

[Rax]

Not to push my luck too much on this, but does anyone still have any excess boards lying around in the aftermath of their "LTZ1000 reference design adventure," or maybe gerbers or KiCad files, or whatever the design files were?

PM works as well, if you prefer.

[Rax]

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         

OMW, Frank!... Order from OSH Park pending (thanks to Andreas/Jason!), I should be up and running in a couple of weeks.

Meantime... Is there a BOM available? Or at least, which parts are more critical? I am aware some of the resistors are.

[Rax]

It looks like R1 through R5 have enhanced requirements for tempco. I think Frank used 3ppm types, and I'm combing the internet for affordable ones in the US. Anything under 5ppm is an arm and a leg though... 

[alm]

Have a look at the Sfernice resistors from: this Polish seller. Shipping to the US shouldn't be too bad. Initial tolerance doesn't matter much. I don't know how many values they still have in stock, but note that for most positions it's the ratio, not absolute value, that matters. Details are on the forum, but I don't have a link handy. I wrote a script for that a long time ago that would find suitable resistor pairs. I could probably dig that up if you're interested.

[Kleinstein]

In the normal design the most critical resistors are R4 and R5 setting the oven temperature. Here the ratio is what matters and an array with 8 equal resistors (e.g. TDP type or NOMC) with resistor combinations is an option for a few ratios.

The next critical ones are R1 and R2. R3 is already less critical and does not have to match R2 - it is just convenient to have a shorter BOM. Also the value for R2/R3 does not have to be exactly 70 K some 47 K or 75 K would be OK too.
There is a table on how much the drift of the resistors is attenuated.

By far the most critical resistors will be the one for the 7 to 10 V step, if one only has a 10 V output. With both a 7 and 10 V out one may be able to measure this separately and thus correct for this.

For the resistors a 5 ppm/K and even 10 ppm/K would be OK. The point is more with the long term stability, but this often not specified very well. A low TC is more used as an idnicator for good quality, though the TC does not directly correlate with long term drift. It is more that high excess noise may indicate a tendence for long term drift. For this reason, I no longer like the PTF56 series very much. Similar the NiCr NOMC version is perferred over the TaN NOMCA version.
If SMD is OK, I would consider the Susumu  RG series as a relative afforable option, possibly even the 10 ppm/K ones.

The op-amp is usually still a LT1013 - keep in mind the unusual pinout of the SO8 version. I personally see not need for one of the better versions here.

[3roomlab]

OSOP 12x array ?
it allows the 12.5:1 ratio
eg digikey : OSOPTC2001AT0

In the normal design the most critical resistors are R4 and R5 setting the oven temperature. Here the ratio is what matters and an array with 8 equal resistors (e.g. TDP type or NOMC) with resistor combinations is an option for a few ratios.

The next critical ones are R1 and R2. R3 is already less critical and does not have to match R2 - it is just convenient to have a shorter BOM. Also the value for R2/R3 does not have to be exactly 70 K some 47 K or 75 K would be OK too.
There is a table on how much the drift of the resistors is attenuated.

By far the most critical resistors will be the one for the 7 to 10 V step, if one only has a 10 V output. With both a 7 and 10 V out one may be able to measure this separately and thus correct for this.

For the resistors a 5 ppm/K and even 10 ppm/K would be OK. The point is more with the long term stability, but this often not specified very well. A low TC is more used as an idnicator for good quality, though the TC does not directly correlate with long term drift. It is more that high excess noise may indicate a tendence for long term drift. For this reason, I no longer like the PTF56 series very much. Similar the NiCr NOMC version is perferred over the TaN NOMCA version.
If SMD is OK, I would consider the Susumu  RG series as a relative afforable option, possibly even the 10 ppm/K ones.

The op-amp is usually still a LT1013 - keep in mind the unusual pinout of the SO8 version. I personally see not need for one of the better versions here.

[Rax]

I could probably dig that up if you're interested.

Yes, please. Thank you!

[Rax]

In the normal design the most critical resistors are R4 and R5 setting the oven temperature. Here the ratio is what matters and an array with 8 equal resistors (e.g. TDP type or NOMC) with resistor combinations is an option for a few ratios.

The next critical ones are R1 and R2. R3 is already less critical and does not have to match R2 - it is just convenient to have a shorter BOM. Also the value for R2/R3 does not have to be exactly 70 K some 47 K or 75 K would be OK too.
There is a table on how much the drift of the resistors is attenuated.

By far the most critical resistors will be the one for the 7 to 10 V step, if one only has a 10 V output. With both a 7 and 10 V out one may be able to measure this separately and thus correct for this.

For the resistors a 5 ppm/K and even 10 ppm/K would be OK. The point is more with the long term stability, but this often not specified very well. A low TC is more used as an idnicator for good quality, though the TC does not directly correlate with long term drift. It is more that high excess noise may indicate a tendence for long term drift. For this reason, I no longer like the PTF56 series very much. Similar the NiCr NOMC version is perferred over the TaN NOMCA version.
If SMD is OK, I would consider the Susumu  RG series as a relative afforable option, possibly even the 10 ppm/K ones.

The op-amp is usually still a LT1013 - keep in mind the unusual pinout of the SO8 version. I personally see not need for one of the better versions here.

Well, I did some forensics in my parts bin (and several projects on the back burner) and found some stuff:

• A bunch of PTF56 and 65s, such as: 1k (.05%/10ppm); 100R (.1%/5ppm); 20k (.01%/10ppm); 200k (.1%/5ppm); 100k (.05%/10ppm); 10k (.05%/5ppm); 50k (.1%/10ppm); 100k (.1%/10ppm); 600R (.1%/5ppm)
• A TT RC55Y: 100R (.1%/15ppm)
• Z201: 100R (.005%/.2ppm)
• These are in a loan of sorts, but may be able to use them: Z102k(?): 4k (.02%/2.5ppm?); 1k (.05%/2.5ppm); VHS102: 40k (.1%/2ppm); Dale WWA-23: 30.1k (?/?)

I'll research the topic of ratios I need to be seeking, but that's what I have at hand (list for myself, which I thought won't hurt posting here).

[alm]

I could probably dig that up if you're interested.

Yes, please. Thank you!

I couldn't find what I used for the resistance ratios, but I did find what I used to extract the list of values. if you open the developer console (right click -> inspect, then go to console tab) on an eBay page with a list of resistance values like this, and paste this line will print out a list of all in-stock values on that page as CSV. Do this on every page in the shop, paste the results in a CSV file and then do some spreadsheet manipulation (probably pivot table) to make it into a neat list with one value per row.

Code: [Select]

options = []; for (option of document.getElementById("x-msku__select-box-1000").childNodes) { if ((option.attributes.disabled?.nodeValue != "disabled") && (option.childNodes.length > 1)) { options.push(option.childNodes[1].data) } }; console.log(document.location.href, ',', document.getElementsByClassName('x-price-primary')[0].innerText, ',', options.join(','));


[Rax]

this Polish seller[/url]. Shipping to the US shouldn't be too bad.

Wow, that's a really good source (particularly assuming all those are genuine, which I take as the case due to you recommending it - not that I assume any guarantees, but at a minimum good experience from an exigent customer ). With the exposure here, I wonder how long they'll have what I assume is a limited stock.

Shipping charges are not a buck more, nor less than what I pay to my usual domestic large distributor (where I purchase 90% of my parts). Of course, transit will be distinctly different, but at least the overall cost of the project can be kept at bay.

[Rax]

In the normal design the most critical resistors are R4 and R5 setting the oven temperature. Here the ratio is what matters and an array with 8 equal resistors (e.g. TDP type or NOMC) with resistor combinations is an option for a few ratios.

From the precision resistors I already have here, the only combo with a 12.5x ratio I can see is the 50k/4k combo. But I assume being different types (different TCs, etc.), that's actually a very poor fit.

[Rax]

By far the most critical resistors will be the one for the 7 to 10 V step, if one only has a 10 V output. With both a 7 and 10 V out one may be able to measure this separately and thus correct for this.

The R12-R15+R18 is a very confusing set. I assume this is for fine setting the voltage out for 10.00000V. Not sure which value for each resistor (for instance, R12 vs. R12A, as they're not supposed to be populated at the same time, correct?) is supposed to be used. Particularly as I'm not looking to get 7V out. I'm only looking for 10V out of this.

[Kleinstein]

In frank's schematics the resistors with the A are alternative footprints to allow different resistor types on the same PCB. Usually they overlap and thus no parallel use sensible. The value of R14 depends on the individual reference voltage and maybe the exact values of some other resistors. The resistor combination is used to get most of the gain from 2 very stable resistors, than a small part  (e.g. 1-3%) for a coarse trim (R14) and finally the fine trim from the trimmer.

There are different ways to combine resistors. With THT parts chances are one could also fit a slightly different configuration (e.g. one more trim resistor) on the same PCB.

I would consider it good to have not only acess to the 10 V output, but also the 7 V - ideally with a 2nd buffer amplifier. This would allow to check for drift of the 7 to 10 V stage.

P.s.: the 7 V output from the FB divider may be good enough, as this is essentially only for testing either the 7 or 10 V, but usually no both at the same time.

[Rax]

In frank's schematics the resistors with the A are alternative footprints to allow different resistor types on the same PCB. Usually they overlap and thus no parallel use sensible.

That's what I thought, but in some cases (R12, R13) the main and the alternate seem to have different specific values. Are those there just to point out a couple of options for the same resistor?

[Rax]

P.s.: the 7 V output from the FB divider may be good enough, as this is essentially only for testing either the 7 or 10 V, but usually no both at the same time.

I assume this means JP9, correct? It seems almost a "free" 7V output, and I assume no trimming is really necessary for it (other than gain considerations). I guess relative examination of JP8 and JP9 would allow for monitoring the drift of the "7V to 10V"stage.

For reference, I am adding here the v2 schematic (I think this is the latest and greatest), and the PCB "parts side."

[Kleinstein]

The different values for R12 / R12A and so one are different example combinations, not bond to the form factors. It is mainly about the ratio and staying in a reasonable range.

I would ideally consider an added resistor (could be a simple one) between the 7 V raw reference and where C9 and the LTC1052 meet, so to really keep away switching spikes, though the LTC1052 is a low current noise one and thus only relatively small spikes. The resistor would also avoid capacitive loading the LT1013.

Not much trimming for the 7 V - if the non A version is used one may want a selected R10 to fine trim the TC. The suitable value depends on the units and details like the thermal setup and set temperature. The 400 K in the DS are more like an order of magnitude number. The actual best value may be from some 100 K to 1 M.

[Grandchuck]

Hi Rax.  Regarding your Reply #19, I have three of those.  That board and circuit are a good choice.  My three are better than my Agilent 3458A, which has a measurement uncertainty of 3.64 ppm on the 10 volt dc range (I have the Agilent calibration report).

[alm]

My three are better than my Agilent 3458A, which has a measurement uncertainty of 3.64 ppm on the 10 volt dc range (I have the Agilent calibration report).

How can you tell measurement uncertainty from a calibration report? The calibration report will state the deviation at the time of calibration (which is not an uncertainty but an actual measurement) and the uncertainty of the calibration, which is more a statement about the calibration procedure and the lab, than the meter. If you sent your LTZ-based reference to the same lab and it got calibrated using the same equipment, you'd likely get a very similar uncertainty.

Of course the specifications of your 3458A will state an uncertainty, but so will the LTZ datasheet. In either case you can develop a better estimate using multiple calibrations over a period of time.

[Grandchuck]

Thanks alm.  After years of measuring and expermenting, it is clear that the Dr. Frank references are more stable than the DMM.  I have some collected data that, to me, support my assumption.  I could be wrong, of course.  I am not in a position to buy more stuff or employ professional calibration services.  I have sealed off that rabbit hole 
I just might be a recovering voltnut.

[alm]

Thanks alm.  After years of measuring and expermenting, it is clear that the Dr. Frank references are more stable than the DMM.  I have some collected data that, to me, support my assumption.  I could be wrong, of course.

I would agree that's likely, and I'm not challenging your assertion, but I'm confused how you can look at a cal certificate and derive the current measurement uncertainty of the instrument from that. And in particular how you can learn anything about the stability of an instrument unless you have multiple calibration certificates (which I agree is an expensive hobby).