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

[Grandchuck]

Not claiming to "derive the current measurement uncertainty of the instrument from that".   I feel that my 3 references are more stable than the DMM.  That is all.

[Rax]

In looking at low-TC resistors, I came across these (see enclosed datasheet). To me, they look like naked z-foils (meaning - the enclosure is stripped off), which I can't imagine being good for their TC... What's any air movement doing to their exposed resistive core? Supposedly though, they have the same TC as their "dressed" siblings (.2ppm/C), but to me that sounds like at best wishful, at worst a sprinkle of fairy dust or snake oil.

Has anyone tried these by any chance? If the observed performance is as stated on the datasheet, I may be aware of a relatively affordable source (vs. performance point).

[Vgkid]

A TC test on the VAR
https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg879467/#msg879467

[Rax]

A TC test on the VAR
https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg879467/#msg879467

So not very good. It looks like an order of magnitude higher than the Vishay claims on TC specifically.

[TUMEMBER]

A TC test on the VAR
https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg879467/#msg879467

So not very good. It looks like an order of magnitude higher than the Vishay claims on TC specifically.

https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg879820/#msg879820
"Re. Z-foil humidity drifts, I had initially used standard (non hermetic) as references, and have seen drifts of about 5-10ppm over the year. That was the end of these references... The hermetic ones were up to Vishays spec (2 ppm in 6 years, low load, iirc).
So if you want to use standard z-foils in the voltage divider (multiplier) of an LTZ design to step up to 10V, they are probably not worth it. I have not done any tests with dissicators or putting them into oil, which my be worth a try, so cannot comment.
The other option /besides hermetic resistors/dividers) that may be more stable is dividers in one package. This way the humidity impact affects both resistors on the substrate (made from the same material) the same way and one would expect a more symmetrical behaviour."

[Rax]

The point is more with the long term stability, but this often not specified very well.

One US manufacturer that seems to provide this data is PRC. Their HR/HVA series - their highest precision and stability series - show a 10ppm/year "greater stability." TCR can be as good as 1ppm/C, or "matched" to .5ppm/C.

Datasheet enclosed.

[Rax]

I'll continue posting here as I find more similar data.

My intention with this "new" LTZ1000 10V reference build thread is to make it as useful and as simple and as I can, organize as much as possible information accrued trough the efforts here (which should be for the benefit of others down the line from me), and to introduce new information where possible.

For instance, I don't think information on these "long term rated" PRC resistors was presented here before (anywhere on the Metrology section).

[Rax]

Research results this far below. Please note this is relative to Frank's design.

• PCB and schematic - see https://www.eevblog.com/forum/metrology/agilent-keysight-03458-66509-dcv-reference-pca-for-3458a/msg5200593/#msg5200593
• R4/R5 set the oven temperature. These should be under 2.5ppm/K TCR. The recommended value is:
  
  • LTZ1000 - 45-50C: 12k/1k
  • LTZ1000A - 55-60C: 12.5k/1k or 10k/800R, depending on resistor available values for a certain type
• R1 (120R) also needs to be under 2.5ppm/K
• R10 is only needed for the LT1000 (per datasheet). Tweaking the value is subject to experimentation. I don't yet have this nailed down.
• R12/R13 - the schematic at the link above shows two values available for two types of resistors. Also under 2.5ppm/K. The relevant part is the ratio - 1.4 - as this sets 10V out from the Vref actual output. For instance:
  
  • FLCY: 10k/4k
  • econistors: 15k/5.6k

In the US, these resistors are pretty pricey, but the only reasonable solution I was able to find is Texas Component Corporation. They sell S102s on the C and K variety, and also Z102s. Some of the values above will need to be custom, some others are available stock.

[Kleinstein]

There is no hard 2.5 pm/K limit for the resistors. The resistor drift is attenuated by a certain factor, that varies for the different resistors in the circuit.
The attenuation ratio is on the order of 100 and if the demand is not too high one may well get away with 10 ppm/K resistors. This is especially with the non A version, when R10 is used to trim the resudual TC anyway. The relevant point is anyway more long term drift and not actually the TC. A precision reference is usually used near room temperature.

It is only the R12/R13 ratio that has much less attenuation (around 3 fold) and here even 2.5 ppm/K may not be good enough.

R4/R5 and also R12/R13 are only relevant as a ratio, so the relative TC / dirft is what matters. This can make quite some difference with resistor arrays.
For both ratios it is possible to use resistor arrays like TDP, e.g. as  6S:2P or 2S:6P for a 12:1 ratio. The specs for the relative TC are not that great (5 ppm/K), but this includes the more tricky low resistance versions. With the relevant 2K or 5 K per resistor the matching is typical much better and averaging also helps a little.

[Dr. Frank]

Rax,
I found my  table 3_, where I collected the measurements from other volt-nuts concerning these attenuation factors.
That means, that the T.C, and the timely drift is attenuated by the sum of these factors for the completed circuit.
The numbering R1 .. R5 refers to the original  LT datasheet.
I have found similar results, and selected my econistor resistors, after I measured their individual T.C.s.
As you can see from table 4_, the calculated overall T.C. of the circuits differ from the measured ones, in case that you don't use this compensation resistor R9 (it's R10 in my schematic).
I think, that the residual T.C. is dominantly determined by the individual LTZ1000.
Andreas made detailed experiments with longer or shorter legs, and was able to show, that the T.C. also depends heavily on that.
Therefore, the ultimate quest for ultra low T.C. of these resistors is not necessary.
Medium stability, lower cost resistors like these econistors (compared to those ultra stable Vishay VHPxxx, hermetically sealed, oild filled ones) do the job perfectly.

I also can tell, that all these references perform very stable and predictable over time (since 2016), as all of those show an initial, annual drift of < 1ppm/y. after a short run-in, and w/o any pre-ageing / baking of the LTZ.  Meanwhile, all of them show drifts between -0.25ppm/y .. -0.5ppm/y. only.

Frank

[TUMEMBER]

There is no hard 2.5 pm/K limit for the resistors. The resistor drift is attenuated by a certain factor, that varies for the different resistors in the circuit.
The attenuation ratio is on the order of 100 and if the demand is not too high one may well get away with 10 ppm/K resistors. This is especially with the non A version, when R10 is used to trim the resudual TC anyway. The relevant point is anyway more long term drift and not actually the TC. A precision reference is usually used near room temperature.

It is only the R12/R13 ratio that has much less attenuation (around 3 fold) and here even 2.5 ppm/K may not be good enough.

R4/R5 and also R12/R13 are only relevant as a ratio, so the relative TC / dirft is what matters. This can make quite some difference with resistor arrays.
For both ratios it is possible to use resistor arrays like TDP, e.g. as  6S:2P or 2S:6P for a 12:1 ratio. The specs for the relative TC are not that great (5 ppm/K), but this includes the more tricky low resistance versions. With the relevant 2K or 5 K per resistor the matching is typical much better and averaging also helps a little.

Very similar conclusions can be drawn from this post
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg1051957/#msg1051957

[Rax]

In the US, these resistors are pretty pricey, but the only reasonable solution I was able to find is Texas Component Corporation. They sell S102s on the C and K variety, and also Z102s. Some of the values above will need to be custom, some others are available stock.

Not to advertise too strongly, but the other advantage to TCC is they have differentiated pricing for resistors from a series, where less precision is cheaper. For instance, the Z201s (.2ppm/C) are available at decreasing cost from .005% to .25%. Hence, where relative TC is critical (ratio of two resistors across temperature differentials), this is very advantageous as one can order less precision but very high TC for a relatively low price.

[julian1]

I can report good experiences ordering from texas components also.

Turnaround time, from order placement, to *custom* manufacture, to having the parts in my hands was ten days.

For the product lines they carry z201, s102, vsmp, smnz etc - and unless somebody tells me otherwise - I suspect they are the original and sole manufacturer of these parts that are more commonly known,sold, and marketed under the Vishay/VPG name.
Presumably it was decided there was still a good business-case to carry on trading under their own name, even after they were acquired/folded into the Vishay group.

Edit. clarify potentially misleading point.

[Rax]

courier options limited and expensive.

I am not quite seeing that - the shipping options I'm presented with are two (FedEx 2nd day and "domestic 2nd day," which I assume is USPS Priority) and they're roughly at the price I'd expect them to be retail. Though maybe julian1 is looking at international shipping which may be an entirely different business.

[Rax]

But turnaround time, from order placement, to *custom* manufacture, to having the parts in my hands was ten days - which is what actually matters.

The other positive aspect to them is this custom resistor option. So one can ask for any value they want/need and they'll make it as ordered. If the turnaround on that is 10 days, that's pretty dope. And at a price that's lower than the regular, large, off-the-shelf retailers in the US (for the same exact product).

[julian1]

I fear I've given the wrong impression from my comment.
Ordering is very simple and streamlined, and international courier pricing is not unreasonable. 
The point I wanted to convey was not to be put off, if the UI experience isn't as slick as digikey or mouser.
They are a *very* handy vendor to know about.
hopefully they will continue taking small-lot custom orders from the general public, even if they probably only just break even.

[Rax]

The relevant point is anyway more long term drift and not actually the TC. A precision reference is usually used near room temperature.

I found and provided some spec data on the PRCs, and found some reports of 1-2ppm/yr drift for the Z201s (which I can pretty easily source in the US, alongside the S102s). So the Z201s seem to fare pretty well.

I'd be interested to learn what other resistors may have returned in similar tests. Namely, whether there are some standouts in a world where this data is seldom specified.

[Kleinstein]

Data on the long term drift are rare and even if one finds something this is often for accelerated tests at higher temperature or similar and the question is if all units behave the same. Most data-sheets are written when the parts are new and at that time there are naturally no long time data available.
There are some long term dirft data for the Susumu RP SMD resistors, though it is hard to read the graphs. I consider these resistors more at the lower end for what makes sense with the LTZ1000.

Anecdotal data from more normal users of the resistors can be tricky as they can be biased (getting more of the extremes) and the conditions are more real life and often less comparable. There is more than just time and temperature to effect the long term drift. Temperature and humidity cycles and maybe PCB stress can have an effect.

For the resistors directly at the LTZ1000 the Z201s are overkill . For the 7 to 10 V gain part they can be justified.

[Dr. Frank]

 
I can confirm what Kleinstein stated: long term drift is rarely specified, and if done, it's often not reliable, or the drift specification is barn-door wide.

Especially all claims from Vishay have to be treated with utter care!

I purchased 5 EA, 10kOhm, 0.0005%, with measurement protocol, oil filled, hermetically sealed VHP202Z from Vishay, which were "specified" to age < 2ppm/ 6 years.

I don't know until today, what went wrong at Vishay, but those resistors, which I wanted to use as secondary standards, already had already drifted out of specification on arrival.
The annual drift, until today, was always -1ppm/year only. The promised low T.C. of the Z - technology, as of typical (!) 0.05ppm/K, or so, was not met at all, in reality it is between 0.3 .. 1pppm/K, which is following their more realistic maximum specification.

On the other hand, in my hp3458A from 2000, they already used those VHP101 for the internal 40kOhm standard, which is hermetically sealed, probably oil filled, definitely rock stable, as seemingly it is not drifting at all / any more, or not more than +/- 0.5ppm over at least 6 years. I precisely monitor my resistors over many years by aid of ab precision, btw.

This VHP101  consists of two resistor chips in series, with C and K characteristics, i.e. their T.C.s nearly cancel, to give a very low overall T.C., which my monitoring data also reveal. If I remember correctly, the T.C. seems to be below 0.2ppm/K.. I'll check that
I always recommend to order VHP101 if intended for such high stability  applications.

Those are not necessary for the LTZ1000 circuit.
Frank

[Rax]

Thank you both.

From my research this far, other than TCC, every other place I talked to asks for volume and/or takes weeks to source (if you meet the first criteria). Pricing is not available until you meet the first two criteria. Honestly, my time is more valuable than the price difference from average cost to great resistors exceeding the requirements that cost a bit more. When looking at off the shelf resistors, they are not available at needed value, and they're typically more expensive (in some cases far more expensive - we're talking some getting close to $200, cause why not?  ) than the custom ones from TCC (which are not in all cases higher cost than their "off the shelf" ones?... go figure).

Then at TCC specifically, the price difference between Z201s and any of the S102s is either small or even at the advantage of the Z201 in some cases. I ended up getting a set of Z201s for about $20 each.

It's true, Rhopoint in UK has 2ppm resistors for about $7-8 (though not all values in stock, and those not in stock need 20+ volume and have a lead time of 28 weeks...), but by the time I factor in the cost of shipping I'm just about breaking it even with TCC.

PRC sent me around to their distributors and that led to more useless email exchanges and that just piles more wasted time.

Just the quirkiness of today's markets and availability of products and supply chain shortages and who knows what else.

[Rax]

Also, btw, I couldn't find the combination of values needed at the recommended ePay sellers either. Being that the relative TC is critical, I'm keen on making sure any pairs are of the exact the same kind of resistors, including manufacturing batch.

[Andreas]

hmm,

please read:
https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/

many resistors of the same batch with different outcome.
in the single digit / sub ppm T.C. range every resistor looks individual.
The amount of cement to bond the metal to the substrate is never the same nor equally distributed.
And small differences in packaging have large effects.

with best regards

Andreas

[Andreas]

Hello,

another recommended reading:
https://www.eevblog.com/forum/projects/vishay-bulk-foil-drift-after-soldering/msg445297/#msg445297
so solder with care.

with best regards

Andreas

[Jendas]

Hello,

with inquiry about precision resistors I tried to contact this manufacturer: http://www.direct-token.com/en/resistors-ppm/index.html. They make a wide variety of various resistors. I send them a email about UPCS resistor type (similar to Vishay S102 or Z201 etc.. ) with this response on specific values:

UPSC 70K T C10 P, US$6.5/Pc for 30Pcs/MOQ.
UPSC 12K T C10 P, US$6.5/Pc for 30Pcs/MOQ.
UPSC 1K T C10 P, US$6.5/Pc for 30Pcs/MOQ.
UPSC 120R T C10 P, US$6.5/Pc for 30Pcs/MOQ.
Lead Time is 5~6 weeks after order and remit payment confirmed.

All resistor are C10 = ±2 ppm/℃,  and T = ±0.01%
Datasheet: http://www.direct-token.com/en/pdf/resistors-ppm/ultra_precision_resistor_upsc.pdf

with best regards
Jendas

[alligatorblues]

Regulator ICs like the LTZ1000 make possible very stable voltage standards. But after only a short time futzing with 2 of them, I realized a precision oven is required, because the external components add a lot of temp drift. I completed one, fully enclosed, very stable at 10VDC. but temp drift of 1.0ppm/C. Even though the regulator is temp regulated, and even if the heater control is well implemented, the other components have to be inside a precision oven to obtain the results experimenters want.