Resistor Set for LTZ1000 positive standard 7V circuit

[pitagoras]

Hello,
LTZ1000A + LT1013 are on their way. Several of them. 
Meanwhile I read and read, and re-read again the datasheet as a kid waiting for Santa.
My main problem now is the resistor set.
I asked a quote to Mr Pettis but first I want to understand exactly (well...) what I'm doing. Because budget is not infinite... and I could privilege building "many" references over "ideal" ones.

I understand that R1 (120) is very critical as it sets the current through the zener. I guess it has been chosen very near the sweet spot. I understand that changes in this resistor (either due to temp or long term drift) would obviously change it's voltage drop and thus affect the output voltage... So for this resistor, the best tempco, humidity coefficient, long term drift, etc. that one can get will pay in the end.  So far so good (but see below).

Then there are R4 and R5. They set the operating temperature. For this the datasheet indicates to use the lowest temperature consistent with the environment, adding 10°C for LTZ1000A. "Consistent" here means (if I understand correctly) take a margin for the ±10°C variation due to production. So all this implies making it work at
Ta_max + 10°C [production variability worst case] + 5°C [extra safety] +  10°C [if LTZ1000A]

From this information and the note below the circuit, one might conclude that the exact ratio is not *that* important, provided the ratio is as stable as possible. So if I had 1% resistors with exactly the same tempco (and other drifts) [Of course I understand that probably no such thing exists] , is better than 0.01% resistors with very different tempcos.

Then there are the two 70k resistors (R2, R3). Their drifts impact 300 (500 for R3) times less. So drift for those does not look like a super critical parameter. What I don't know is how critical is their nominal value (as 70k is a rather hard to find value).

So to resume, if I had not an infinite wallet to bet:

1) Put as much as possible in R1
2) Make sure R4 and R5 are the same kind, their values must respect the ratio giving the desired temperature, and selected for same "drift due to the elements". . This can be tricky, so better put some money in those too.
3) if money is left, bet on R2 and R3. Else just put some cost reasonable metal film,  "about 70k" each.


So, is this a question? I hope so, and not a flame war. To summarize, and avoid unnecessary casualties, I think what I'm asking is
1) The rationale of resistor selection when in a budget is correct? (in a limited budget)
2) What is all this 13k stuff anyway? I'm aware that there are discussions about the 13k value, for example, discussions whose origin I don't know, but apparently it's not the datasheet?
3) is 70k important. Would 50k do? and 72k?
4) bonus track: why HP used 111 for R1 and 15k/1k for R4/R5  My guess is they changed the operating temperature with R4/R5 and then selected a new sweet spot with R1, but most people here knows better for sure...

[Andreas]

Hello,

This has been discussed in the LTZ1000 thread many times. So the summary is:

R1 is not the most critical resistor.

https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg833226/#msg833226

https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg656529/#msg656529

So put your money into R4/R5 first.

You will at least need 0.1% resistors. Otherwise you have to put a further safety margin for the temperature setpoint.
(-2mV per deg C against the resistor variations).

the 13K / 1K gives about 60 deg C temperature setpoint.
12K5 / 1K around 53 deg C so this can be used with a LTZ1000A for a operating temperature up to around 35 deg C.

The 70K is not that critical. It is mainly to meet the 100uA collector current for which the device is specced in the data sheet.
But since the unheated tempco of the LTZ increases about 10% when reducing the value to 50K I would rather use a 68K-75K resistor.

with R1 you want to set the 5mA setpoint for low noise.
the 120R with 12K5/1K give around 4 mA only.

So if increasing the temperature in HP with 15K/1K
you would have to compensate for the -2mV/K.
(havent calculated it yet)

with best regards

Andreas

[Dr. Frank]

I want to confirm completely, what Andreas stated already, especially that the stability of R1 is not critical, also not its absolute value.

Use precision wire wound resistors,  either from Edwin Pettis, which may take a while, or from G.R., the econistors, which are all available from stock, 0.1%, T.C. 3ppm/K, and use a set of 12k5/1k, 120 Ohm, and 2x 70k. This will also give the least cost.

Argentina, especially Buenos Aires, I assume to be located in a moderate climate, so 35°C ambient temperature max. for the circuit @ about 50°C oven temperature should be fine. I also recommend constant laboratory temperatures, for example in your basement, for all precision measurements.

HP originally designed the reference for up to 55°C ambient temperature, which requires an oven temperature of about 90°C.
So they violated the LTZ1000A, and operated it practically like the LM399, so that the LTZ1000A also would have a similar drift rate like the LM399. Only by run-in and selection, they could guarantee these - very mediocre - 8ppm/yr.
As the base-emitter voltage of the reference transistor decreases by about 60mV at 90°C, compared to the regular 60°C oven temperature, the zener current would also decrease by about 10%, so they decreased the 120 Ohm to compensate for that effect.


In a stand alone reference, and in real metrological environment, you might even decrease the oven to 45°C for a LTZ1000, non-A version, typically then having < 1ppm/yr. from scratch.

Frank

[MK]

Dont forget that the 1K/13K or 1K/15K ratio is just that, a ratio, that resistor chain is running at 700uA, to stabilise a transistor running at 100uA, so it could easily go to 2K/26K or 2K/30K, also 5K/65K would do, it would help if you want to run it on batteries. going much higher would make it more sensitive to rfi.

when I made a spice model, using 5087? 5089? as the small signal transistors the whole circuit was very sensitive to rfi, even prone to oscillating.

I have three ltz1000a's but in amongst other projects and work currently not enough time to lay out the PCB. I will definitely be using Edwin's resistors for R2, R4, R5 but suspect I may just use RC55's for the others.

One thing I thoght about is to limit the heater power so that the D(temp)/D(time) is controlled to reduce the turn on stress to the die as I suspect that is possibly the cause of slight changes after a power cycle that has been noticed by some.

[try]

Hi Frank,

I want to confirm completely, what Andreas stated already, especially that the stability of R1 is not critical, also not its absolute value.

Is that true?
Why bother buying high end resistors then?

Regards
try

[MK]

Hi Frank,

I want to confirm completely, what Andreas stated already, especially that the stability of R1 is not critical, also not its absolute value.

Is that true?
Why bother buying high end resistors then?


Regards
try

Because as hobbyists it is actually cheaper to overspecify a bit than to fit a resistor then later have to replace it and spend again because the unit is found to be not stable enough.

And also a few people just copy what has been done before and have not done all the error budget calculations for themselves.

[Kleinstein]

The importance of R1 seems to be usually lower than specified in the LTZ1000 data-sheet. If taking the values from the data-sheet R1 would be about as important as the R4/R5 divider. However experience from build circuits showed lower sensitivity to drift in R1. This could be due to the LTZ1000 getting better or a rather conservative specification. So the important resistors are R4/R5 and less important by about a factor of 3-5 the other resistors (120 Ohms, 70 K).

The 70 K value is kind of a compromise in low TC of unheated reference (would prefer larger resistor) and low noise (would prefer smaller resistor).  Both aspects are only marginal for the overall performance. So no need to exactly get 70 K: something like 50 K to 100 K should be acceptable. The low noise circuit shown in the DS even uses 30 K.

[Dr. Frank]

Hi Frank,

I want to confirm completely, what Andreas stated already, especially that the stability of R1 is not critical, also not its absolute value.

Is that true?
Why bother buying high end resistors then?

Regards
try

Yes, it is not as critical, as given in the LTZ1000 datasheet.
This has been proven by several volt-nuts consistently.
The real, measured attenuation factors for drifts are about +/-90 for R4/R5, -600..-800 for R1, -250 for R2, and about -1000 for R3.
The factors mitigate the resistors T.C.s and their timely drift, but all 5 drifts sum up, of course.

As you want to achieve < 0.05ppm/K for the whole reference, it's obvious, that you also want the less critical resistors to have low T.C.s.
So, with a matched set of PWWs @ <5ppm/K , you'll achieve these 0.05ppm/K quite comfortably. Below that, it's getting a tough job.

Same goes for the timely drift, the sum of all the resistors shelf life drift (attenuated by their individual factors) should be far less, than the typical -0.8ppm/year @ 45°C for the LTZ1000 itself.
This parameter is specified very rarely.

Therefore, only good quality resistors with about < 10ppm/year drift should be used.
Make a simple calculation and variation on your own, to get a feeling, how less well specified resistors will influence the performance.

On the other hand, choosing only highest quality and price resistors (e.g. Vishay hermetically sealed, low T.C. MBFs)  is an engineering and monetary overkill, as you'll anyhow end up with a residual T.C. which you may further compensate by several other techniques.

Frank

[Andreas]

Why bother buying high end resistors then?

because we can!

And of course because the AN86 specifies on page 46 the VHP-100 resistors with 0.1%.
(That information is "cut out" in the data sheet).

with best regards

Andreas

[Andreas]

Hello,

by the way.
has anyone tried a thin film resistor array for this?
A 4 resistor array with 2*1K+2*10K could give a 12.1 : 1 ratio for a LTZ1000 (non A) version.

https://www.digikey.de/product-detail/de/vishay-thin-film/ORNTA10-1T1/ORNT-10K-1KCT-ND/1771027

with best regards

Andreas

[feedback.loop]

Hello,

by the way.
has anyone tried a thin film resistor array for this?
A 4 resistor array with 2*1K+2*10K could give a 12.1 : 1 ratio for a LTZ1000 (non A) version.

https://www.digikey.de/product-detail/de/vishay-thin-film/ORNTA10-1T1/ORNT-10K-1KCT-ND/1771027

with best regards

Andreas

They seem to have 25ppm/C tempco

[Andreas]

But not for the ratio

with best regards

Andreas

[Kleinstein]

The TCR matching is specified at +-5 ppm/K. So this might be acceptable for a lower cost version (not super stable). However the long time drift specifications are no not that good: 150 ppm for 2000 h at 70 C. So if useful at all it would need an extensive burn in to get rid of much of the initial aging at least. Also the SMT case might be somewhat sensitive to board stress.

[MK]

OT

[MK]

OT

[MK]

OT

[pitagoras]

I like very much your discussion about LM399, but it's not somewhat off topic here?
I opened this thread because there are 80+ pages of LTZ1000 thread, and this one focuses on resistors only. If it starts deviating then it would not be useful anymore.
Your discussion is very interesting, would you mind opening a new topic please? I will subscribe to it 

[lars]

If we go back to the resistors for the LTZ1000.

If I use simple thin film resistors for the 120 and 70kohm and even the 1k and 13k resistors do I risk a lot except long-term drift and temperature coefficient? For example LF noise, jumps? As far as I understand HP used simple thin film resistors for the 3458A.

As I think I have said before my tests of RC55 in through hole and RN73 in SMD0805, ERA6 and PCF0805 in the range 1-100kohm have been less than 100pm drift the first year and after that less than 20ppm/year. Temperature coefficients has been within maximum specs from manufacturer (sometimes quite close). Humidity sensitivity has been in the same range as wire wounds I have tested, that is < 2ppm/%RH typical 0.5-1ppm/%RH for 10kohm. This has been solder to FR4 proto boards and without power in my normal lab room (16-32C).

Should be interesting if others have long term data from real tests. Anything I have missed on the internet?

Lars

[lars]

The TCR matching is specified at +-5 ppm/K. So this might be acceptable for a lower cost version (not super stable). However the long time drift specifications are no not that good: 150 ppm for 2000 h at 70 C. So if useful at all it would need an extensive burn in to get rid of much of the initial aging at least. Also the SMT case might be somewhat sensitive to board stress.

Two discrete resistors with max +-3ppm/C will give +-6ppm/C ratio so worse than +-5ppm/C. If the about max 0.05ppm/C that will give to the LTZ design is not good enough I probably would go for a better resistor set or trim the TC of the complete LTZ board before measuring and selecting individual resistors.

For me it seems that data from the resistor data sheets are very difficult to apply to the conditions of an LTZ1000 based std in a normal hobbyist lab say 15-35C. So probably 150 ppm for 2000 h at 70 C says nothing?

For R4-R5 it is probably a good idea to have something in the same package like the ORNTA or NOMCA resistor networks. Another possibility is to have a series/parallel combination of the same value. I have at home 10 pcs 3kohm PCF0805 5ppm/C from Digikey that I intended to test in a combination to give 1k:13k (7pcs will give 1k:12k) (let’s see if I ever do the test). My tests of other resistors with the same values seems to indicate that both drift and humidity sensitivity will be in the same direction (but not TC) so a “statistical array” would be of use I guess.

Curious if someone has any data on bending on boards with eg SMT resistors? How likely is it with bending in a DIY home built LTZ design eg similar to  the KX? If this is a problem in the LTZ design with the small sensitivity from the resistors it should be a large problem in other applications?

Lars

[lars]

I have several times read that you should not have the resistors radial in the LTZ design but how much will it actually affect the performance? For me it seems you will have a gradient but it will not change a lot if the board is in a case and the box is not tilted. If the box is tilted is it better if they are mounted axial??

Lars

[Dr. Frank]

If we go back to the resistors for the LTZ1000.

If I use simple thin film resistors for the 120 and 70kohm and even the 1k and 13k resistors do I risk a lot except long-term drift and temperature coefficient? For example LF noise, jumps? As far as I understand HP used simple thin film resistors for the 3458A.

HP indeed used thin film resistors for both 70k, 10ppm/K.
The others were Vishay BMF resistors, 1.3k/ppm.

Why do you bother about noise and jumps?
The LTZ1000s purpose is superior stability.

As I think I have said before my tests of RC55 in through hole and RN73 in SMD0805, ERA6 and PCF0805 in the range 1-100kohm have been less than 100pm drift the first year and after that less than 20ppm/year. Temperature coefficients has been within maximum specs from manufacturer (sometimes quite close). Humidity sensitivity has been in the same range as wire wounds I have tested, that is < 2ppm/%RH typical 0.5-1ppm/%RH for 10kohm. This has been solder to FR4 proto boards and without power in my normal lab room (16-32C).

Should be interesting if others have long term data from real tests. Anything I have missed on the internet?

Lars

PWW and molded BMF resistors are specified to have shelf life stability of < 20ppm/yr, so the complete circuit is more stable from the beginning, than yours, after one year.
I can't imagine, why you're using such unstable resistors with the LTZ1000, that's simple a waste.

Frank

[lars]

Why do you bother about noise and jumps?
The LTZ1000s purpose is superior stability.

That is the reason I bother about noise and jumps from the resistors. Of course I know you can have outliers of both resistors and the LTZ for all kind of parameters. My objective is to solve if normal thin film is not useful at all.

PWW and molded BMF resistors are specified to have shelf life stability of < 20ppm/yr, so the complete circuit is more stable from the beginning, than yours, after one year.
I can't imagine, why you're using such unstable resistors with the LTZ1000, that's simple a waste.

Frank

That is the specification. From real life with about the same conditions soldered on FR4 board I have had up to 30ppm drift the first year for S102. Some S102 tested at the same time had very low drift so the variation was very large. Also remember that both BMF and WW in epoxy have humidity sensitivity that is much worse than the drift. With 1ppm/%RH and a 40 %RH variation you will have a seasonal variation of 40ppm not getting better with time. So even if I have seen a lot of WW and BMF probably having 1-2ppm/year, I say probably as with the large humidity dependence I have had difficulty to get the long term drift.

A side note: My intention two years ago was to make 3-4 LTZ1000 based references but I only have one prototype with thin film resistors and BMF's so far. The reason is I bought two sets WW (except the least sensitive 68k) but found they had up to 2ppm/%RH. So it stopped the project.

With a price of up to 10USD for WW compared to about 1USD for thin film 5ppm/C or 10ppm/C resistor from Digikey, Mouser and others I think it is relevant to see how much each type worsens the complete design. Of course hermetic BMF's for about 50USD each will make the ultimate design?? Sorry I couldn´t resist I know from all your previous post you have a serious attitude.

Lars

[Dr. Frank]

..
With a price of up to 10USD for WW compared to about 1USD for thin film 5ppm/C or 10ppm/C resistor from Digikey, Mouser and others I think it is relevant to see how much each type worsens the complete design.

Of course hermetic BMF's for about 50USD each will make the ultimate design?? Sorry I couldn´t resist I know from all your previous post you have a serious attitude.

Lars

By using T.F. resistors, you very probably have worsened the design upfront, if not by humidity sensitivity, then by their other mediocre stability parameters.

And your further writings just puzzles me, maybe you're not so serious about me?
I have used PWW resistors only in my LTZ1000 circuits, as I've always stated, that BMF resistors have no real advantage, maybe the hermetical ones, but these are engineering and monetary overkill for that design goal.

Frank

[Kleinstein]

As the drift sensitivity for the resistors is in the range of 1/100 and less, noise of the resistors should not be a real problem.

With TF resistors there is also a large variation between different versions. The more difficult properties are long term drift and humidity - the TC is just the parameter that is relatively easy to measure. The humidity effect can be less critical if used in an always warm circuit.

The high quality hermetical sealed TF resistors can be better in stability than many PWWs. Of cause the price is also high and availability is difficult with these.

It somewhat depends on the application whether long term drift is more important than noise.

[lars]

Hello Dr.Frank,

I am absolutely serious about you! Sorry if I missed that you only used WW. I really appreciate what I have read from you.

But I really want to know about T.F not just that they just are worse.

The LTZ in itself are not perfect even for stability and noise and the design of the LTZ is such that it allows worse resistors to some extend as both resistance shift and noise is suppressed. For the resistance shift we have found the attenuation coefficients but not for noise as far as I can see. Also the data sheets for resistors reveals just part of the truth I think.

Having resistors that cost 5-10 times more and add upp to some 25-50USD needs to be considered. Having all wire wound seems not as the final solution if you are cost conscious.

Lars

[lars]

As the drift sensitivity for the resistors is in the range of 1/100 and less, noise of the resistors should not be a real problem.

With TF resistors there is also a large variation between different versions. The more difficult properties are long term drift and humidity - the TC is just the parameter that is relatively easy to measure. The humidity effect can be less critical if used in an always warm circuit.

The high quality hermetical sealed TF resistors can be better in stability than many PWWs. Of cause the price is also high and availability is difficult with these.

It somewhat depends on the application whether long term drift is more important than noise.

I fully agree. But the same seems to apply for WW about humidity at least , and that I seldom see specified. And yes, with about 13C higher temperature the humidity drift is about half is my experience. The problem in my LTZ box with two aluminium boxes, with a peltier between (for future use and temperature tests), is that the resistors are only about 2-3C above room temperature.

For me the RN73, PCFxxx, ERAxx seems to be good candidates from testing, I guess PTF56 might be ok for through hole but I have not tested them yet.

Lars

[Andreas]

Hello,

what about UPF50/UPF25 resistors?
at least they had a very low hysteresis in my T.C. tests.

The two 1K resistors were around 2 ppm/K.

https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg462301/#msg462301

Would be interesting how they behave with humidity.

With best regards

Andreas

[cellularmitosis]

Hello,

I decided to summarize all of the results which have been linked to in this thread.

In the first table, the values are the percentage of drift which will appear in the output.  I.e., for 1%, that means a 100ppm change in R will result in a 1ppm Vz change.

In the second table, the results are normalized against R4/R5, so this is "importance of each resistor relative to R4/R5".

References:

• Andreas links
• zlymex results
• janaf results
• Andreas results
• lars results

Note: these results do not include any "R60" results.

Edit: the "average" column is only the average of the community-measured values -- it does not include the datasheet values.

[cellularmitosis]

Just got my quote back from Rho point about the econistors.  These are the green wirewound resistors seen in Dr. Frank's LTZ1000 reference build.

https://www.eevblog.com/forum/metrology/mx-reference/msg1297126/#msg1297126

(Note that the quantities aren't correct -- there's only one 70k listed in the quote).

(Note also that shipping is half of the total!)

Edit: oops, I cropped out the shipping and total.

Edit2: Oh wait, actually the part number visible in Dr. Frank's photo says "8G16A", not "8G16D".  It looks like that is a different series (which might have better long-term stability?), but is only available in values 60R to 247R from stock: https://www.rhopointcomponents.com/components/resistors/precision-through-hole/rtd-simulation-resistors-100c-to-400c-8e16a-series.html.  It looks like he used that for the 120R?

[Andreas]

It looks like that is a different series (which might have better long-term stability?),

Hello,

according to the ordering list:
https://www.rhopointcomponents.com/components/resistors/precision-through-hole/wirewound-econistor-3ppmc-8e16-8g16-series.html

A = 0.01% tolerance
D = 0.1% tolerance

0.1% tolerance is sufficient for a LTZ-Reference.
(see  application note AN86 Figure I2)

With best regards

Andreas

[Dr. Frank]

Edit2: Oh wait, actually the part number visible in Dr. Frank's photo says "8G16A", not "8G16D".  It looks like that is a different series (which might have better long-term stability?), but is only available in values 60R to 247R from stock: https://www.rhopointcomponents.com/components/resistors/precision-through-hole/rtd-simulation-resistors-100c-to-400c-8e16a-series.html.  It looks like he used that for the 120R?

A few of the 0.1% resistors (D) had been out of stock, so I needed to order some 0.01% (A).

0.1% or even 1% is fully sufficient.

Frank

[Andreas]

0.1% or even 1% is fully sufficient.

Hello,

at +/-1% for the setpoint divider you should have another 5 deg C headroom for the setpoint against 0.1%.

With best regards

Andreas

[cellularmitosis]

Followers of this thread will also be interested in this quote I just received for some Vishay hermetic foil resistors from Texas Components.

I asked about the following resistors and got a prompt reply:

Hello,

...

VHP202Z 120R 1% Y6071120R000F0L
VHP202Z 70k 1% Y607170K0000F0L
VHD200 12.5k / 1k divider 1% Y5076????F0L

For the VHD200 hermetic dividers, is there a Z-foil version of those?

Thanks,
Jason

Hello Jason. Thanks for this new opportunity.

We can offer you the 3 items as you have them here.

The Z Foil exit for the 300144 series, but we don’t have chips in USA. They will be 25 pc. MOQ and 11 weeks ARO. You let me know if want them.

Meantime, let me quote what we can do here, with 3 weeks ARO, maybe sooner.

See below for unit prices,
Nelson

VHP202Z 120R00 1.0% (Y6071 120R000 F0L) @ $27.90 each
VHP202Z 70K000  1.0% (Y6071 70K0000 F0L) @ $40.95 each
VHD200 12.5k / 1k divider 1.0% Abs%Mat (Y5076 V0599 FF0L) @ $46.50 each

For the VHD200 hermetic dividers, is there a Z-foil version of those? Yes. See note above.

I was also thrilled to hear that Edwin Pettis is resuming resistor production!  Soon I'll be able to compare LTZ specimens with Vishay, Ultrohm, and Econistors.   

http://www.texascomponents.com/store/VishayFoilResistors.asp

[beanflying]

It has been nearly 2 weeks since I had a communication from Texas Components  I was being forgiving as Christmas New Year is a PITA for everyone however I know have boards and LTZ's so ...... Hurray up 

[cellularmitosis]

For some reason, their reply to me was flagged as spam, so even though I thought they hadn't replied for a week, it turned out they had replied same-day.  Can you check your spam folder?

[beanflying]

I had been to and fro a few times to the one human but yep email from them on the 23rd of December in my Gmail spam folder  to google it seems 

All good they only got back a day or so back anyway. 

Also making a Vishay based Harmon divider, looking at it may have been cheaper to buy a s/hand fluke 

Wasn't any urgency when the LTZ's were due in Feb......

[cellularmitosis]

Communication with Rhopoint seems a bit difficult.  When I edited my quote request to increase the quantity of the resistors, they updated the quote and more than doubled the price of the 12.5k resistors.  When I replied and asked about this, they just deleted the quote.  Strange.

Perhaps they have 2k and 25k in stock, and I can use those instead.

EDIT: This was a mistake on their part -- they meant to type "6.90" and instead typed "16.90"

[Edwin G. Pettis]

Cellular:

There is absolutely no reason that a 12.5K resistor should cost more than a 70K, it should be ~$3.00 less give or take....I find sometimes that RhoPoint quotes are silly.

[Vgkid]

If you didn't need them immediately, you are running close to the the minimum pricing needed from prime technology (general resistance)

[cellularmitosis]

Thanks, I didn’t know about Prime Technology!

http://www.primetechnology.com/PartsDetails.aspx?PartsId=176&Parentid=82&ParentType=C

[Edwin G. Pettis]

RhoPoint get's their stock from General Resistance, I believe GR has something of a minimum order amount, you might not like it.

[HalFET]

I ended up using platinum rtds once when trying to save cost on a LM399 circuit. If you build the circuit to be ratio based and thermally couple all the RTDs it did seem to work and showed almost no humidity sensitivity since the RTDs were glass passivated on alumina substrates, temperature stability was fairly good since these are manufactured with low tolerances.  Might be worth checking if the same trick could work for the LTZ1000 if you have too much time on your hands like I did back then 

[mimmus78]

Cellular:

There is absolutely no reason that a 12.5K resistor should cost more than a 70K, it should be ~$3.00 less give or take....I find sometimes that RhoPoint quotes are silly.

Maybe they consider 12.5k custom built one :-).

[cellularmitosis]

Paul from Rhopoint reached out to me -- this was just a mistake.  He meant to type "6.90" and instead typed "16.90".

Prime Technology got back to me -- the minimum order is $250.  I've asked for a quote anyway, just to find out how many people would have to do a group buy in order to make it work.

[rigrunner]

Communication with Rhopoint seems a bit difficult.

I've got a request in with them for some FLCY resistors. How long did it take for your first quote to be responded to?

[Edwin G. Pettis]

A set of LTZ resistors from me is $37.90, I give discounts, no minimums.  The question is; why are you fussing with Rhopoint and their nothing special resistors plus high shipping?

[cellularmitosis]

Well, my intention was to have a set of econistors, a set of Vishay BMF's, and a set of your resistors and compare them all, but it looks like I'll be waiting on the econistors for a while yet.

[Andreas]

Well, my intention was to have a set of econistors, a set of Vishay BMF's, and a set of your resistors and compare them all, but it looks like I'll be waiting on the econistors for a while yet.

Hello,

with one set alone of each you cannot judge whats really going on.

Currently I am measuring some econistors.  (3 sets)
Unforunately they have a relative large stray of parameters (at least the 70K see latest post).
And if you think it cannot get worse: wait until I have measured the 8G16 120R resistors.
(I am not really happy with the first result).
And also Frank had its issues with some low ohmic econistors in the past.

So from the PWWs I´d prefer Edwins resistors.
If you handle them with care (using pliers for bending the leads) they are your friend.

with best regards

Andreas

[cellularmitosis]

SO FAR I have  ordered 3 sets of everything.  Not really statistically significant, but more informative than just one set.

[Andreas]

SO FAR I have  ordered 3 sets of everything.  Not really statistically significant, but more informative than just one set.

Hello,

Ok then you should be able to select the best. (after measurement).

with best regards

Andreas

[cellularmitosis]

Quote from Prime Technology.

[cellularmitosis]

I wrote some scripts to crawl the ebay pages of seller "hifi-szjxic" (http://stores.ebay.com/hifiaudioic/) and list all of his resistors, dividers, etc, in formats which are a bit easier to browse.

These scripts run every day just before 9AM central, so these lists will be updated daily.

Here's a list of all of his metal foil resistors, sorted by resistance value.  The format is (resistance value, description of the listing).  I haven't added support for links yet, so if you see something you want, you'll have to copy/paste the description into his store search box.

https://ssl.pepas.com/hifi/resistors.txt

Here's the above list, but only entries which contain the letter "Z":

https://ssl.pepas.com/hifi/resistors-z.txt

Here's the list of ratios (between 12:1 and 13:1) which can be made using pairs from the above list of resistors:

https://ssl.pepas.com/hifi/resistor-ratios.txt

Same thing, but where both resistors in the pair contain the letter "Z":

https://ssl.pepas.com/hifi/resistor-ratios-z.txt

Finally, here's the list of all of the dividers he sells, sorted by ratio:

https://ssl.pepas.com/hifi/dividers.txt

Hopefully these will be useful for other LTZ nuts on a budget

[TiN]

Now you just made it that easier to clear his precious resistor inventory  

[cellularmitosis]

Now you just made it that easier to clear his precious resistor inventory  

I did hesitate selfishly for a moment there

On the other hand, he has over a thousand listings of resistor values — he needs all of the help he can get .  The real rare commodity were the dividers, and he has been out of dividers useful for the LTZ for a while now.  The next most important value is one of the 70k resistors, but that value isn’t specific, so he will have options there for a long while.  The next most likely to run out is 120R, but that is only 15% as important as R4/R5, and he has a number of options left for those.

[rigrunner]

SO FAR I have  ordered 3 sets of everything.

Did you pay Rhopoint anything extra to get a response?  My quote from the beginning of January still says STATUS_REQUEST and emails to sales@ are seemingly ignored.
 

[cellularmitosis]

No, but I did have about two weeks of back-and-forth with quotes until I was finally able to place an order.  There were a few mistakes in the quotes and it took a few days for them to respond each time.  I also had to bug green after the fact about having paid with PayPal (they sent me a bill in the mail, and PayPal said my payment was still “pending” because they had not accepted it).

Everything did work out with enough persistence though!

[rigrunner]

Surprisingly, a little after posting my earlier message i had a response from a guy here in the UK.  He apologised for the delay and said he'd look into it on Monday.  So far they don't seem to be the easiest of companies to give money to 

[cellularmitosis]

Hello,

A fellow board member messaged me asking for some advice about where to find metal foil resistors for the LTZ1000, and I thought my reply might be useful to others who are just getting started, so I'll copy my reply here (and also bump this thread back to the first page

Yup, here's the deal:

The 400k resistor is a value which is used to "tune" the temperature coefficient of the whole assembly.  The board works without it (I haven't populated it yet), then you measure the temperature coefficient of the whole circuit, then try a value (maybe start with 390k), see how that works, then try a little higher like 470k, then a little lower, etc.  You don't need to worry about that resistor yet.

For the metal foil resistors (R1 through R5), there is an ebay seller in china who has a stockpile of metal foil resistors, for about 1/3 of the price of new metal foil resistors, for example look at this listing:

https://www.ebay.com/itm/1x-Z201-100K00-0-01-Vishay-Z-Foil-Series-Metal-Foil-Resistors-Y1453100K000T0L/111862789497

It is a bit of a pain to comb through all of his listings looking for the values you need, so I wrote a script which looks through all of his listings every night, then publishes a sorted list of all of them:

https://ssl.pepas.com/hifi/resistors.txt

That's a list of all of his metal foil resistors.  To buy one, you can copy a bit of the text and paste it into ebay's search bar, e.g. "RNC90Y 100K00 VR", and the listing will come up.

Here's a list of his dividers:

https://ssl.pepas.com/hifi/dividers.txt

It looks like the "12.345294:1" ratio is the only one he currently has which is useful for this circuit.

If you don't want to use a divider, you can use two discrete resistors to make the ratio.  Here's a list of all of the resistor combinations which make ratios between 12:1 and 13:1:

https://ssl.pepas.com/hifi/resistor-ratios-12.txt

I've been using 12.3:1 or 12.5:1 in my circuits so far, which works fine if your reference will be at room-temperature.  13:1 is the default value recommended in the datasheet circuit, but that runs a little hotter, in case you need to run the circuit in a slightly hotter environment (i.e., inside of an instrument which gets a bit warm, like 35 or 40C).  In the 3458A meter, HP uses a ratio of 15:1 which runs very hot.  This means the inside of the meter can get up to around 60C and the reference still works.  However, the hotter you run the reference, the faster it drifts.  That's why I try to use 12.5:1.

This ebay seller has two different grades of resistors.  The regular metal foil (S102C, RNC90Y, etc) are going to be about 5ppm/C tempco.  The "Z-foil" resistors (Z201, RNC90Z, etc) will be under 2ppm/C tempco.

These are more expensive ($10 to $15 each for the z-foil, the regular ones are like $6 to $9 each), and I keep a separate set of lists for only z-foil resistors:

https://ssl.pepas.com/hifi/resistors-z.txt

https://ssl.pepas.com/hifi/resistor-ratios-z-12.txt

There is also an ebay seller in Poland who has a stockpile of RCK02 metal foil resistors (also by Vishay, but blue).  These are about equivalent to the "regular" resistors from the "hifi" seller (about 5ppm/C), but they are even cheaper ($4 to $6).

Here are his listings:

1%, 5ppm, 5R to 148k, $2.77 https://www.ebay.com/itm/1-5ppm-0-5W-High-Precision-Stability-Foil-resistor-RCK02-VARIOUS-VALUES-150R/132261333295

0.1%, 5ppm, 2R to 3.3k, $4.15 https://www.ebay.com/itm/0-1-5ppm-0-5W-Very-High-Precision-Vishay-SFERNICE-Foil-resistor-values-2R-3K3/132092526341

0.1%, 5ppm, 3.44k to 169k, $4.15 https://www.ebay.com/itm/0-1-5ppm-0-5W-Very-High-Precision-Vishay-SFERNICE-Foil-resistor-values-100K-/142274715625

0.05%, 5ppm, 10R to 147k, $4.85 https://www.ebay.com/itm/0-05-5ppm-0-5W-Very-High-Precision-Vishay-SFERNICE-Foil-resistor-values-10R/132121105979

0.02%, 5ppm, 6R to 99k, $5.54 https://www.ebay.com/itm/0-02-5ppm-0-5W-Very-High-Precision-Vishay-SFERNICE-Foil-resistor-VARIOUS-VALUES/132099860265

0.01%, 5ppm, 60R to 180k, $6.24 https://www.ebay.com/itm/0-01-5ppm-0-5W-Very-High-Precision-Vishay-SFERNICE-Foil-resistor-VARIOUS-VALUES/142282995078

For this circuit, the precision of the resistors doesn't matter (1%, 0.1%), only their long-term drift and tempco matters, so if you use the blue resistors, try to get the cheaper 1% ones.

Of these resistors R1 - R5, some matter more than others.  Several EEVblog members have measured how important each resistor is, and I've compiled there results here:

https://www.eevblog.com/forum/metrology/resistor-set-for-ltz1000-positive-standard-7v-circuit/msg1386014/#msg1386014

https://github.com/pepaslabs/px-ref/blob/master/kicad/releases/v2.4/README.md#r1-r2-r3-r4--r5-r45

The R4/R5 ratio's drift is attenuated by about 100:1.  So if that ratio drifts by 100ppm, the voltage of the LTZ will be moved by 1ppm.

Even with the cheap LTZ's (5ppm/C), they would have to drift by 20C to see the voltage move by 1ppm (7uV).  But maybe the two resistors of the ratio drift in opposite directions, which means only 10C would make a 7uV drift...

Next most important is R2, which is 3x less important than the ratio, then R1.  So it would have to drift like 300ppm to caus a 7uV drift.  Then R1 is less important than that, and R3 is less important than that.

So, if you aren't planning on trying to measure the temperature coefficient of your board, don't bother with the z-foil resistors, just get any metal foil resistors and you will be fine.

If you are planning on trying to dial in a very low temperature coefficient for your board, then maybe try to use a divider or z-foil for the R4/R5 ratio and for R2, then use the cheaper regular metal foil for R1 and R3.

oh, wait I just realized that divider has a weird lead spacing:

https://www.ebay.com/itm/1X-300191-16K0674-1K3015-High-Precision-Voltage-Divider-and-Network-Resistors/121921159091?epid=2095964793&hash=item1c63113fb3:g:NY8AAOSwu1VW4u~i

ok, so forget the divider.

For z-foil, use these two for the ratio ($9 each):

https://www.ebay.com/itm/1x-RNC90Z-15K000-BR-Vishay-RNC90-Series-Metal-Foil-Resistors-Y118915K0000BR0L/121779420465?hash=item1c5a9e7d31:g:mTkAAOSw4HVWErRg

https://www.ebay.com/itm/1x-RNC90Z-1K2000-BR-Vishay-RNC90-Series-Metal-Foil-Resistors-Y11891K20000BR0L/111790693166?hash=item1a073e9b2e:g:mPIAAOSwKIpWE~JL

and use this for R2 ($9):

https://www.ebay.com/itm/1x-RNC90Z-68K100-BR-Vishay-RNC90-Series-Metal-Foil-Resistors-Y118968K1000BR0L-/121804687045

Then use a regular one for R1 ($7):

https://www.ebay.com/itm/1x-RNC90Y-120R00-BR-Vishay-RNC90-Series-Metal-Foil-Resistors-Y0089120R000BR0L/111760942444?hash=item1a0578a56c:g:78MAAOSwMmBV5p7A

and for R3 ($7):

https://www.ebay.com/itm/1x-RNC90Y-71K500-BP-Vishay-RNC90-Series-Metal-Foil-Resistors-Y008971K7500BP0L/121794231241?hash=item1c5b807bc9:g:x-8AAOSwl9BWKIAL

Cheers!

Of course you can also get a custom set of Ultrohm resistors made by Edwin Pettis, which are a very good value!  (Excellent long-term drift, and the tempco will typically be under 2ppm/C, and the are usually under $8).

[hwj-d]

Hello,

A fellow board member messaged me asking for some advice about where to find metal foil resistors for the LTZ1000, ...

Maybe the same fellow ask me that too, 
and i answered him to directly search for these ebay article-numbers:

• 112337652641
  (Y0007120R000T0L)
• 121749693656
  (Y00891K00000VR0L)
• 121777259014
  (Y008913K0000BP0L)
• 121794231241
  (Y008971K7500BP0L)

But your lists are of course much more extensive 

[Andreas]

Hello,

perhaps interesting for volt nuts in EU:
I have found that Reichelt stocks some 8G16D resistors.
At least the 1K resistor and 50K (if you can live with the deviation to 70K).

https://www.reichelt.de/drahtwiderstand-axial-0-33-w-1-kohm-0-1-gen-8g16d-1k0-p233202.html?

https://www.reichelt.de/wirewound-resistor-axial-0-33-w-50-kohm-0-1-gen-8g16d-50k-p233208.html?

https://www.reichelt.de/drahtwiderstand-axial-0-33-w-100-ohm-0-1-gen-8g16d-100r-p233199.html?

with best regards

Andreas

[Andreas]

Hello,

I have asked for a quote for 5 resistor sets (120R, 1K, 12K5, 70K) + 10K/25K for the 10V transfer.
The answer was that they only sell the remaining stock and do not want to sell the resistor in future.

with best regards

Andreas

[niner_007]

I have something to contribute

Use VPG 303381, for best factory effort VHD200 12.45K/1K, with maximum 0.5ppm/C over the 20C to 30C range

[Kleinstein]

A super low TC of less than some 5 ppm/K is not really needed. The resistor TC is attenuated by about a factor of 100. The more difficult factor is the long term stability. If a resistor ratio of 1:13 and a higher resistance is OK, one could consider an array like NOMCA for the set temperature.

If it is about the 10 V output, the resistors that really matter are those for the 7 to 10 V step. Here something like the VHD200 can be justified.