Stability of 5V references compared to AD586LQ

[Andreas]

Hello,

I widely use battery powered ADCs (LTC2400 based) with AD586LQ (temperature compensated) as work horses for my measurements.
The reason why I rely on them can be seen in the attached diagram: ADC13, ADC15, ADC16 are built with AD586LQ.
ADC17 is based on LT1236AILS8-5 which is typically about a factor of 2 less stable.
And even the LM399 based 6.5 digit DMMs (the K2000 with additional temperature compensation) are not so stable when reading my most stable 7V reference LTZ#4.

Since the AD586LQ is now near unobtanium (in small quantities) I am looking for a replacement.
Of course a heated reference could do the job but I want to operate the systems battery based so power consumption of the whole cirquit should stay below 5 mA.

with best regards

Andreas

[Andreas]

Hello,

first try is with AD586MNZ (the same reference in DIP8 plastic package)


I built ADC29 + ADC30 with AD586MNZ for testing.
Note: I am measuring the LTZ#4 at the input of the LTC2400.
The 5V reference is at the reference input.
So if the reference drifts to positive values the readings of the LTZ#4 on the input of the ADC go down.

Since the reference is still not pre-aged much I measured the average ageing of the references
ADC29:  +0.517 ppm/kHr
ADC30:  +0.366 ppm/kHr

Unfortunately the plastic package of the reference suffers from humidity sensitivity
(only the GND pin is soldered to the PCB the others are connected with thin wires so the PCB has no influence)

ADC29: +0.41 ppm / %rH with a time constant of ~2.61 days
ADC30: +0.42 ppm / %rH with a time constant of ~2.13 days

The diagrams show the readings from day 0 of measuremen in red.
The "prediction values" based on ageing and %rH in green.
And the filtered %rH values which are measured all 15 minutes and PT1-filtered by a time constant of 2 days or 3 days in blue.

As next candidate I want to evaluate a REF7050 in LS8-package. (still waiting for the PCBs)

with best regards

Andreas

[deepfryed]

MAX6350 seems better on paper at least, though only available in the plastic DIP packages. Also the DIP versions are EOL, but still available from Digikey.

[The Soulman]

ADC17 is based on LT1236AILS8-5 which is typically about a factor of 2 less stable.

I would expect those to be better than that, also after the first month of use? Do you see correlation with temperature or humidity?

[zepto]

What about LT1027? Advantest used it in their AD7461A which allowed for integration periods up to 10 seconds.

[Andreas]

Hello,

MAX6350 seems better on paper at least, though only available in the plastic DIP packages. Also the DIP versions are EOL, but still available from Digikey.

MAX6350 has a large ageing drift (initially ~10 ppm/year for the first 5-7 years). After 10 years I now have ~5 ppm/year which is still much higher than AD586 (< 1 ppm/year).
Additionally seasonal changes due to humidity of ~5 ppm.
https://www.eevblog.com/forum/metrology/tales-from-the-mouth-of-a-voltage-reference-ageing-box/msg3618924/#msg3618924

ADC17 is based on LT1236AILS8-5 which is typically about a factor of 2 less stable.

I would expect those to be better than that, also after the first month of use? Do you see correlation with temperature or humidity?

The parts in my ageing box are read under constant conditions (heated up to 50.0 deg C before measurement each day). ADC17 is temperature compensated.
I fear it has to do something with the die attach (EPOXY) which creates hysteresis over temperature changes.
https://www.eevblog.com/forum/metrology/tales-from-the-mouth-of-a-voltage-reference-ageing-box/msg3618927/#msg3618927

What about LT1027? Advantest used it in their AD7461A which allowed for integration periods up to 10 seconds.

which package? (the metal can package is no longer available).
I had used LT1027CCN8 (DIP8) in ADC4 + ADC8 with about 0.5 ppm/%rH humidity drift.
This was solved when using the AD586LQ in ADC13.
The LT1027DCLS8 (LS8 ceramic) in ADC21-ADC24 showed the following.
- many of the samples show large hysteresis on temperature changes -> you have to select them to be < 1ppm.
- if soldered directly to the PCB there is again a large temperature hysteresis -> so it can be only used "dead bug" style.
but even then the ageing drift after 5 kHrs is still too high for my needs.

https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1040415/#msg1040415
https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1349435/#msg1349435


I have been asked wether I have already tested molding of the plastic reference in paraffine.
But I have no experience with that. And I fear that over my temperature range of 16-34 deg C I will not get it really tight.
So as with other conformal coating I might get only a increased time constant for the humidity.
Does someone else have experience with paraffine?

with best regards

Andreas

[guenthert]

Well, availability seems to be about zilch, but the ADR1001 comes (will come?) with a 5V output, doesn't it?

[iMo]

Yep, the ADR1001 comes with 7V and 5V and/or -5V and/or 10V out, and all the stuff is in ceramic and hermetic/ovenized (default=70C, you may set the temperature with a single ext resistor from aprox 40C to 120C).
Finetuning of the output voltage possible with a single ext 110ohm resistor (the zener current).
You would need buffers for some configs. The performance wise perhaps better than the 399 (let us wait on the final release). Hopefully we will get the official release and DS soon ..
Will not be cheap (but much cheaper than $$ to get 5V off the ADR1000 or ADR1399), however (perhaps at the ADR1000 level, the estimated price in fall 2022 was $60).

PS: the notorious link https://www.eevblog.com/forum/metrology/adr1001-ovenized-voltage-reference-system/

[deepfryed]

Wow that sounds nice, looking forward to getting my hands on one.

[iMo]

@Andreas: some times back I had an email conversation with the guy (in CZ) producing the nixie tubes, asking him whether it would be possible to encapsulate a small electronics module into a small evacuated glass tube like the nixies he makes. He indicated basically doable with several technology challenges, as usual.. 

https://www.daliborfarny.com/

[Conrad Hoffman]

Way back when I did my reference I found plastic packages unsuitable. AFAIK, there is nothing you can pot things in, including paraffin, that will solve the problem, just slow it down. The old GR mica capacitors were sealed in an aluminum box using resin and they included a generous tube of desiccant. Probably good for a long time, but not forever. It might be a suitable answer for a reference, since you have to re-cert it every so often. IMO, the tube is a great idea. I've been going to try and duplicate the old vacuum thermistors used for oscillator stabilization, but never seem to get around to it. They have to be evacuated to a pretty decent level and I don't have a good enough pump, but a reference would be less fussy. It could even be back-filled/flushed with dry nitrogen or argon, no pump required at all.

[Andreas]

Hello,

sorry but the ADR1001 is out of scope (5 mA limit)

Nice idea with the tubes.
Optimum would be with copper wires soldered into Kovar eyelets like in the vishay VHP resistors.
The usual Kovar leads will cause additional thermal EMF.
 
with best regards

Andreas

[magic]

@Andreas: some times back I had an email conversation with the guy (in CZ) producing the nixie tubes, asking him whether it would be possible to encapsulate a small electronics module into a small evacuated glass tube like the nixies he makes. He indicated basically doable with several technology challenges, as usual.. 

You could empty a TO-3 can, mount stuff inside, add desiccant, and solder it shut.

[iMo]

The tube with some traditional electronics components inside cannot be evacuated to the same level as with the nixies. The vacuum will be several orders of magnitude worse.
But that is not a big issue, imho. What is important it will be hermetic and partially evacuated (lowering the thermal flow inside).
The wiring - that is one of the issues as the tube's pins cannot be soldered but welded (there is the large heat during the process in that part close to the pins). Or to use long wires instead of the pins.
Also the module itself has to be placed at the very top of the tube to be away of the heat (you will be melting the glass at the very bottom). Perhaps a longer tube will be required, or to use some thermal insulation between the bottom and the module at the top (or cooling of the top side somehow).

[iMo]

@Andreas: some times back I had an email conversation with the guy (in CZ) producing the nixie tubes, asking him whether it would be possible to encapsulate a small electronics module into a small evacuated glass tube like the nixies he makes. He indicated basically doable with several technology challenges, as usual.. 

You could empty a TO-3 can, mount stuff inside, add desiccant, and solder it shut.

There are feed through pins with glass-to-metal seal available.. You may make a small metal box, drill the holes, solder the feeds in, solder it shut..

[Echo88]

https://youtu.be/N1F4duEZR_s?t=1359 Like this one. Maybe there are fitting/cheap OCXOs to gut.
A glass tube ref isnt really feasible due to the glass fusing heat, but would be really sexy. 

[iMo]

Would be interesting to compare the performance of his hybrid "ADR1002" with the cheapo ADR1001

[Gyro]

The problem with most hermetic solutions, particularly the glass tube is the materials used for the leadouts (Kovar / Dumet?) and thermocouple effects. The only thing that gets around this is the hollow sleeve type glass hermetic seal where you can solder a copper wire right through without interruption. These used to be available NOS from former Soviet countries on ebay but the listings seem to have dried up. Whether this is because the warehouses are all in Ukraine, or supplies have really been exhausted, I don't know but they are worth keeping a eye out for.

Using such feed-throughs, it's really easy to make hermetic enclosures using soldered copper or brass sheet. As Conrad says, flushing with dry Nitrogen or Argon is much easier than evacuation. Argon is the most easily available inert gas (welding supplies), it has half the specific heat capacity of air though. I'm not sure whether that's actually good or bad - less cooling vs less effect from convection.

[Conrad Hoffman]

With proper design, you can seal off a glass tube without putting too much heat into the contents. I can envision an LS reference package wired dead-bug. I think there's even a way to get copper wire through glass by flattening it and keeping it small, though I may be thinking of some other wire type.

edit- look up Houskeeper glass seal (note the spelling).

[Kleinstein]

Beside covar wire another option may be a tungsten wire, as is used with some (especially halogen) light bulbs. The thermal expansion may be good enough and also the thermal emf is not that much different from copper (~ 1 µV/K difference).

With constant, relatively low power and symmetry the thermal EMF should not be such a big problem. It can add a little to the reference but a mianly constant part.

Instead of a TO3 case, I would consider a THT crystal oscillator can - some NOS ones can be optainted quite cheap (or used one found) and the space may even be enough for a SO8 case inside.

[magic]

You could empty a TO-3 can, mount stuff inside, add desiccant, and solder it shut.

There are feed through pins with glass-to-metal seal available.. You may make a small metal box, drill the holes, solder the feeds in, solder it shut..

Same thing, but transistor (or oscillator) cans are off the shelf items available everywhere that only require simple modding to put them to use.

Regarding Kovar, AD586Q uses it too. Intuitively, it shouldn't be too big a problem if the reference is a low power one and isolated from external heat sources.

[Gyro]

Some solder-in ceramic feedthrough capacitors could possibly be considered hermetic to, I'm not sure but I've certainly seen glazed ones. Parasitic capacitance might cause issues on some nodes if not isolated but would be a positive benefit on the supply pins. I don't know what wire they use but it might be copper.

I guess, within reason, the chamber should be as large as possible so that all relevant components - scaling resistors etc can be kept in an inert atmosphere, with just the connections for a fine trim pot brought out.

[deepfryed]

Is AD587 an option ?
 https://www.digikey.com.au/en/products/detail/rochester-electronics-llc/AD587KQ/12094126




Ah 5V references, should've double checked

[The Soulman]

Andreas,

Thermal hysteresis is a real concern PITA but I have not noticed such on lt1021 in H-package (tested 10 pcs, always settle back within a single ppm after a +50 dgr. C. thermal excursion).
Sadly the H versions are discontinued (although Arrow have some in stock including the 5V version (€€..)).

Also, when looking at the adr1399 datasheet it seems that the ceramic package performs (much) worse on thermal hysteresis compared to the to-46.
If this is a inherent function of a ceramic package (more mechanical stress on the die as you already mentioned), a metal can v-ref is still the way to go in 2024.
Sadly not much on offer.

Or.. avoid thermal fluctuations and keep a steady (controlled/raised) temperature.

[David Hess]

I imagine trying to solder a lid down onto copper clad board and then evacuating it, but even if the FR4 material could hold a vacuum, it would put enormous stress on the board so the reference would need to be mounted to a separate board inside, in which case why not use a real enclosure, like a copper pipe with copper caps and evacuate that?  The high thermal conductivity of copper helps maintain an isothermal environment.

And that leaves how to get the signals out and in, but solderable hermetic feedthroughs exist, or feedthrough capacitors.