Stability of 5V references compared to AD586LQ

[Andreas]

Hello,

Obviously they sell not to every country:
"This Shop has no inventory available in your location"

On the other side: I still have some AD586.
The only question that arises: what to do when my stock is depleted.

with best regards

Andreas

[Gyro]

Strange, because they list shipping to just about every other EU country, eg. £10.29 to the Netherlands. I think they simply forgot Germany.

It's a good price for an LQ though. I picked up one of them a while back but haven't got around to testing it yet. I wish I'd bought up some bare SVR PCBs when Joe Geller was shutting up shop!


Edit: Oh, "Excludes: Afghanistan, Austria, Germany, Libya, Nauru, Russian Federation, Somalia, Switzerland, Yemen". You're in interesting company!

[magic]

I see only 1pc available and are you sure that it's legit?
You can get dozens of AD586-not-really-L from AliExpress, anyone can import and resell them.

[Gyro]

From the seller's other items, my ebay-fu says yes, probably.

[Andreas]

Hello,

fortunately some ADR4550D showed up datecode #2351
Two of them mounted dead bug style on the REF7050 PCBs.
Hoping that the hysteresis will be much lower.

with best regards

Andreas

[argintviu]

Regarding hermetic enclosures, I was thinking to make one for a NOMCA network used for a 7->10V amplifier as it's the only non hermetic part in the reference. At first I thought to make a square hole in the PCB right under the SOIC and pour refined beeswax under the package then over it. This way the package would be surrounded with wax. From what i know it is supposed to be quite water resistant.

The second cheapest thing (I think) would be to cut, unroll and flatten copper pipe to make a small box and solder it. Then carefully remove the bases of 2 x HC49 THT crystals (to get 4 wires), make holes in the copper lid for the HC49 bases and solder them on. Then solder the resistor divider to the wires, put the lid on and solder the cap to the box. 3 wires are enough for the divider, the 4th could be tied to the case. For HC49 crystals, the seals are usually Kovar-glass and the case plating is nickel (according to the material declarations) so it should be easy to solder it to copper. Wax could also be poured inside. Might be fiddly though .

[iMo]

Wax itself will not be hermetic (we talk here water vapors resistant).
The package made of metal like the copper foil with hermetic feeds through would be the solution. No need to pour wax inside (wax may contain water as well). I would heat up the complete package with resistors or reference chip inside with a small hole made in it for a while (say 100-150C such the water on the inner surfaces evaporates) and then I would shut the hole up with a solder drop (while the package is still hot). Under normal conditions all surfaces are covered with a thin (called mono-molecular) layer of water (and sometimes with other stuff as well) here on our planet..

PS: another option is to put the package (with the small hole in it) into a high vacuum and solder the hole shut inside the vacuum after a while, but that would be a pretty difficult exercise for a hobbyist, imho.

[argintviu]

I see. It's probably the same with silicone oil, right? Can't they be desiccated by heating before using?

Regarding the assembly, it would probably need to be baked for a few good days at 100C to get it dry before sealing. One could also use a piece of pipe directly like David Hess suggested and put the HC49 feedthroughs on each of the 2 caps, but then it would be difficult to hold them at the same temperature. Maybe use a larger diameter pipe and have both feedthroughs at the same end and mount it vertically. The hard thing is separating the bottom part of the crystal without breaking the seals.

[argintviu]

To make things easier, one can solder a capillary tube to the case, draw the air out, mechanically crimp/flatten it and then solder the top for a final seal. I saw this being done on older fridges after filling in new refrigeration gas.

[iMo]

You may buy the ceramic hermetic feeds through on ebay or else. I DIY in past such I soldered the feeds through off the old bulky metal germanium and silicon diodes. And yes, as I wrote above the complete package has to be baked at those temperatures for some time to get rid of the water inside, then sealed up with a drop of tin. No oil or wax needed when the package is soldered properly.
The epoxy package needs to be baked at 150C for 48-72 hours to get rid of the water (5% of the volume, btw).

[argintviu]

Unfortunately the cheaper, soviet hermetic seals are long gone from the bay. The only other thing I found that seems to have the hollow type solderable kovar seals are the K40Y-9 paper-oil capacitors. But I have no idea how they're built and if they can be dismantled in a convenient way.

[Kleinstein]

There is no need to get the inside of a hermetic chamber dry. The problem with humidity is not the water by itself, but the changes in the humidity. A temperature change effects the RH. With a circuit that gets warm in use and is used on and off this will cause some hysteresis type variation from humidity dirven out from the PCB when hot and back in when clod. So sealing with a low humidity does have benefits. It would be less of an issue for a circuit that runs 24/7.

Wax and plastics are not 100% humidity tight, so they won't keep humitidy out over a very long time. The first target would be to slow down the humidity variation over a year. Once this is filtered out, humidity would no longer change very much. With large enough a container and absorbing material, one could keep humitiy out for an extendet time even with plastics seals.

[Andreas]

Hello,

first result on ADC#35 (ADR4550D + LTC2400) measuring a ADR1000A reference: (first NTC calibration of the on board sensor)
the initial drift ~4-5 ppm for the first 12 hours is now settling at ~1 ppm per 12 hour temperature cycle. (0.12 deg C / minute ramp)

Hysteresis seems to settle at ~4ppm (+/-2 ppm) which is better than ADR4550 in SO-8 plastic package (~16ppm) but still more than I hoped.

with best regards

Andreas

[miro123]

Seems promising Andreas.
The normal sequence is
1. Ageing tests  - then aged components are used for further tests
2. TC tests
3. Hysteresis tests
On such way you exclude counting of multiple errors at once.

[Andreas]

Hello,

in my world:

0. NTC calibration of on board sensor (but why not peek for ~T.C. and hysteresis)
1. after ~1 week first T.C. test (at least if I have stable buried zener references like AD586). Hysteresis is measured at the same time.
2. T.C. of the Offset of the 2:1 divider
3. INL of the ADC (with T.C. and offset calibrated out)
4. further tests like noise (popcorn), PSRR ...

so NTC calibration was the main task

with best regards

Andreas

[PCB.Wiz]

I see TI have a part code for REF54500, but no stock visible ?

Looking at Digikey, I also see Masach Tech Ltd. and Würth Elektronik have seamless RF shields, some for clip over a mounted base, some for solder.
https://mm.digikey.com/Volume0/opasdata/d220001/medias/images/987/MFG_MS102-10S.jpg
Available in trays so production suitable, they could give a simple cavity.

I guess they need open vias during soldering, which could be soldered or taped closed manually later.
 

[Andreas]

Hmm,

I am not shure what you want to tell me with that.
I am not a native english speaker.

But I think that a plastic packaged device is not suitable for my needs.
With or without external cavity.
Or do you have experiences with that device regarding humidity?

with best regards

Andreas

[Andreas]

Hello,

next initial measurement this time with ADC36 with 2nd ADR4550D.

It shows lower T.C. and lower initial ageing drift.
But slightly higher hysteresis of 6-7 ppm.

with best regards

Andreas

[miro123]

Hi Andreas,
What type of temperature curve are you using?
Ramp-up Ramp-down rate? Do you apply dwelling time for every measure temperature point?

[Andreas]

Hello,

the Setpoint ramps with 0.12K/minute with some plateau at minimum and maximum temperature. (so one complete cycle lasts ~12 hours)
Since temperature is never the same at 2 different points the actual temperature on the PCB differs somewhat from the heat spreader
especially when the refrigerator is switched on/off.

see attached picture

with best regards

Andreas

[iMo]

It seems to me your temp speed is still too high (0.12C/minute).
Would be interesting to see what happens when you do for example 3-4 temp points in each direction only and wait say 2 hours on each temp point till the temp at the specific temp point settles..

[Andreas]

Hello,

might be an option to reduce the hysteresis somewhat under controlled conditions.
But I fear that it will not help under my "real life" conditions.
I have in summer gradients up to 3 deg C/hour.
One extreme gradient in winter (forgot to close the window) was 6.5 deg C/hour.
And there are other references which show < 1 ppm hysteresis under this conditions.

On the other side I have recognized that in the ADR4550 datasheet they "cheat" with hysteresis.
They have introduced the terms "half cycle hysteresis" (~100 ppm) and "full cycle hysteresis" (~8ppm)
So the "low thermal hysteresis" is only related to the "full cycle hysteresis" which I would call "ageing drift" instead of hysteresis.
The full cycle hysteresis is practically useless.
After transport of a reference I can measure the actual temperature but not the "history" wether it was higher or lower before.

I made a comparison of the last temperature cycle from NTC adjustment (0.12 K/minute) with a preliminary T.C. adjustment (0.06 K/minute).
The problem is that the ageing drift is still much too high (now 1.5 ppm/day). But I fear the ramp speed has little influence on the hysteresis.
(should say 2*half cycle hysteresis).

with best regards

Andreas

[miro123]

Hi Andreas,
I dont want to discourages you but I want to share my little experience.
I was searching to good Vref candididate voor my sigma-selta adc acqusition board. Requirement ware small size not ovenized Vrefs for 2,5V. tested few of them two LTC6655,  two REF5025, two REF6025,  four ADR431, two ref3425. Nor of them give me sufficient results. Sometime results ware inconsistent due to influence of many parameters. aging, humidity just to name few of them.  Finally I give up.
I lowered my requirement and start testing six LM399 and four ADR1399
Long sotry short -
1. ADR1399 in TO packade is definitly suprerior against LM399.
2. Both LM399 and LM399 are another league of devices. - I get consistent results with good and bad performing devices.
3. I like testing of ADR1399 due to the tighter tolerances.
At the moment I'm processing the RAW data.
See Attached - I have measured the voltage difference between two ADR1399 - The volatge diference is roughly 12mV - Data is captured with DMM6500 - 17200sec~5Hours.
Two Notes
  - some TC difference appears from my test jig -
  - those ADR1399 are pushing my test equipment to the limit.
  - ToDo list - I want to do cross correlation of results between my - HP3456,HP34970, DMM6500 and M3500. Then I can extract more info about the source properties.


 

[Andreas]

Hello,

I know that LM399 / ADR1399 would fit my precision needs.
But I want to have low power consumption too.

Perhaps you should have tried LT1236AILS8-5 with a LTC1043 or LTC2053 precision divider to get 2.5V.
Most of them have a low hysteresis even on the very first temperature cycle. (see table of selection)

The only downside is a ~ -3ppm/K tempco on most devices.
I want to evaluate how good this can be compensated by measurement of the internal chip temperature at the trim pin.
The Trim pin is amplified and then measured by a 12 Bit ADC to get ~0.03 K resolution.

with best regards

Andreas

[Andreas]

Hello,

first results from LT1236AILS8-5 devices.

I had chosen #08 and #13 from the table.
#08 had low T.C. but higher hysteresis and is populated on ADC#33 now.
#13 had ~2ppm/K T.C. but low hysteresis and is populated on ADC#34 now.
As temperature sensor I use the TRIM-PIN of the LT1236.
The pin has ~ 4.0V with ~ 1.7 mV/K and 100K source resistance.
To evaluate that I use a amplifier to scale 10-40 deg C to 1.5V - 3.5V for the 12 Bit ADC of a PIC.

Against my NTCs with 10 Bit ADC the temperature is relatively linear.
But the 12 Bit ADC appears a bit more noisy (+/-1 count around current value) whereas the NTC was stable except for the transitions.
Now the question is if it is related to the additional high ohmic amplification or if the higher resolution ADC is the guilty.

Peeking to the T.C. of the LT1236 it seems that now both have around +/-2 ppm (4 ppm total) hysteresis.
#08 has near zero T.C.
#13 has 2.2 ppm/K
But both have some "jumps" at the lower temperature ends during each temperature cycle.

when comparing to the old measurements 10 years ago I see that obviously the measurements where made at 0.3K/minute temperature gradient. Whereas today I use the more realistic 0.12K/minute. Can this be the reason that the hysteresis was lower in the old measurements? Does the die attach creep more with lower temperature gradients?

with best regards

Andreas