Stability of 5V references compared to AD586LQ

[Kleinstein]

There is no need to evacuate. The point of a hermetic seal is keeping the humidity constant inside The actual humidity level choosen should not be that critical. Starting near the normal environment may even help to make residual leaks less relevant. So an extra gas like argon also has downsides.

Even without vacuum it would make sense to have a seprate PCB and enclosure to avoid stress from variable air pressure.

[David Hess]

Besides atmospheric effects, evacuation would also remove the thermal effects from convection.

[magic]

There is no need to evacuate. The point of a hermetic seal is keeping the humidity constant inside The actual humidity level choosen should not be that critical.

Not sure if long term temperature variations could shift humidity distribution between the chip and surrounding air inside the can?
My first instinct would be to fill it with desiccant to be sure.

[iMo]

The epoxy packages are full of water (like 5% of the volume). With temperature variations the water and water vapors will mess with the package, imho. I would get rid of the water before closing an epoxy package (or the epoxy pcb) into the hermetic package. Afaik 2 days at 150C shall do the trick..

[Kleinstein]

If the temperature changes, there is some shift in where the humidity goes preferably. However in most cases the difference is not very large as the binding energy for the water seems to be not that variable. As an example the RH in equilibrium with salt solutions does not depend much on temperatur. This also applies to many solid materials (e.g. wood at a given level of humidity).
Adding some desiccant can be helpful as it increases the amount of water the system can hold - so a small leak wout not have that much effect and one may start rather far on the dry side if wanted. One may get some initial drift from getting the plastic parts in equilibrium with the closed cell.

One may still get a shift of humidity if there are temperature differences in the can with parts that can hold humidity. This could be an argument against adding desiccant if it gets a different temperature than other parts inside.

[Conrad Hoffman]

Most voltnuts don't care about size, at least I don't. I wonder about the feasibility of buying dice and mounting them to a simple alumina substrate that could be put in the hermetic housing of our choice? Alas, I have no wire bonder. Or, buy some easy-to-seal package from somebody like this- https://www.jitai-electronics.com/hermetic-package-ic/ No experience in this for many years, so I don't know if it's a crazy idea or not.

[iMo]

Such a package will cost you an arm and a leg.. Perhaps more than an LTZ1000A with all the resistors around. My guess..

[Andreas]

Hello,

very interesting ideas here.
But I fear the only one that I might test is the suggestion with the paraffine.
(hoping that the time constant will be > 1 Year so that I cannot distinguish it from normal ageing).

In the mean time the PCBs for the REF7050 have arrived and I started with population of the first sample (missing only some parts).
The next sample I will try with a dead bug mounting.

with best regards

Andreas

[F64098]

I mentioned in long time ago in another thread. The Morion MV180 contains a MAX6350ESA in the inner housing to generate the reference voltage.
So it's double hermetic contained and also precisely temperature controlled. Maybe it's not the best reference at all, but for me it looks like a "reference" for
other, unsealed references in view to their reaction to air humidity or air pressure.

Best regards

Frank

[iMo]

There are "(Ultra) High Vacuum Sealants and Adhesives" for usage with vacuum gear, afaik.
Pretty expensive, but somebody told me in past you may seal a glass tube with it hermetically..
Would be great to investigate..

[Conrad Hoffman]

The usual concern for high vacuum stuff is outgassing. The NASA list here- https://ntrs.nasa.gov/api/citations/19970027853/downloads/19970027853.pdf is useful if you look for low total mass loss (%TML) and volatiles. The problem with good epoxies is you either have to buy a rail car full or they're quite expensive, with high minimum orders. Interestingly, some of the RTV rubbers are quite low, probably seal well and are easily available. I don't have any info on the permeability of paraffin vs epoxy vs RTV vs the hot resins that so many things used to be sealed with. GR standards were sealed with hot tar-like resins (or maybe they were tar!) as were ordinary D-batteries. IMHO, it's hard to beat a soldered joint!

[Zoli]

My small contribution:
1. I've built two references, based on LT1236-10, schematics and picture attached. Details on the construction: the inside aluminum box is sealed with rubber "ring" on the top, and RTV along the wire exit; is suspended in hard foam within the fibreglass enclosure. Is powered from 2 9(8.4 Li)V batteries. They agree for the last 3 years with my 34401A's within the last(sixth) digit; I consider them  5&1/2 digit references.
2. Vacuum grease: I've worked on a few(~10) vacuum related projects; not high, down to 10^-3 Torr range. The goto vacuum grease for those projects was Dow Corning 111(or the current equivalent).

[Conrad Hoffman]

Very nice- is that a Rose enclosure?

[Zoli]

Very nice- is that a Rose enclosure?

The fibreglass is Hammond. The aluminum is unknown origin, of which I had two lying around; originally was some kind of remote sensor, the cable exit is an SMA hole.

[iMo]

The next sample I will try with a dead bug mounting.

I would highly recommend.. Use thin wires (say 0.1mm), such the package has got some freedom of movement (a trampoline  ).
Like a small 12x12mm cutout in your pcb, with pads for the wires.
Btw, why not having a temperature sensor on your pcb (wired to the mcu)?

PS: I would trampoline the LTC2400 as well..

[DavidKo]

What about using the enclosure with some desiccant inside. Before closing leave the whole setup at ~125°C to let the water evaporate and than seal the container. You can use the silica gel or zeolite.

Glass metal interfaces are always tricky. What I remember from vacuum lectures the glass is usually named after the metal to be compatible. If the tube design should be used. One can use the viton sealing around the legs (copper for example), evacuate for longer time period and fill the tube with some inert gas with small over pressure. water will have hard time to get in and the device will have "constant" atmosphere.

[Andreas]

Hello,

I would highly recommend.. Use thin wires (say 0.1mm), such the package has got some freedom of movement (a trampoline  ).
Btw, why not having a temperature sensor on your pcb (wired to the mcu)?
PS: I would trampoline the LTC2400 as well..

The sensor was one of the missing parts. (I wanted to see the golden reference on the photo).
Now populated. (orange golden blob on the reference).
Hoping to get the same temperature stability (< 1 ppm over 10-40 deg C) like on my AD586 references.

Trampolining the LTC2400 is useless.
The LTC2400 does a self calibration (offset + full scale) on each single measurement.
So it behaves like a chopper amplifier.

For the reference: I typically solder the GND-Pin of the reference directly to the NTC pin to get some stability and sense the temperature.
The other pins with thin VERO-Wire (~0.2 mm)

My small contribution:

Nice design.
Did you do any T.C. measurements (hysteresis) or rH% measurements with these samples?

[Zoli]

...

My small contribution:

Nice design.
Did you do any T.C. measurements (hysteresis) or rH% measurements with these samples?

Thanks for the appreciation; the build reflect what I've had available at the time, and try to respect common sense and best practices, both in the enclosures and electronics.
Measurements: unfortunately I didn't had/have the possibility to make any of those measurements(lack of access to environmental chamber), only pressure/vacuum test: passed ±50kPa for 1h.
As bonus, original resolutions close-up/bare PCB(I still have 3 left ).

[iMo]

@Zoli: perhaps add the diode in your next pcb revision (see below). Also the RV1 trimmer - generally it is not recommended to use those plastic trimmers with the precision references..

[GigaJoe]

did play a lot trying to seal plastic chips .

major problem of plastic it do affect the crystal from any angle , as it molded and surrounded,  comparing to the metal \ ceramic case, where crystal attached to one side.

so first and foremost, as little as possible minimize any forces \ tension during mount \ soldering \ fixing chip on board  ;  should be like a float design. 

at second as a result from above,   anything that apply any forces to the plastic surface -  not work at all.  any solid material such as paraffin , rosin , create a forces that transferred over plastic to the crystal , basically any solid filling that contrast or expand, don't work at all ,

the best case i got -   floating design, thermally compensated by set of diodes,  heater +55C all submerged in , transformer oil ,  in the thermos.
as liquid -  Silicone or fluorocarbon-based oils would be the best,  then transformer oil , then mineral oil ...

an alternative: DIP8 , glued to the owen +85C,  and very thick around few cm Styrofoam box.  some kind headache to adjust temperature compensation for this temperature.

finally - small tin can , Italian canned fish, edge tinned , and cap are dual copper FR4 , copper wire through,  soldered on both sides, can soldered around edges to PCB cap.   -  more  less

heated oil bath - i have better then other result

[Zoli]

@Zoli: perhaps add the diode in your next pcb revision (see below). Also the RV1 trimmer - generally it is not recommended to use those plastic trimmers with the precision references..

Thank you for your concerns; let's see what the datasheets has to say:
The LDO

FEATURES
Reverse-Battery Protected
No Reverse Current
No Protection Diodes Needed

ABSOLUTE MAXIMUM RATINGS
IN Pin Voltage .........................................................±20V
OUT Pin Voltage ......................................................±20V

APPLICATIONS INFORMATION
Protection Features
The LT1761 regulators incorporate several protection
features which make them ideal for use in battery-powered circuits. In addition to the normal protection features
associated with monolithic regulators, such as current
limiting and thermal limiting, the devices are protected
against reverse input voltages, reverse output voltages
and reverse voltages from output to input.

The reference

Trimming Output Voltage
A 10kΩ potentiometer is recommended, preferably a 20 turn cermet type with stable
characteristics over time and temperature.

The trimmer is a Bourns 3296W:

Features
Multiturn / Cermet / Industrial / Sealed

Additionally, if you take a closer look to the previously posted picture(Close-up.jpg), on the top you can see a diode in DO-41 case connected between the switch and the low voltage detection board. It's a 1N5817, whose sole role is to protect the LP2951 on that board, even if it supplies the reference board, too.

[andrewtaylor]

In the mean time the PCBs for the REF7050 have arrived and I started with population of the first sample (missing only some parts).
The next sample I will try with a dead bug mounting.

The REF70xx are very nice, I also recommend the dead bug style which I use in all my reference boards.
I recommend you take a few (3..6) REF7050 and compare them first for the one that best suits your needs (drift, ageing, etc.) before choosing one for your final PCB.
All REF70xx are good, but some variation exists in the batch you'll get.

[iMo]

..The trimmer is a Bourns 3296W:

Features
Multiturn / Cermet / Industrial / Sealed

..

Imho that is not an indication for a suitability with precision voltage references..
I was using those blue bricks with several references and found out with temperature sweeps (like 20-50C) it made small voltage jumps. I've been still using one in my 2xLT1021 ref with a wax drop put over the adjustment screw.

Also what I've learned - those trimmers have pretty large "dead zone" when turning the adjustment screw up/down. After I adjust the proper voltage I always turn the screw a little bit back, such it is positioned in the middle of the dead zone. Thus the screw is not engaged with the internal gear. It seems to me that helps a bit with stability. Mind the plastic gear inside (afaik) changes its shape with temperature/aging..

[Svgeesus]

1. I've built two references, based on LT1236-10, schematics and picture attached.

Could I ask about the design of your trimmer circuit?

The LT1236 datasheet suggests a trim circuit with a diode between the trim output and the wiper of the cermet trimmer, says that this is related to maintaining a low tempco, and mysteriously hints that for this reason the resistor values should not be altered (while not giving resistor or trimmer tolerances, tempco or any other details).

[Svgeesus]

Oh - I hadn't noticed that the 10V and 5V versions use quite different trim circuits.