Best out-of-the-box 10V reference?

[pitagoras]

Hi,
I would like to build my first "Mini Metrology Lab" (Conrad Hoffman).
Which would be the preferred way today for building a reference that can provide 10V without calibration?
I mean, I don't have easy access to a precision DMM... so it would need to have a reasonable 10V without adjustment.
Thanks

[Galaxyrise]

budget?

[Andreas]

Hello,

from datasheet
VRE310 (1 mV)

others: (2.5 - 5 mV)
REF102C
AD587U
LT1236A

I would take a AD587UQ

with best regards

Andreas

[Conrad Hoffman]

LT1236 might be interesting, 0.05% and 5ppm tc. Not very expensive. I'd be careful to float it on 3 points to avoid mechanical stress of the package. The 587 is also very good, but costs a bit more.

[pitagoras]

budget?

I don't think I can set any specific budget beforehand. Let's say $50?
The idea is to have a reference to calibrate many other things by, given that I don't have access to precision instruments.

[pelule]

How about AD588B reference, 10V +/- 3mV (+/-0.03% or +/-30ppm)?
Specs: http://www.analog.com/media/en/technical-documentation/data-sheets/AD588.pdf

I bought just for fun two of the cheap offered boards from China (15 EUR).
Link: http://www.ebay.de/itm/AD588-High-Precision-Voltage-Reference-Module-Multiple-Output-AD588BQ-/252181862385?hash=item3ab735d7f1
I had planned to use just the board, as I have 1x AD588BQ and 1x AD588JQ.

When the boards arrived, I tested them and found both well in spec.
Vref at 10V (+/-10ppm), Tempco less 0.8 ppm/°C and so far long term drift (yet) of less than 1ppm/1000hrs.

PeLuLe

[e61_phil]

How about AD588B reference, 10V +/- 3mV (+/-0.03% or +/-30ppm)?

3mV or 0.03% are 300ppm on 10V not 30ppm

Perhaps, anybody can measure such a reference and send it to the TO

[guenthert]

How about AD588B reference, 10V +/- 3mV (+/-0.03% or +/-30ppm)?
Specs: http://www.analog.com/media/en/technical-documentation/data-sheets/AD588.pdf

I bought just for fun two of the cheap offered boards from China (15 EUR).
Link: http://www.ebay.de/itm/AD588-High-Precision-Voltage-Reference-Module-Multiple-Output-AD588BQ-/252181862385?hash=item3ab735d7f1
I had planned to use just the board, as I have 1x AD588BQ and 1x AD588JQ.

When the boards arrived, I tested them and found both well in spec.
Vref at 10V (+/-10ppm), Tempco less 0.8 ppm/°C and so far long term drift (yet) of less than 1ppm/1000hrs.

PeLuLe

Uhm, 0.03% are 300ppm.  The AD588 is good, but not *that* good.  There are variations of the module listed above on Fleebay.  I like the "KKMOON" one, comes with a fairly nice case, 4mm banana plugs, convenient rechargeable (via USB plug) batteries and someone went through the trouble of measuring the output (with a HP34401a in unknown calibration state) and write the values down.  As far as I can tell works as advertised. 

[IconicPCB]

Hello Pitagoras,

It does look like a useful project and quite within range of possibilities. The 10V reference is a major issue.

In fact It does not need to be so much of a problem if a well known reference of any voltage is available. The ten volt source could be referenced and checked against any other well known source and characterized.
Later the ten volt source could be used as initially postulated in the article . But in reality 10V is an arbitrary choice where any other voltage would do.

A chatter named TiN ( working on HP 8.5 digit HPDVM has been able to offer in the past LTZ1000A based source. Unfortunately he has run out of printed circuit boards and apparently is not in a position to offer the reference any more. May be if we were to aggregate a number of people and purchase the PCB ( 4 layer affair ) , TiN might be persuaded to build more references. 

[TiN]

There is public open PCB of my design order available on OSH Park. Note they are not 10V, but 7.XXXXXX V sources, without any robustness (no protection against overvoltages/shorts/power polarity errors, etc).
However PCBs are lesser problem, compared to resistors (and their cost) required. So far I used only VPG resistors (with few internal test only exceptions) on my boards, and those set back from 120 to 350$ a set to build one PCBA. So it's rather expensive feat, and does not quite fit "Mini Metrology Lab" budgets/requirements.

Also LTZ module can't be used to check DMMs without calibration, which is whole another story (which takes fair bit of time and require at least a month of aging before calibration can be done). 

[pitagoras]

A chatter named TiN ( working on HP 8.5 digit HPDVM has been able to offer in the past LTZ1000A based source.

Yes I've followed somewhat his thread. Correct me if I'm wrong but LTZ1000 is the most stable source, BUT it does not provide a KNOWN voltage out of the box. So we would need a calibrated LTZ1000 source? Would he do that? Aged + calibrated LTZ1000-based reference?

[IconicPCB]

My  misunderstanding. Apologies.

[Conrad Hoffman]

You can also build the MML voltage source just as originally designed, but use the plastic package, the only one available now. It will be quite accurate if you match the gain resistors on the opamp. I don't think there are many references trimmed for initial accuracy that are going to be substantially better. AD588 can be 0.05%, but IMO, they're way too expensive.

[David Hess]

I like the idea of buying a bunch of references and building something but you can get an already calibrated 0.01% voltage reference for $30 from VoltageStandard.com.

[pelule]

    **
How about AD588B reference, 10V +/- 3mV (+/-0.03% or +/-300ppm)?

Sorry, was a typing error...

Conrad Hoffman
I don't think there are many references trimmed for initial accuracy that are going to be substantially better. AD588 can be 0.05%, but IMO, they're way too expensive.

Thats, why I tried that cheap Chineese (crap) offer at 15 EUR.

When the boards arrived, I tested them and found both well in spec.
Vref at 10V (+/-10ppm), Tempco less 0.8 ppm/°C and so far long term drift (yet) of less than 1ppm/1000hrs.

Thats my measurment results so far, still ongoing...
Just my idea for a cheap "out-of-the-box 10V reference" - compared to the cost of the LTZ1000 based references.

[Conrad Hoffman]

Unless you have a better meter than 5.5 or even 6.5 digit, there's little need for an LTZ1000 based circuit. FWIW, the original MML boards, once aged, only moved a couple ppm per year, sometimes even less. I checked one after a decade that was still that good, so those buried zener reference chips may be better than you think. Whatever you build or buy, it should be possible to find somebody to measure it, maybe some local company that has a common 6.5 digit Keysight meter or similar.

[SvanGool]

@Conrad Hoffman

You can also build the MML voltage source

Forgive me  my ignorance, can you explain what an MML voltage source/board is?
EEVblog search doesn't process three letter acronyms 

[The Soulman]

@Conrad Hoffman

You can also build the MML voltage source

Forgive me  my ignorance, can you explain what an MML voltage source/board is?
EEVblog search doesn't process three letter acronyms 

Mini Metrology Lab

http://conradhoffman.com/mini_metro_lab.html

[SvanGool]

Sorry, should have known that, temporary black-out 

[Conrad Hoffman]

Sorry, that's the problem with TLAs!

[The Soulman]

Sorry, that's the problem with TLAs!

Ok, what's a TLA? 

On a different note, is it correct that (buried) zener references have better long term stability than bandgap references?

[bingo600]

Sorry, that's the problem with TLAs!

Ok, what's a TLA? 

On a different note, is it correct that (buried) zener references have better long term stability than bandgap references?

Three Letter Acronyms ??

IBM had a thick book , just explaining their version of them

/Bingo

[The Soulman]

Sorry, that's the problem with TLAs!

Ok, what's a TLA? 

On a different note, is it correct that (buried) zener references have better long term stability than bandgap references?

Three Letter Acronyms ??

IBM had a thick book , just explaining their version of them

/Bingo

Thanks... 

[Andreas]

On a different note, is it correct that (buried) zener references have better long term stability than bandgap references?

Hello,

in principle: yes (if you compare the same package and the bare reference element).
In practice: there is also a influence of the scaling resistors which are used to get from the 6-7V or 1.22V to the final output voltage.
The best packages are hermetically sealed ones like metal can or CERDIP.
From the plastic packages DIP8 packages behave usually better than smaller (SMD) packages.

so for best performance you should use a 6-7V temperature compensated buried zener in a hermetically sealed metal can package.

With best regards

Andreas

[retrolefty]

A non-volt-nut here with a question.

 Why must there be a 10vdc output standard with all the added complexity and variation of the opamp stage. Why not just output the zener's natural reference voltage? Just measure and record it's voltage upon manufacture of the standard for future tracking with proper calibration standards.

One just ends up with a voltage number either way suitable for measurement, trending with time, etc, and should be usable via mapping calculations for any user's need? Is there not value in eliminating sources of variation altogether. OK maybe a buffer X1 opamp might still be desirable?

[The Soulman]

A non-volt-nut here with a question.

 Why must there be a 10vdc output standard with all the added complexity and variation of the opamp stage. Why not just output the zener's natural reference voltage? Just measure and record it's voltage upon manufacture of the standard for future tracking with proper calibration standards.

One just ends up with a voltage number either way suitable for measurement, trending with time, etc, and should be usable via mapping calculations for any user's need? Is there not value in eliminating sources of variation altogether. OK maybe a buffer X1 opamp might still be desirable?

I'm having the same idea, long term stability is a must, and it appears a part based on a zener and in
a hermetically sealed case are best.
Maybe LM399? https://www.conrad.at/de/pmic-spannungsreferenz-linear-technology-lm399hpbf-shunt-fest-to-46-4-1256340.html

Or actually in my case I would only like to know if my 30.000 count bench meter is still accurate (on 3 volt dc range) I know it's close
and it is stable when compared to my 20.000 count meter, they only differ 0,01% at most (no I'm not a voltnut and not planning on to becoming one..).
So for me I'd rather build a small simple and stable 2,5 volt (With 2,5 volt I use most of the meters resolution)  reference and have it measured by someone who is a voltnut, or maybe someone that is still in denial and "only" has a 7,5 digit meter.
Are LM136AH-2,5 any good?

For 6000 or 60.000 count meters a 5 volt reference would be better suited.

[guenthert]

A non-volt-nut here with a question.

 Why must there be a 10vdc output standard with all the added complexity and variation of the opamp stage. Why not just output the zener's natural reference voltage? Just measure and record it's voltage upon manufacture of the standard for future tracking with proper calibration standards.

One just ends up with a voltage number either way suitable for measurement, trending with time, etc, and should be usable via mapping calculations for any user's need? Is there not value in eliminating sources of variation altogether. OK maybe a buffer X1 opamp might still be desirable?

Generally speaking 10V is just a convention (in the age of Weston cells it used to be 1.018V).  Important is only that you have multiple references with the same nominal voltage so that you can compare them using a Null voltmeter.  A null meter is much simpler in construction and (hence) much cheaper than a long scale DMM for the same sensitivity.  It also doesn't need to be accurate other than at 0V (and you can easily calibrate that yourself, which reduces maintenance costs).

If all you want to do is to verify that a given DMM operates still within specification (of a given DC voltage range), then you need only a voltage reference calibrated to a remaining uncertainty (some argue four times, some argue 10 times) smaller than the DMM.  For a 4.5 digits DMM that can be a cheap AD584 which offers multiple voltages ...

[The Soulman]

For a 4.5 digits DMM that can be a cheap AD584 which offers multiple voltages ...

A AD584KH "out of the box" has maximum deviation of 0,16% on 2,5 volt range, my meter is specced at 0,05%.
Yes a AD584 can be used to verify there isn't something majorly wrong with the meters accuracy, but adjusting
a meter like this would require something much more accurate, max 0,01% or so.

[David Hess]

Are LM136AH-2,5 any good?

For less cost there are references with initial accuracy much better than the LM136's 1%.  At 1/10th the price, an 2.5 volt LM4040A would have 10 times better initial accuracy.

An expensive hermetic through hole package would make more sense for a more precise reference like an LM339 or LTZ1000 where physical strain would be more apparent.

For "out of the box" reference accuracy, the accuracy and temperature coefficient of the internal voltage divider is already taken into account in the specifications.  So it may pay to use a higher voltage reference if it results in a better initial accuracy expressed in parts per million.

I would look for something like a 5.000 volt MAX6250A in an 8 pin plastic DIP package which is 0.02% and 1ppm/C at 5 volts for about $10, and then put it into a circuit which operates it under the nominal conditions specified in the datasheet.

My actual search for a part would involve a spreadsheet with price, tolerance errors from various sources, temperature coefficient from various sources, and then maximum total unadjusted error over some nominal temperature range.

[The Soulman]

To the OP, the AD584 has the capability to output four different voltage's, 2,5, 5, 7,5 and 10Volt
and are available in different accuracy grades (like most voltage reference ic's).
The AD584TH and AD584KH are the most accurate versions and have maximum 0,1% deviation on the 10 Volt range.

http://www.analog.com/media/en/technical-documentation/data-sheets/AD584.pdf

[The Soulman]

Are LM136AH-2,5 any good?

For less cost there are references with initial accuracy much better than the LM136's 1%.  At 1/10th the price, an 2.5 volt LM4040A would have 10 times better initial accuracy.

I'm not the OP and come to think of it maybe I'm kinda hijacking this thread 
For me initial accuracy of even 0,02 is not good enough and would like to get below 0,01%.
My plan is to build something around a stable 2,5 Volt zener reference and ship it to someone who is
able to measure it within that 0,01% uncertainty and then have it shipped back and compare it to my 4,5 digit meter.

Maybe something like this would also work for the OP.

[pitagoras]

The original post is about accuracy: "out of the box" without calibration known voltage.
Then there is the second important aspect that is precision, which is related to drift, short term and long term.
I think in this last aspect it is somewhat well stablished that LTZ1000 is king and LM399 is the prince. However you need some mean for calibrate them (which I don't have).
On the other hand, the zener discussion is also very interesting and might well continue here, but if you decide to open a thread, let us know
So zener is better than the king?

[Conrad Hoffman]

FWIW, my Data Precision / Analogic 6.5 digit voltage standard is quite good, drifting a couple ppm per year at most. What does it use for a reference? LM399!

[The Soulman]

The original post is about accuracy: "out of the box" without calibration known voltage.
Then there is the second important aspect that is precision, which is related to drift, short term and long term.
I think in this last aspect it is somewhat well stablished that LTZ1000 is king and LM399 is the prince.

It indeed appears to be so.

However you need some mean for calibrate them (which I don't have).

I've been searching online and looking at many different datasheets and it appears 0,02% is the best you can get "out of the box" (the 5 Volt MAX6250A as suggested by David Hess).
Other option might be a build up module and measured by voltagestandard.com, or one of the onehunglow equivalants.

On the other hand, the zener discussion is also very interesting and might well continue here, but if you decide to open a thread, let us know
So zener is better than the king?

No, there are two different types of voltage reference IC's (that I know of, I'm not an expert) one that uses a bandgap (common in almost everything that needs a low power and ok voltage reference) and the other (including LTZ1000 and LM399) use a topology around a zener diode (inside the IC) and the rest of the IC circuitry is there to pamper that diode to the max.
There are also actual plane bare zener diodes designed specifically to be used as voltage reference but must be driven
by a stable constant current and kept at a (fairly) constant temperature to perform well, couldn't
find those at all in online stores

What type of volt meters do you have and what accuracy do you want?

[guenthert]

For a 4.5 digits DMM that can be a cheap AD584 which offers multiple voltages ...

A AD584KH "out of the box" has maximum deviation of 0,16% on 2,5 volt range, my meter is specced at 0,05%.
Yes a AD584 can be used to verify there isn't something majorly wrong with the meters accuracy, but adjusting
a meter like this would require something much more accurate, max 0,01% or so.

Ah, I guess I missed the "out of box" qualifier.  I don't know of any voltage references which have initial accuracy anywhere close to their long term stability.  You'll find voltage references for less than $50 which are claimed to have been calibrated (their actual value measured and noted) using a sufficiently precise (and hopefully calibrated) DMM.  It's a bit of a trust issue though.  Best to bring that module to a friend with a calibrated DMM ...

[bingo600]

I would take a AD587UQ

with best regards
Andreas

I'd go with Andreas suggestion , Joe Geller used it.

Or if on a tight budget , maybe try the - AD587KNZ DIP-8
http://www.ebay.com/itm/AD587KN-AD587KNZ-DIP-8-/270986206419

Get an AD587 that's "Old Stock" , rumours are that the newer ones are not as stable


http://pdf1.alldatasheet.com/datasheet-pdf/view/48072/AD/AD587JQ.html


/Bingo

[SvanGool]

@guenthert

I like the idea of buying a bunch of references and building something but you can get an already calibrated 0.01% voltage reference for $30 from VoltageStandard.com.

Well, you might want to read  https://www.eevblog.com/forum/metrology/calibratory-d-105-dc-precision-voltage-reference-standard/ first.

The VoltageStandard.com have, in general, good feedback. I think you are mixing up with http://www.ebay.com/itm/10-Volt-10-V-DC-Prec-Voltage-Reference-Standard-Nulled-to-Fluke-732A-or-732B-/251756273626 which is indeed in detail discussed in the thread you mentioned.

If not, can you elaborate a bit?

[pelule]

http://www.ebay.de/itm/AD588-High-Precision-Voltage-Reference-Module-Multiple-Output-AD588BQ-/252181862385?hash=item3ab735d7f1
I would take that AD588BQ board for EUR 15,88 EUR and free shipment

[guenthert]

@guenthert

I like the idea of buying a bunch of references and building something but you can get an already calibrated 0.01% voltage reference for $30 from VoltageStandard.com.

Well, you might want to read  https://www.eevblog.com/forum/metrology/calibratory-d-105-dc-precision-voltage-reference-standard/ first.

The VoltageStandard.com have, in general, good feedback. I think you are mixing up with http://www.ebay.com/itm/10-Volt-10-V-DC-Prec-Voltage-Reference-Standard-Nulled-to-Fluke-732A-or-732B-/251756273626 which is indeed in detail discussed in the thread you mentioned.

If not, can you elaborate a bit?

Yes indeed I have and I'm terribly sorry about that.
Really odd blunder, as I do actually have his "DMM check" product, which includes a 5V reference, which is well constructed and for all I know works as advertised.

[IconicPCB]

@ Conrad Hoffman

First of all Thank You for a very common sense approach to the metrology lab project.

I was wondering if You had any words on  aging the resistors and components prior to selection in order to ensure a more predictable outcome at the end of assembly?

And now for another three letter acronym... TIA

[David Hess]

I'm not the OP and come to think of it maybe I'm kinda hijacking this thread 
For me initial accuracy of even 0,02 is not good enough and would like to get below 0,01%.
My plan is to build something around a stable 2,5 Volt zener reference and ship it to someone who is
able to measure it within that 0,01% uncertainty and then have it shipped back and compare it to my 4,5 digit meter.

The selection process for the reference would be different in the case where you are going to trim and/or measure it against a standard.  Drift and various other errors become more important than absolute tolerance from the manufacturer.

[Andreas]

I'd go with Andreas suggestion , Joe Geller used it.

Get an AD587 that's "Old Stock" , rumours are that the newer ones are not as stable

Hello,

Joe Geller used the AD587LQ (0..70 deg C). But this grade is now obsolete (no longer produced).
The AD587UQ is specified from -55 to 125 deg C.
And it is clear that when you trim the overall (average) T.C. at the temperature extremes
a device with a larger temperature range is likely to have a larger gradient (slope) near room temperature.

I think the rumours result from the different behaviour of the LQ and UQ type at room temperature.
But this is only relevant if you are looking into ppm stabilities.

The selection process for the reference would be different in the case where you are going to trim and/or measure it against a standard.  Drift and various other errors become more important than absolute tolerance from the manufacturer.

Thats true. In this case I would select for zero T.C. / hysteresis / ageing drift and low noise.


With best regards

Andreas

[Conrad Hoffman]

...words on  aging the resistors and components prior to selection

One can get really crazy with this stuff, as evidenced by the LTZ1000 threads. I don't have the equipment to do much beyond about 1-2 ppm. Even when I had ovenized standard cells, I'm not sure I could have done much better. I heat with wood, so my lab temperature tends to vary by 5-10 degrees. All I can say is the original MML voltage references remained very stable over time, 1-2 ppm/year, using common 1% metal film resistors selected for matched tempco. One has to be careful soldering, as it often causes a permanent value change. I would typically power the units and age them for a couple weeks before final calibration. Even if you aged the individual parts, you'd still have to age the assembled unit, so it wouldn't gain you much.

It's too bad the metal can references are getting scarce. They have the thermal emf problems of Kovar leads, but are mechanically pretty good. The plastic versions are mechanically terrible, changing voltage with any flexing or stress on the pc board. What you have to do is float them above the board on three (or whatever the minimum needed is) wires, each with a small stress relieving "s" bend. Then, mount the pcb with three wires and no other supports. It's all about kinematics and avoiding bimetal bending effects as the temperature changes.

Don't underestimate individual temperature compensated zeners if you can find them. The original common one was 1N821, but there are many other similar ones. If you do the classic opamp circuit with an op-07 or similar, that supplies the zener current from the regulated output, and ovenize it, you can compete with some of the best ICs. Stay with the optimum zener voltage of something like 6.8 V and experiment with current to find the zero TC point.

[pitagoras]

Don't underestimate individual temperature compensated zeners if you can find them. The original common one was 1N821, but there are many other similar ones. If you do the classic opamp circuit with an op-07 or similar, that supplies the zener current from the regulated output, and ovenize it, you can compete with some of the best ICs. Stay with the optimum zener voltage of something like 6.8 V and experiment with current to find the zero TC point.

Do you have some information on this? Specially in building the "oven".

[Conrad Hoffman]

There should be info on ovens on-line somewhere, and again the level of crazy is up to you. My old JRL standard cell oven consisted of an inner box, aluminum, with some resistance wire wrapped around it, or maybe flat heater pads. Control was with a mercury thermoregulator, but today you'd use a thermistor or other sensor of your choice, and a control circuit. They then put that inside another aluminum box, wrapped with resistance wire and controlled the same way. The inner oven ran a bit hot, 37C I think, and the outer oven a few degrees less. It's like using double voltage regulators- no ambient is going to affect that inner box. I've built simple single ovens that held a few millidegrees, and I don't think that's needed here. Look for a Jim Williams Linear Technology app note with a temp control circuit. Hewlett Packard made a 1N821 based voltage standard years ago, visually similar to the Fluke 731- so old I can't find a picture of one. The diode circuit was on a small and not-too-special pcb that lived inside a small drawn aluminum can- wrapped with resistance wire and stuffed in a block of foam for insulation. You just need to get a few degrees higher than the highest ambient you ever expect, and hold it within a tenth degree or so. If I were doing one today I'd probably get some 1" copper pipe and copper end caps. Wrap it with insulated resistance wire (or use a Kapton heater pad- eBay is loaded with them), glue a sensor to it, and make a simple proportion control. I wouldn't use a nice PWM control because of the noise. Install some feedthrus for wire and stick it a block of Styrofoam for insulation.

[pitagoras]

Look for a Jim Williams Linear Technology app note with a temp control circuit.

This one?
http://cds.linear.com/docs/en/application-note/an89.pdf

[David Hess]

Look for a Jim Williams Linear Technology app note with a temp control circuit.

This one?

http://cds.linear.com/docs/en/application-note/an89.pdf

That would work but he was thinking of something a little simpler.  Check figure 5 on page 3 of Linear Technology application note 12.

[Andreas]

Hello,

although I think that for your needs a oven is overkill ...

the simplest cirquit for a small oven I have found is at the bottom of this page:
http://www.techlib.com/electronics/ovenckts.htm

with best regards

Andreas

[pitagoras]

Ok... 2 pieces AD587KR are on the way...
Having 2 pieces can be used to improve noise figures? Where can I read about that?
Thanks

[Andreas]

Hello,

what application do you have that noise plays a role?
What stability do you want to reach?

Individual references may have up to factor 10 difference in noise.
(see the LT1027DCLS8-5 thread with popcorn-noise).

https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg970876/#msg970876
against
https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg972153/#msg972153

So buying 2 references and hope that the averaging will reduce noise by a factor 1.4 makes no sense for me.
I would at least buy 10 references and select the 2 with the lowest noise for averaging.

with best regards

Andreas

[Kleinstein]

If one has references of different noise, one can still do an average - however one should give a different weight to the references with the highest weight to the lowest noise one. If uses resistors to combine the references the best resistor value would be proportional to the square of noise (measured as a voltage).

If a references has significant higher noise it will not contribute much, and there maybe not much sense in using that one at all.

[RandallMcRee]

I don't think its that simple.

There is both drift and noise.  Averaging, at least theoretically, could improve both of these. For improving drift I would think equal value resistors are best.

Just a thought.

Randy

[pitagoras]

The application I have in mind is a Mini Metrology Lab.  Just to enter the game.

At first, it will be my best shot at calibrating the rest of the lab under construction (simple DMMs, PSUs).
I'm interested in experimenting with old-fashioned zener references and FET ones.

The rest of the game includes experimenting with sigma-delta converters ("discrete").

So whatever makes the ref "better" in some sense or another.

I agree that, at least intuitively, it looks like drift would be improved by averaging.

I will study the threads on low freq noise and popcorn.

Thanks for your valuable time.

[tggzzz]

I agree that, at least intuitively, it looks like drift would be improved by averaging.

Consider a source that drifts monotonically from 10V to 9V over the course of a year. What averaging would be necessary to improve that, and what would the improvement be?

Ditto a source that drifts during the day as temperature changes?

Averaging only works in some circumstances, and understanding those conditions will make you a better engineer.

[pitagoras]

Averaging only works in some circumstances, and understanding those conditions will make you a better engineer.

Yeah. I'd like to understand better the conditions. I understand the following:
1) "popcorn" noise is related to "imperfections" in the construction (or inherent characteristics of semiconductors). Mitigated by the manufacturer in the construction process
2) There is "climate" drift related to temperature (mainly mitigated with ovens)  and humidity (mainly mitigated with ceramic packages)
3) There is some kind of drift related to mechanical stress. Mitigated by "floating" the component.
4) There is long term drift related to aging. Mitigated by pre-aging and selection of parts.
5) There is low frequency noise, a.k.a. 1/f, 0.1 to 10Hz noise... this one I suspect this is related to some oscillation or "ringing" in the implicit control system... but I don't really know here.

What else?

[pitagoras]

hey tggzzz, funny enough, I happened to land in the link in your signature earlier today, by chance, while googling weston cells

[tggzzz]

Averaging only works in some circumstances, and understanding those conditions will make you a better engineer.

Yeah. I'd like to understand better the conditions. I understand the following:
1) "popcorn" noise is related to "imperfections" in the construction (or inherent characteristics of semiconductors). Mitigated by the manufacturer in the construction process
2) There is "climate" drift related to temperature (mainly mitigated with ovens)  and humidity (mainly mitigated with ceramic packages)
3) There is some kind of drift related to mechanical stress. Mitigated by "floating" the component.
4) There is long term drift related to aging. Mitigated by pre-aging and selection of parts.
5) There is low frequency noise, a.k.a. 1/f, 0.1 to 10Hz noise... this one I suspect this is related to some oscillation or "ringing" in the implicit control system... but I don't really know here.

What else?

Forget the physical mechanisms; averaging (and limitations) is a mathematics/statistics issue. After you understand the maths/stats, you can start see how it can/can't help you with the physics.

[tggzzz]

hey tggzzz, funny enough, I happened to land in the link in your signature earlier today, by chance, while googling weston cells

That's gratifying

I now have 4 saturated weston cells, and a couple of 10V "Trancell" zener-based references => the too many clocks problem

[pitagoras]

the too many clocks problem

Come on, one can never have too many clocks
The only problem here is not having a calibrated 3485A...

[Andreas]

2) There is "climate" drift related to temperature (mainly mitigated with ovens)  and humidity (mainly mitigated with ceramic packages)

Hello,

so for me it looks you are on a good way.

But one point: you mentioned the KR version above which should be the SO-8 epoxy-version and not a ceramics dip version (which would have a Q in the 2nd suffix).

with best regards

Andreas

[Kleinstein]

The low frequency (1/f) noise can be similar to popcorn noise, just on a different level and many processes in parallel.
Averaging helps primarily against noise: so white noise, 1/f noise and popcorn noise as well.

With drift it is well possible that most refs if the same type show drift to the same direction and thus averaging similar refs will not help very much against drift. It can help a little with linear TC - but already the second order TC is usually not at all random. For things like long term drift with multiple refs one usually looks at the ref units individually and than decides which one to trust and include in an average.

[pitagoras]

Thank you for your answers.
Yes, I got epoxy version because of current budget restrictions.
Could/Should I ovenize the reference?
I will have 2 units so I can experiment with one ovenized.
But to check drift I need something more stable, like building another reference based on LM399...

[Andreas]

Could/Should I ovenize the reference?

Hello,

that depends on your stability needs and the individual T.C. near room temperature of your references.
You can also try a temperature compensation by NTC like in the Geller SVR-T reference.

The DIP8 epoxy package has after my experiences a humidity coefficient up to 0.5 ppm/%rH. (LT1027CN)
Lars sent me some measurements for AD587LN with up to 0.3ppm/%rH.
According to a MAXIM datasheet the SO8 packages may be less stable by a factor of 2-3.

With best regards

Andreas

[not1xor1]

Could/Should I ovenize the reference?

Hello,

that depends on your stability needs and the individual T.C. near room temperature of your references.
You can also try a temperature compensation by NTC like in the Geller SVR-T reference.

The DIP8 epoxy package has after my experiences a humidity coefficient up to 0.5 ppm/%rH. (LT1027CN)
Lars sent me some measurements for AD587LN with up to 0.3ppm/%rH.
According to a MAXIM datasheet the SO8 packages may be less stable by a factor of 2-3.

With best regards

Andreas

I have 3 MAX6250 I bought a few years ago and never used
they are in DIP package so would probably be affected by ambient humidity too
do you know if anybody ever succeeded in solving/reducing the problem by enclosing the circuit with some sort of sealant?
thanks

[Gyro]

It's really not hard to make a hermetic enclosure. Some copper sheet (foil) or even PCB material. Hermetic feedthroughs - either feedthrough caps or glass-metal seals off ebay and fully solder all the seams. Fix a silica gell sachet inside if you want a dry, rather than constant humidity, environment inside. Just make sure you've done all your circuit tweaking first.

[ap]

There is a difference between a hermetic enclosure and a hemetic case for an electronic part. There are different mechanisms, one is e.g. the epoxy case 'moving', causing voltage changes, the other is the continuous invluence of remaining humidity causing chip changes on die/crystal level. Silica e.g. gets humidity down to say 20 or 30%, the remaining humidity inside, especially with temperature changes, is enough to have a continuous impact on the reference. There is no way arround a hermetic case on die level for a stable reference. A hermetic enclosure may make things a little better, but the same money is easier spent on the right part.

[tszaboo]

It's too bad the metal can references are getting scarce.

Well, TI sells the REF50xx series as a DIE option, so technically nobody is preventing us from buying that, and package it in a metal can. It even has a TEMP pin. One of my "virtual" projects is making an external heater with temperature controller for that chip. In the meantime, I have this:

Good enough. Unfortunetly it has a fatal flaw, so  I should be working on version 2 already.

[Andreas]

Hello,

and you think you can get a single DIE without buying a complete wafer?

On the other side: a REF5050 has typically a factor 5 more noise (3 ppm/V) than a comparable buried zener (0.6 ppm/V)
And there are still some of them available in hermetically package in single quantity. (AD586LQ/AD587UQ/LT1021CCH...)

with best regards

Andreas

[tszaboo]

Hello,

and you think you can get a single DIE without buying a complete wafer?

On the other side: a REF5050 has typically a factor 5 more noise (3 ppm/V) than a comparable buried zener (0.6 ppm/V)
And there are still some of them available in hermetically package in single quantity. (AD586LQ/AD587UQ/LT1021CCH...)

with best regards

Andreas

Probably not. It should be a group buy! 
I am aware of the issues with modern IC voltage references, I had my share of problems with it. Noise is a problem, if you cannot set the integration time high. At 100 NPLC, 10 Hz noise is not an issue.
The biggest issue I have with buried zener is the driving strength. You need to buffer it, if you want to use it as a reference for any ADC, and you will end up with similar noise than the IC references. And tempco is worse.
And I would write more, but it is almost 1 PM...

[pitagoras]

You can also try a temperature compensation by NTC like in the Geller SVR-T reference.

The link to the schematic for SRV-T is broken here
http://www.gellerlabs.com/Voltage%20References.htm

Does anyone have it at hand?

[lars]

You can also try a temperature compensation by NTC like in the Geller SVR-T reference.

The link to the schematic for SRV-T is broken here
http://www.gellerlabs.com/Voltage%20References.htm

Does anyone have it at hand?

Got a request offline so here is my copy. /Lars W

This schematic is based on the original SVR that I just added the NTC + series resistor to. I have done temperature compensations to many AD587, REF102, LT1021 and LT1236 and my preferred way today is to have a separate trimpot for temperature compensation with a NTC+ series resistor.

The AD587JQ from Digikey (or Mouser) is right now my preferred DIY10V ref as it isn´t humidity sensitive. The last years I have bought AD587JQ´s a couple of times and no one have been over 5ppm/C (probably been lucky but am not sure). So it should be quite easy to get them down below 0.5ppm/C with some temperature tests and NTC compensation.

As pin 5 on the AD587 is at about 7V I have had two 20 k trimpots in series between 0 and +10V (pin 6). The upper has been connected to the NTC (680+680k SMD near pin  5)) in series with a resistor of 1.5M. This gives a range of about +-10ppm/C adjustment if I remember correct. If the temperature coefficient is positive I sometimes calculates a higher value resistor to get some second order compensation, this also gives lower sensitivity to the components (TC, res, pot). The other pot is connected with a 10Mohm resistor to pin 5 to give about +-135ppm trim. A third resistor is connected between 0 or 10V to get the trim range for the pot.

This compensation scheme works well with the other IC´s mentioned but values will be different. If the trim pin is at +5V (eg LT1021+1236) don´t put the trimmers in series but instead in parallell. To set up your own calculation just hook up a 1Mohm resistor to the 0v respective 10v and see the change in ppm. Use these values together with say values for the NTC at 20-25-30C and put in a spreadsheet.

Another important thing that is missing in the schematic is a 100nf capacitor on the output (pin 6 to 0V). Without that the SVR is very EMI sensitive as I have seen a lot of times in my lab before I understood that. I also normally use ferrite beads to the connecting pins (in and out)

[Andreas]

Hello,

made some T.C. measurements.

First on a AD587UQ#1 date code 1631
I had bought 2 samples but unfortunately I killed one of them by wrong polarity.
So no statistics here.
T.C. is linear with 7.4 ppm/K


Then I decided to go for the JQ grade since lars reported having below 5 ppm/K.
Yesterday they arrived and the very first measurement looks promising.
AD587JQ#01 date code 1613
T.C. is near linear with -1.5 ppm/K
so a factor 5 better than the UQ -version near room temperature.

On the first measurement there is some drift. But this should stabilize after a few cycles.

With best regards

Andreas

[HighVoltage]

My most stable 10V reference at this time is an original Geller SVRT, about 5 years old and was turned ON most of the time.
The picture here shows a recent measurement over night with "cheap normal cables" and around 2 °C change over night.

Actually I am surprised how stable the SVRT really is.
When it is turned off for a few days it needs about 30 h to stabilize again.

[Andreas]

Actually I am surprised how stable the SVRT really is.
When it is turned off for a few days it needs about 30 h to stabilize again.

Hello,

it also depends a bit on luck how good the final cirquit is.
you can really get better results from a device that has < 1ppm/K without external compensation than from one with > 5ppm/K.
That´s what I´m looking for: a device like this sample.

AD587JQ #03 Datecode 1342

#03 has very low T.C. around 25 deg C.  -0.14 ppm/K

But unfortunately this device has some pop corn noise of up to 3uV measured on oscilloscope
 which is also visible in the T.C. measurement (e.g. near 25 deg C)

with best regards

Andreas

[orin]

Another important thing that is missing in the schematic is a 100nf capacitor on the output (pin 6 to 0V). Without that the SVR is very EMI sensitive as I have seen a lot of times in my lab before I understood that. I also normally use ferrite beads to the connecting pins (in and out)

This can show up when you reverse the leads at the DMM... From my discussion about it at the time: "To my surprise, I read -9.989XX (I don't recall the XX).  With the leads the normal way, I'm seeing 9.9998X with the X depending on temperature drift in the 'lab'".

After using twisted pair leads for both power and output to the DMM (an HP 3455A), I found a 4.7 nF film capacitor on the SVR output was sufficient to fix it in my case, but before the twisted pair leads, using standard mini-grabber leads, it took 0.33uF to make the 3455A read the same with the leads swapped!

I never really worked out why the AD587/HP 3455A did that - I had no problem with an HP 3456A.

[Conrad Hoffman]

Don't know if it would work well with the meters, but with my bridges I make several passes through a big tape wound toroid core with the detector coax to kill common mode stuff.

[pitagoras]

Well, my two AD587KR finally arrived. 

Those are SOIC 8 epoxy, so not ideal... I know I'll have to live with humidity drift...

In the other aspects I think there is still something that can be done.
1) For temp drift I'll be building the thermistor compensated circuit.
2) I'd like to know if, to mitigate mechanical stress, for the mounting I should be doing something different than for any other standard SOIC 8

Before digging into the PCB design, I accept all suggestions.
Cheers

[Kleinstein]

Mechanical stress can be a problem with SO8. Stress can be from bending the board and expansion (thermal and humidity) of the board.

One way to reduce stress is to put the chip close to the corner / edge of a board or even better use cut out on 3 sides. Another more DIY like option would be to try a kind of through hole mounting of the SO8 with extra wires. So floating the chip something like 3 mm above the board. Chip on flex might be an option for a more production like solution.

[David Hess]

Another important thing that is missing in the schematic is a 100nf capacitor on the output (pin 6 to 0V). Without that the SVR is very EMI sensitive as I have seen a lot of times in my lab before I understood that. I also normally use ferrite beads to the connecting pins (in and out)

This can show up when you reverse the leads at the DMM... From my discussion about it at the time: "To my surprise, I read -9.989XX (I don't recall the XX).  With the leads the normal way, I'm seeing 9.9998X with the X depending on temperature drift in the 'lab'".

After using twisted pair leads for both power and output to the DMM (an HP 3455A), I found a 4.7 nF film capacitor on the SVR output was sufficient to fix it in my case, but before the twisted pair leads, using standard mini-grabber leads, it took 0.33uF to make the 3455A read the same with the leads swapped!

I never really worked out why the AD587/HP 3455A did that - I had no problem with an HP 3456A.

I have seen the same kind of problem with high resolution DMMs when reading low impedance sources like strain gauge bridges and strain gauge amplifiers.  Besides adding LC filtering at the DMM, one solution was to use passive speed control on the fans in the temperature chamber so no EMI was generated.

Two problems I have run across include:

1. While the inputs are floating, they are not symmetrical.  The common input has a lot more common mode coupling to ground.  They make special meters with differential inputs and a separate common for shielding the test leads but they are not general purpose instruments.

2. Some "modern" DMMs and especially handheld ones suffer from excessive charge pumping at their inputs just like chopper stabilized amplifiers.  Usually I see this show up when two DMMs are connected to the same source (like each other) and intermodulation of the charge pumping causes a DC offset but some circuits can produce the same problem.  I keep a couple of old DMMs which have JFET input buffers around to make measurements when I need a sanity check.

[tszaboo]

I have seen the same kind of problem with high resolution DMMs when reading low impedance sources like strain gauge bridges and strain gauge amplifiers.  Besides adding LC filtering at the DMM, one solution was to use passive speed control on the fans in the temperature chamber so no EMI was generated.

Two problems I have run across include:

1. While the inputs are floating, they are not symmetrical.  The common input has a lot more common mode coupling to ground.  They make special meters with differential inputs and a separate common for shielding the test leads but they are not general purpose instruments.

2. Some "modern" DMMs and especially handheld ones suffer from excessive charge pumping at their inputs just like chopper stabilized amplifiers.  Usually I see this show up when two DMMs are connected to the same source (like each other) and intermodulation of the charge pumping causes a DC offset but some circuits can produce the same problem.  I keep a couple of old DMMs which have JFET input buffers around to make measurements when I need a sanity check.

A fan have rotating magnets, or magnetic field, with coils in it, so so much for not generating EMI.

And yes, DMMs can be nasty loads. Just place a DMM and a scope on the same testpoint in a circuit, something with high source impedance.

Mechanical stress can be a problem with SO8. Stress can be from bending the board and expansion (thermal and humidity) of the board.

One way to reduce stress is to put the chip close to the corner / edge of a board or even better use cut out on 3 sides. Another more DIY like option would be to try a kind of through hole mounting of the SO8 with extra wires. So floating the chip something like 3 mm above the board. Chip on flex might be an option for a more production like solution.

In my experience the 3 way milling is enough. The other important thing is to get rid of ceramic capacitors near high impedance nodes, like any RC filter, and place there film capacitors. Panasonic has reasonably priced SMD film capacitors in 1210 and smaller packages.

[pitagoras]

In my experience the 3 way milling is enough. The other important thing is to get rid of ceramic capacitors near high impedance nodes, like any RC filter, and place there film capacitors. Panasonic has reasonably priced SMD film capacitors in 1210 and smaller packages.

What do you mean by 3-way milling?

[tszaboo]

The PCB around the reference is milled on 3 sides, in a U shape, so it flexes less. "3 way" is probably not the good wording for it.
http://cds.linear.com/docs/en/application-note/an82f.pdf
Figure 11.

mod: fixed link

[Vgkid]

Heres one most have heard about.
Burr Brown Ref101. All the down to 1ppm/c and .005v accurate.

[ocw]

The REF101 has long been discontinued, is hard to find, expensive when you do find and because of that stands a good chance of being counterfeit.
A REF102CP is still made and is available from reputable stores for around $10.  Since it is +/-2.5 mV 2.5 ppm/C it will be more accurate "out-of the-box" than a REF101 at normal room temperatures.  The one that I have measured 10.00051 volts without any correction efforts being made.

[Vgkid]

I have a k-specced 101 sitting in its ovenized tomb, where it was sitting for decades.

[David Hess]

In my experience the 3 way milling is enough. The other important thing is to get rid of ceramic capacitors near high impedance nodes, like any RC filter, and place there film capacitors. Panasonic has reasonably priced SMD film capacitors in 1210 and smaller packages.

What do you mean by 3-way milling?

Mill a channel around 3 sides of the part so it sits on a peninsula.  Now any strain in the printed circuit board is directed around it.

[ocw]

I have a k-specced 101 sitting in its ovenized tomb, where it was sitting for decades.

While that ovenized 101K will have better frequency stability, its specification sheet shows less accuracy promised (unless it has a correction circuit which violates the "out-of-the-box" condition) than a $10 REF102CP sitting at typical room temperature.

[not1xor1]

The REF101 has long been discontinued, is hard to find, expensive when you do find and because of that stands a good chance of being counterfeit.
A REF102CP is still made and is available from reputable stores for around $10.  Since it is +/-2.5 mV 2.5 ppm/C it will be more accurate "out-of the-box" than a REF101 at normal room temperatures.  The one that I have measured 10.00051 volts without any correction efforts being made.

what about AD588BQ?
it is in ceramic case so should be insensitive to humidity variations
according to the datasheet it should produce 10V +/- 1mV, +/-1.5ppm/K (0-70°C), 15ppm long term stability (and/or other voltage values according to the connections)

I bought several of them from various ebay suppliers
so far I got only one from Germany and it looks like a real one
it has a 1325 mark... 25th week of 2013?

unfortunately my best DMM is just a fairly new Brymen 867S which measures 10.0017V (and 5.00038-5.00080 from 3 different new MAX 6250ACPA+ with 0.02% initial accuracy)

now I'm waiting for some more from China and Hong Kong, they should be second hand and so already aged (provided they are not fake)
I just hope I can average the values to have a rough idea of the real voltage
so far it looks like the Brymen is within specifics but a bit of a mV on the high side

[Vgkid]

I have a k-specced 101 sitting in its ovenized tomb, where it was sitting for decades.

While that ovenized 101K will have better frequency stability, its specification sheet shows less accuracy promised (unless it has a correction circuit which violates the "out-of-the-box" condition) than a $10 REF102CP sitting at typical room temperature.

True, those ref102's are impressive.

[tszaboo]

The REF101 has long been discontinued, is hard to find, expensive when you do find and because of that stands a good chance of being counterfeit.
A REF102CP is still made and is available from reputable stores for around $10.  Since it is +/-2.5 mV 2.5 ppm/C it will be more accurate "out-of the-box" than a REF101 at normal room temperatures.  The one that I have measured 10.00051 volts without any correction efforts being made.

You know, that the REF102C does not mean ceramic package?
The picture I have in the thread is a REF102 reference, it is OK, not brilliant.

If you want a modern, hermetically sealed 10V reference, there isn't one. You need to make your own.
Its actually not that hard, just place two LTC6655BHLS8-5 on top of each other (in series... well not really). By the looks of it, it does not like load, so buffering the bottom one will be necessary. Since it has a sense pin, you can negate the output buffers offset voltage. It is somewhat analog black magic, but precision analog was always difficult, and it requires some crazy BOM cost. Starting with 28EUR just for the two reference.

[Kleinstein]

If you don't need modern, there are still a few 10 V references in a metal can: LT1013, AD581.

Modern reference chips are more like 2.5 or 5 V and often not in a metal can any more. Two of the 5 V refs in series is an option.

The LTC6655 is good when it comes to noise and TC, but not that good when it comes to hysteresis and long term drift. So it depends on the requirements.

[tszaboo]

If you don't need modern, there are still a few 10 V references in a metal can: LT1013, AD581.

Modern reference chips are more like 2.5 or 5 V and often not in a metal can any more. Two of the 5 V refs in series is an option.

The LTC6655 is good when it comes to noise and TC, but not that good when it comes to hysteresis and long term drift. So it depends on the requirements.

Because they are made for ADCs and DACs obviously. long term drift can be solved by aging, and temperature cycling. hysteresis with keeping it indoors. I think it is a lesser issue, my home has only 5-10 degrees temperature change during a year. And I'm only saying this for others, because you will know this.

[pitagoras]

Ok I will be building the circuit attached, if I interpreted correctly @lars indications.
PCB will be 2 layers, one of them ground.
Will try to mill the "U" te relief stress.
The reference will be trimmed at a local EE faculty metrology lab. I'll try to perform some stress bending as suggested in the link by @NANDBlog

[tszaboo]

If you want to hear my 2 cents:
Adding a 7815 on the input can reduce the line regulation issues almost entirely.
Adding a TVS diode on the input and output makes sure you dont fry the circuit. An ESD event can drift your reference, and you never notice it, until calibration.
Make sure you can place a film capacitor for C2, at least as an alternative.
Place an LED on the input
Leave space for extra input output capacitors.
RV2 should be 10 turns pot
none of these is necessary for proper operation

[pitagoras]

Yes I want as many cents as you have
I attach new version of schematic.

[The Soulman]

Do you need the adjustment options?
How and when are you going to dial it in just right?
If not set just right they're potentially a source of error.
I'm not an expert, just asking.. 
Also might want to add a polarity reversal protection diode on the input (1n4001 or something).
 

[Andreas]

If you don't need modern, there are still a few 10 V references in a metal can: LT1013, AD581.

Modern reference chips are more like 2.5 or 5 V and often not in a metal can any more. Two of the 5 V refs in series is an option.

The LTC6655 is good when it comes to noise and TC, but not that good when it comes to hysteresis and long term drift. So it depends on the requirements.

Hello,

the AD581 is only a bandgap reference. So it has about factor 5 more noise (with comparable current) and more ageing drift than a buried zener.
LT1031 (not 1013) and LT1021 (H-package) are metal can buried zener references.
 
When I measure a LTC6655 (5V) it usually has about the same noise than a  buried zener 5V reference. (all in hermetically LS8-packages).
But the hysteresis is really bad for the LTC6655 even near room temperature.

The AD588BQ should play in the same game as AD587xQ. But if you do not need a dual voltage output the price is too high.

Ok I will be building the circuit attached, if I interpreted correctly @lars indications.

Depending on positive/negative T.C. of the AD587 you might have to put the thermistor either to ground or the output of the reference.
The resistor/trimmer in series to the 100K thermistor needs probably more than 20K.
The output voltage trimmer is around 200R to 1K depending if you take the default values of the other 2 resistors or if you carefully adapt them to the raw output voltage.

see also parts list of standard SVR reference:
http://www.gellerlabs.com/SVR%20M.html

Adding a 7815 on the input can reduce the line regulation issues almost entirely.

I second this. I am usually using a LT1763 low noise regulator as low drop/low power pre regulator (thermally isolated from the reference). (Note: max 20V input)

Also might want to add a polarity reversal protection diode on the input (1n4001 or something).

of course yes if you do not have a voltage regulator with reverse polarity protection.

With best regards

Andreas

[David Hess]

Adding a 7815 on the input can reduce the line regulation issues almost entirely.

Or bootstrap the reference using an operational amplifier and single transistor if necessary.

[nuno]

If we buy a few chips of same output voltage and initial accuracy, can't we "average" their values and mathematically work "backwards" to get a higher accuracy for each? (assuming there is some minimum degree of variability between them)

[Kleinstein]

With more units one might be able to to reduce the uncertainty to a certain degree. There is no magic way to all unknown factors out (like in measuring noise levels or efficiency of reversible transducers). So with 4 units equal quality but uncorrelated error one might on average end up with half the tolerance  - however the worst case error is not getting better, only less likely.

With 4 equal units from the same type and series, chances are the error is in the same direction - thus averaging will be less effective. The can be (and will be to some extend) systematic errors in a hole series.

Before getting something like 4 refs at $10 each, it might be easier to find someone to do a more or less unofficial calibration.

[nuno]

Yes, but you can check that by the difference in measurements from unit to unit, since you know the tolerance, you can have an idea if the units you have are falling "too much on the same side" or not. But that's a good point, when factoring in the price. LM4231AMF doesn't look very expensive (maybe 4 for $10) with 0.05%, although 10ppm/ºC and some other worse long term specs.

[Kleinstein]

Especially when buying 4 of the same units from one source. Chances are you get some that ran of the same machine one after the other.  So chances are good the 4 units will be within 0.01% to each other - but you still don't know if they are all at the upper or lower end of the span or maybe dead on. Even if one unit is off from the others - you still don't know for sure this is not a bad unit outside the specs. Knowing to have an unusual difference also increases the probability to have this due to an defective part.

So overall the 4 identical unit's will not give a much better accuracy spec than a single unit. To make this work it would be more like using 4 different types or manufactures.

[nuno]

To make this work it would be more like using 4 different types or manufactures.

That would make a good experiment.

I guess that if the probability of getting chips more "far apart" in terms of voltage output were the same for any tolerance, then we could buy the ones with worse tolerance (cheaper) and still get an "average" as good as buying the ones with better tolerance.

[The Soulman]

Voltage references or resistors, the same principle applies:

[Andreas]

Voltage references or resistors, the same principle applies:

Hello,

I have made the first T.C. measurements of some AD587
1 UQ
2 JQ with datecode 1613
3 JQ with datecode 1342

@Lars: I fear you had big luck with your T.C. being below 5 ppm/K.

@Soulman: now try to make your statistics.
I still do not see any gaussian distribution of the output voltage.
Even when the price of the references easily exceeds that of 1000 metal film resistors.

my personal opinion:
even when building a reference with temperature compensation the  T.C. should not be too large.
Usually the temperature sensor is not on the chip. So there is always a temperature difference which can be 1-2 deg C.
So for a excellent device the T.C. should be around / below 1 ppm/K without compensation.

with best regards

Andreas

[Gyro]

I think you'll find that Joe Geller and Lars were using AD587LQ... Those are what Joe fitted to the SVR/SVR-T.

I'm not sure what you would find in comparison, but the LQ, sadly no longer available, was closer initial tolerance and better TC than the JQ. The LQ was specified max 5ppm/'C versus max 20ppm/'C for the JQ.

[Andreas]

The AD587JQ from Digikey (or Mouser) is right now my preferred DIY10V ref as it isn´t humidity sensitive. The last years I have bought AD587JQ´s a couple of times and no one have been over 5ppm/C (probably been lucky but am not sure).

Hello,

I referred to this statement of Lars.

with best regards

Andreas

[Gyro]

Ah, sorry. I missed that quote, lucky it is then!

Chris

[not1xor1]

@Soulman: now try to make your statistics.
I still do not see any gaussian distribution of the output voltage.

but if you average the values you get quite close to 10000mV 
unfortunately the Tc is positive for all the samples

[Andreas]

Hello,

for the diagram I picked the "box" TC (per definition positive).
the temperature gradient is negative for the lower TC values.

with best regards

Andreas

[tszaboo]

Voltage references or resistors, the same principle applies:

I had the privilege of measuring a lot (100+) of LTZ1000 references. It wasnt a gaussian distribution.

[The Soulman]

Voltage references or resistors, the same principle applies:

I had the privilege of measuring a lot (100+) of LTZ1000 references. It wasnt a gaussian distribution.

Than I stand corrected, but than the question: why not?

[Kleinstein]

For the LTZ1000 refs I would expects a more normal distribution, but there still is a chance they come from the same batch and can thus be all similar with maybe a slow movement of the average. So distribution could be a little more even, like square.

For precision 10 V refs, there usually is some checking of quality / grading after the adjustment. So the better units can end up at the higher grade - again having more units close to the limits. The lower grade samples may be missing a significant number of the good ones up to the point of having two peaks in the distribution. Also the specs are often guarantied by testing and rejection the bad ones, nut just by having a low scattering assuming only 0.1% will be off. It may happen for low cost chips / less critical parameters (e.g. TC), that they only test a few samples and rely on not too much scattering for the rest.

[Andreas]

For the LTZ1000 refs I would expects a more normal distribution,

Why that?

the LTZ1000 is a heated device with about 50 ppm/K in the unheated state.
so every little error in output voltage is multiplied by setting a different temperature setpoint.

For the 10V references there are different mechanisms which play a role.
First the zener and then the output voltage divider. (so at least 2 overlapping distributions).

Low T.C. components are often trimmed for tempco by a temperature compensation network which is adjusted during production at 3 temperature points.
Also there I would never expect a gaussian distribution.

with best regards

Andreas

[HighPrecision]

Hi,
got three NOS Burr-Brown REF10KM refs, datasheet is available on web, I haven't found any info here, anybody knows these chips ?

Thanks

[Vgkid]

Here you go.
[urlhttp://www.datasheetcatalog.com/datasheets_pdf/R/E/F/1/REF10.shtml][/url]
That looks like that uses the same datasheet as the ref101, I have.

[tszaboo]

For the LTZ1000 refs I would expects a more normal distribution,

Why that?

the LTZ1000 is a heated device with about 50 ppm/K in the unheated state.
so every little error in output voltage is multiplied by setting a different temperature setpoint.

For the 10V references there are different mechanisms which play a role.
First the zener and then the output voltage divider. (so at least 2 overlapping distributions).

Low T.C. components are often trimmed for tempco by a temperature compensation network which is adjusted during production at 3 temperature points.
Also there I would never expect a gaussian distribution.

with best regards

Andreas

Exactly. The reference itself has a large ~3% variation to begin with. I've used 0.1% resistors to set up the temperature setpoint (very nice thin film resistors from Susumu) , so that has much less tolerance. The Zeners, which had initially larger voltage also run somewhat warmer. But that is not what I saw.
I saw, that I had something, which looked like a normal distribution, where the middle part was missing. Because they bin the parts, and the better ones went to someone, who was paying more for the same part.
If you buy an accurate 10V reference, it will be either:
- Binned, so they will select the best parts and ship it to someone else
- Laser  or electrical fuse, or other electrically trimmed of the internal resistors. Clue: There are NC or DNC pins on the pinout. Trimming uses god know what algorithm to trim it.
- You get surprisingly lucky. Once I asked for 5 sample from a manufacturer, surprisingly, all 5 sample was extremely well behaving much better than datasheet. And then the production parts were much worse. Too bad, I started being skeptic to samples and even datasheets.
So expecting the same part to behave "randomly for tempco" or get normal distribution, or expect it to age in a certain way... Its just wishful thinking.

[HighPrecision]

That looks like that uses the same datasheet as the ref101, I have.

Thanks Vgkid, yes, REF101 is similar but more flexible to old REF10, with some difference in stability @1000hrs. You have the "KM" version ?

[lars]

What I have preferred the last years are actually the AD587 JQ as the LQ isn´t available. That the KQ and LQ isn´t available and that the UQ seems to be trimmed in another way is one reason I think it reasonable to believe a lot of JQ’s will be within 5ppm/C. And of course, I was lucky to get all my AD587JQ within +-5ppm/C. I have nine AD587JQ’s of date codes 1028, 1247, 1517 and 1609 bought from Digikey and RS components.

The datasheet for the AD587JQ says 20ppm/C temperature coefficient and that with the box method. So, at 25C you can have even worse TC. As I, Andreas, Joe Geller and a report from the LISA-project shows the AD587UQ has more like 7-11ppm/C at 25C but the box specification is 5ppm/C.

Joe used the LQ and I think all had the date code 0045. Most of the LQ Joe had were around +2ppm/C at 25C so the simple add-on with a NTC+ resistor worked well.

 I have temperature compensated many 10V references at home (never in a design at work) with NTC’s but mostly used other designs than the SVR-T design. The SVR-T design was just because the trimpot value was already fixed. Enclose a sketch for what I tried to describe earlier as an example for a more general design. It will give an adjustment range to compensate -8ppm/C to +12ppm/C. In this design the sensitivity to drift of all trim components are quite low compared to the SVR-T design. If I know the AD587 have a slight positive TC I can calculate a larger value instead of 1.5M to get less range and some second order compensation. Most AD587 seems to have a slight negative second order component of about -0.03ppm / (°C * °C) around 25C so a larger resistor gives some compensation for this. 1.5M together with 1.36M NTC gives a quite linear compensation, so no second order compensation, but useful for a broad range. I myself has set up an excel sheet for calculation of the NTC compensation.

That I say I prefer the AD587JQ is only true from a hobbyist point and to get a good “transfer standard” to compare and maintain 10V. In my professional work as a design engineer I haven’t designed in an AD587JQ and probably will not. Long long ago I designed in AD688AQ and BQ as they were good choices for +-10V refs with reasonable accuracy in data acquisition systems. As the AD588BQ are similar I guess it is a good choice for 15EUR/USD from eBay if they are not fake. My experiences with AD688xQ compared to AD587xQ are that the 587 is slightly more stable over time and draw much less power (10x) and don´t need dual supplies, so if you don’t need +-10v I prefer the AD587 in all aspects. Having just 2mA current draw is a plus when using say two 9V batteries to power your reference. Today most volt refs I design in are 2.5-5V in SMD of course.

Why 10V even for a hobbyist? I should say because of the traceability chain and uncertainties. If you, like me, after some years comes below 10ppm uncertainty the extra step from say 5 or 7.xV gives extra uncertainty. Not all have a super linear 8 ½ digit DMM or a KVD. My best DMMs are only 6 ½ digits and I never paid more than 100USD for a DMM yet. If you go for a couple of 10V references adjusted close to 10V it is also easy to make a simple x100 amplifier and with just a 3 ½ digit DMM to get 0.1ppm resolution when you regularly compare your references. Of course, after a while you will find it is difficult to measure the absolute long-term drift without repeated calibrations with known uncertainties. But even before that you have learned a lot about drifts between your samples. And temperature coefficients can be checked and adjusted with simple means. My first NTC temperature compensated REF102CM’s I adjusted with just a lamp above the DUT and a 3 ½ digit DMM + amplifier. Three out of four are below 0.1ppm/C at 23C. The fourth I must have made some mistake as it is +0.4ppm/C but as I have a second 10K NTC connected to banana jacks on the front of the box it is easy to compensate afterwards. Some other of my references aren’t compensated but I only compensate afterwards. I prefer the 10V adjusted to within say +-100ppm but some of my better reference have up to 400ppm offset (and 2ppm/C).

As Andreas pointed out you cannot temperature compensate away all TC on the IC. My estimate is maybe 10-20 times for a reference with about 5ppm/C from my experiences with an NTC very close to the IC (and of course mounted in a box, so outside gradients will not affect so much).

As the thread started with suggestion for 10v for MML I would say: DIY with AD587JQ and/or VREF10-003 from voltagestandard.com if you can afford 53USD for it. With the VREF10-003 you get 30ppm for 6 months and as far as I know it is a serious offer compared to many other Vref boards or boxes on eBay. I have only one VREF10-003 but at arrival it was very close and now after more than 1 year it is still just +12ppm. Of course it is humidity sensitive as all plastic refs. It has a cut-out on three sides of the IC but I am not so sure it helps. I think the humidity sensitivity comes from the epoxy in the DIP8 package. Enclose my measurements of it. The result is compensated for temperature (1.3ppm/C).

Having at least three 10V refs is nice as you can compare them. I try to compare my 10V refs each second or third month as that give multiple points over a year, which is nice to see seasonal variations.

Lars

A note: I usually say just “uncertainty” but more mean “expanded uncertainty” as it often is written in calibration protocols with k=2 or a confidence of 95%.

[lars]

Just some other notes from my experiments with 10V refs:

I have tested several LT1031 in metal case (all bought new from ELFA-Distrilec) and was not happy with them. They drifted a lot and especially nasty is that they change a lot if turned off for some weeks after being on continuously for a long time see chart in another thread. https://www.eevblog.com/forum/metrology/new-voltage-reference-from-maxim/msg971413/#msg971413 . The strange thing is that the two LT1021 I have in metal cases (bought on eBay) are much better even if not as good as the AD587xQ for drift and long power interrupts. In my measurements with 3 weeks power off and 3h on  a couple of times the AD587xQ an LT1021CMH is very little affected with less than 1 or 2 ppm , REF102CM about 3-5ppm and LT1031DMH up to 10-40 ppm. Many plastic packages also show 10-30ppm up to maybe 50ppm worst case as I have seen.

Almost all up to 1mA buried zener refs I checked has at some time jumps (pop corn noise) On some it can only be seen at some temperatures on other can be long between that is minutes, days or even months I have seen. (LM399, AD587, REF102, LT1021,1031, 1236 etc). For me it seems that about 0.8ppm jumps is very common but also 0.4 or 1.5ppm. One 6 digit DMM (with LM399) has for 20 years shown 1.5ppm jumps maybe several times per day. As others have shown even LTZ1000 might have “jumps”. So for an uncertainty analysis for 10v refs I always recommend to say 0.8ppm for jumps and also I have seen a step after months or even years of several ppm so getting below 5ppm uncertainty seems not serious for 10v IC refs if you don´t have several of them.

Line regulation is not as bad as the datasheets says in my experience. Last I tested six AD587xQ, two REF102CM and two LT1021CMH with a change of input voltage 15-20-15v the AD587 were best with about 1-3ppm change, the REF102 about 3-5ppm and LT1021 was worst with about 8-10ppm (Note LT1021 had about 5-7ppm/C before compensation the others below 3ppm/C). So datasheets doesn’t say everything. The spec for REF102C is 1ppm/V, LT1021 4ppm/V and  AD587 10ppm/V.

Reverse voltage protection is necessary from bitter experience. I mostly use a schottky diode 40v 1A like the 1N5819 which gives about 0.2v voltage drop at 2mA. Sometimes I have added an LP2951 set for 14v to get better line regulation. The LP2951 is cheap (<1USD) and draws little current and have worked well for me.

Having short-circuit protection with an extra buffer like Alex has with an LT1097 for the LT1021 is probably good but I have never used such protection. One thing to remember with buffers is to test with capacitive loads. Not all can handle that. I have once short-circuited a MAX6350 for a couple of minutes and that gave a permanent shift of -30ppm but that is the only time and I have measured references without protection lot of times. Some time I shall test an AD587JQ short-circuited.. some time…

I have never used protection for over voltage on input or output for my own designs and don´t remember I have had problems either. At work I always design in this but am not sure if it really works. Actually surge or burst tests is seldom needed on outputs like these (if you don´t have extremely long cables).

Having EMI protection is very good as you never know if you have any RF-fields nearby. But it is the same problem as with over-voltage protection. It is not easy to design and verify. Having appropriate bypass capacitors and ferrite beads have worked for me. Have done some EMC tests at home just with an HP8657B signal generator sweeping with a small loop close to the output binding posts from 1Mhz to 2GHz reveals that most of my 10v refs still have susceptibility to relatively small RF fields. My SVR boards and also the 2ppm eBay are worse than the once I have better EMI protection on. I have not tested my VREF10-003. If I remember correct the Fluke 732B has a very low spec of 0.18V/m (needs controlled lab environment!). Normal equipment for home use is tested at 3V/m and the equipment I work with is tested at 10V/m or more. You can really have large errors if the RF signal gets rectified. The really nasty problem with EMI is that you can calibrate your 10V ref in one environment and at home you have different RF fields and may be off many ppm without knowing.

Having the reference in a case is absolutely recommended, mostly due to air drafts. I can´t say that having a metal or plastic case seems to make a difference for me. Neither from an EMC or stability point. One reason the metal case is of little use for EMC is that the noise entering through the binding posts are the same for metal and plastic. Good design of the PCB probably is the best.

I have used different banana jacks and bindings posts including silver and gold plated brass or copper. Except for nickel plated once, I have not seen a difference for my 10v refs. Nickel-plated was a disaster giving easily up to several ppm errors.

Almost all of my 10V refs and resistor references have a temperature sensor. As my reference resistors have no power a PT100 or NTC is my choice for them and to have the same I most often use a 10kohm NTC even on the 10v refs even if I use LM35 on some and it is easier to convert to °C compared to NTC´s.

Lars

[Andreas]

Hello,

I have finished the T.C. measurements on the AD587 which I had in the drawer.
From the remaining references with DateCode 1342 2 were below 3 ppm/K.

If I calculate that from the T.C. the drift over a 10 deg C range can be compensated by a factor of 10
giving 10 uV error for each ppm/K * 10 K / 10
and take the full uncertainity for the popcorn noise
I get the "reference score".

So the yield is 2 excellent devices and 3 fair out of 9 samples.

With best regards

Andreas

[Andreas]

Hello,

just want to suggest a alternative T.C. trimming scheme.
I think this fits more to my relative large temperature range in my "lab".

R2 and RV1 give a linearized temperature dependant voltage of about 1% (100mV) per deg C in a 25 +/- 15 deg C range.
The voltage at TEMP goes down with rising temperature.
If your reference needs it the other way round you have to exchange RV1 and R2.

R1 R5 R4 is the voltage output trimming .
R3 has to be adjusted to the T.C.

But Im also thinking about using a 12 Bit DAC for the fine trimming and the TC correction. This has the advantage that the NTC can be always placed directly at the ground pin which should be connected to the lead frame of the chip.

With best regards

Andreas

[Gyro]

Just a thought, aren't you going to suffer more self-heating of RV1 (~1mW) due to the current through R2? I suppose this could  be good (maybe  ) or bad depending how it mirrors the dissipation of the AD587.

If you delete R2 and connect RV1 to the 10V ref output then you have the SVR-T circuit. Presumably you could still conect RV1 to ground instead to provide opposite TC compensation as you indicated.

Edit: I'm just wondering if R2 actually helps or hinders you.

Chris

[Andreas]

Hello,

the SVR-T cirquit is only linear in a small temperature range.
Since my AD587 references are nearly linear in T.C. over the full 10-40 deg C range I want to have a more linear compensation.
But of course there is nothing for free.

With best regards

Andreas

[lars]

Hello Andreas,

I can´t see why the SVR-T circuit shouldn´t be able to have a linear compensation if needed. With the series resistor set to 80% of the NTC (at 25°C) the compensation will be quite linear over 10-40°C. I had to find my old calculation and add 10 and 40C to the calculation as the old just covered 15-35C. Enclose a chart done with the 2x680k NTC in series with 1078kohm(includes internal resistance of Vref and trimpot node) and the trimpot set to compensate 7ppm/C. The residual is +-1ppm for a compensation range of +-105ppm (10-40ppm). The calculated residual has a max TC of 0.2ppm/C. Probably much better than other errors. I have used the datasheet values for the NTCS0805 680k (from Mouser or Digikey).

Your circuit for me just seems to divide the current between R2 and R3+about 2kohm (from the internal resistance and the trim network). One of my assumption is that the 10v output is very steady relative to ground compared to the change in the NTC-resistor node.
If my assumption is correct the 80% series resistance in your case is R2 in parallell to R3+about 2k and after that your circuit is very similar to the SVR-T. ( I haven´t run a simulation just calculated in my head so I might be completely wrong)

The circuit I showed before has the nice feature that you have a quite linear change of TC of about 4ppm/C per Volt change on the trimpot wiper (TP2). By temporary adding a resistance between the trim pin and 10V you can do more exact calculations by measuring the trimpin and 10V change. My last AD587JQ box had about 4.27ppm/C per Volt calculated.

For those that want to use my circuit, my recommendation is to first set the offset trim pot set to 2.5V (mid). When adjust the difference between the trimpin (TP1) and trimpot wiper (TP2) to zero, the TC compensation will be zero that is very useful for a first temperature test. When adjust the TC trimpot. Check again and maybe do another adjustment. Add R1 or R2 to get within say +-50ppm. Do the offset fine trim. Do a temperature test again to confirm.

This will also work for AD587 in plastic packages but the hysteresis (due to drying) during the temperature tests may be a problem.

Lars

[Andreas]

Hello Lars,

of course you are right according to the SVR-T cirquit.
If you can choose the NTC value freely and adjust it to the T.C. compensation resistor (Trimpot) then you can set it accurately to the 80% rule.

I have not calculated the final values for R3. But it is shurely in the 1 Meg range.
So the 27K is already the 80% of the 33K with typical 0.3-0.4 deg C linearity in the 10-40 deg C range.

But after your 2nd explanation your current cirquit has much more charme.
I overlooked that you can trim +/- TC with one and the same cirquit. Chapeau!! 
So the current cirquit is more practical.

What are the typical values for R1 and R2?
I think you have to trim R1/R2 in a way that the output voltage without trimpots is slightly below 10V.

With best regards

Andreas

[lars]

What are the typical values for R1 and R2?
I think you have to trim R1/R2 in a way that the output voltage without trimpots is slightly below 10V.

R1 changes the output about -1700ppm/(R1 in Mohm + 0.13Mohm) for a typical AD587
R2 changes the output about +4200ppm/(R2 in Mohm + 0.13Mohm) for a typical AD587

So I should be prepared to have resistors in the E12 series from say 1.5M to 10M and maybe 15M and 22M. Of course you can both parallell and series connect resistors if you are missing a value.
I have used cheap thick film resistors like RC0805 or RC1206 (Digikey). They are not very stable but if the resistor moves the output less than 1000ppm, a 1000ppm change on the resistor will be less than 1ppm on the output.

Before you select R1 or R2 you have to adjust the trimpot for the TC as that moves the output by about 50ppm per ppm/C you adjust.
The trim range for the offset pot is about +-130ppm.

Lars

[pitagoras]

Adding a 7815 on the input can reduce the line regulation issues almost entirely.

Or bootstrap the reference using an operational amplifier and single transistor if necessary.

Bootstrap is something like this?

[tszaboo]

Adding a 7815 on the input can reduce the line regulation issues almost entirely.

Or bootstrap the reference using an operational amplifier and single transistor if necessary.

Bootstrap is something like this?

Though I have no idea why this would be better than a 7815 (or low noise equivalent). A good 7815 has a maximum 20mV line regulation, and the AD587 has some 100uV/V. This together would result a maximum 2 ppm error, if you change the input voltage from 18V to 30V. Typical results will be better, and why would you fiddle with the input voltage this much?

[pitagoras]

Adding a 7815 on the input can reduce the line regulation issues almost entirely.

Or bootstrap the reference using an operational amplifier and single transistor if necessary.

Bootstrap is something like this?

Though I have no idea why this would be better than a 7815 (or low noise equivalent). A good 7815 has a maximum 20mV line regulation, and the AD587 has some 100uV/V. This together would result a maximum 2 ppm error, if you change the input voltage from 18V to 30V. Typical results will be better, and why would you fiddle with the input voltage this much?

As it has been suggested by David Hess as an alternative I was motivated to understand the statement and posted the circuit I came up with.
Now... yes, the question is: why it is better (if it is).

[David Hess]

As NANDBlog points out, it is overkill in many applications.  On the other hand it is simple, lost cost, and high performance.  It is more commonly done with shunt references to control the input current but it can be applied to series references as needed.

And a 7815 has more like 150 millivolts of line regulation over temperature and input voltage.  Load regulation adds another 150 millivolts to that.  If you use a premium regulator, then it will cost more than a cascode made from a separate operational amplifier and transistor.

[pitagoras]

OK this is the circuit I'll be building.
trying to agree with everyone, the board will contemplate soldering both 7815 or (exclusive or!) bootstrap circuit.
It contemplates the NTC circuitry as per Lars suggestion.

[Andreas]

Hello,

you will need the test points from Lars cirquit for the adjustment. (at least they would help)
R12 = 1 Meg ?

What happens to the boot strap cirquit in case of a short on the output of the reference?

Bonus question: will the boot strap cirquit ever start up (depending on offset of the OP)?

with best regards

Andreas

[pitagoras]

Hello,

you will need the test points from Lars cirquit for the adjustment. (at least they would help)
R12 = 1 Meg ?

What happens to the boot strap cirquit in case of a short on the output of the reference?

Bonus question: will the boot strap cirquit ever start up (depending on offset of the OP)?

with best regards

Andreas

OK I will add the TPs, thanks!

Does the bootstrap even start?
Well I *think* that even if opamp output is 0V, Q1 is biased and collector is around 1/4 Vsupply. Might be this is not enough for the Ad587 to start?

[tszaboo]

As NANDBlog points out, it is overkill in many applications.  On the other hand it is simple, lost cost, and high performance.  It is more commonly done with shunt references to control the input current but it can be applied to series references as needed.

And a 7815 has more like 150 millivolts of line regulation over temperature and input voltage.  Load regulation adds another 150 millivolts to that.  If you use a premium regulator, then it will cost more than a cascode made from a separate operational amplifier and transistor.

Though, load regulation is insignificant, unless you are planning to go up up to an 1A load on your reference. Anyway, I just like the LM78Nxx parts, they come in nice SO8 packages, can source 100mA, work with ceramic caps and behave as you expect them to behave. Not a lot of power.
I think we already spent too much time on this.

[lars]

As I said above I like the LP2951 with a 40V 1A schottky diode in series.
Some reasons:
-   Around 1USD at Digikey for both. Both available in through hole and surface mount.
-   Can handle +-30V inputs.
-   Voltage drop less than 0.5V for both in series. For the AD587 I set the output of the LP2951 to around 14V so reasonable good margin for 15V supplies. For the LT1021 it can probably be set for an even lower voltage.
-   Only about 0.1mA current extra with a 2mA load gives longer battery life time and less heating. 78 series regulators normally have many milliamperes of quiescent current
-   Line regulation and stability together with AD587 and LT1021 are more than enough. Using two 9V batteries will not be a problem.
-   Noise from the regulator in the 0.1-10Hz region is not a problem as far as I can see. (So I see no need for a low noise regulator)

Lars

[lars]

As Andreas says 1Mohm in series with the two 680k NTC’s gives more linear TC (temperature coefficient) compensation than 1.5Mohm for the AD587. For small TC (< +-2ppm/C) it doesn´t matter a lot and for positive TC it is a benefit to have the slight nonlinearity you get with the 1.5Mohm. For negative TC, I should select 1Mohm or even less if I want to compensate some of the nonlinear TC of the AD587. I have had many AD587 with second order TC of about -0.02 to -0.05ppm/(C*C) but Andreas seem to have more like -0.01ppm/(C*C). But as has been stated before the non-linearity of the compensation probably will be less than other errors.

If you use 2x680kNTC +1.5Mohm the change on the wiper (TP) will change the TC about 4ppm/C per Volt. With 2x680k+1M it will be slightly above 5ppm/C per Volt instead.

Lars

[pitagoras]

Ok just a quick update.
I finally decided to follow Lars about the LP2951.
I attach the schematic and the PCB...


PS: Do you think I should share the project (kicad) on github? Do I have the agreement of the "owners"? With due credit to this thread of course.

[Gyro]

While you're going to the effort, wouldn't you be better using an AD587JQ CERDIP part? Using an SO8 package seems like too much of a compromise in humidity sensitivity and long term stability.


P.S. I missed your post earlier in the thread about already having some AD587KR, but even so...  Maybe you could dual footprint?

[pitagoras]

 

While you're going to the effort, wouldn't you be better using an AD587JQ CERDIP part? Using an SO8 package seems like too much of a compromise in humidity sensitivity and long term stability.


P.S. I missed your post earlier in the thread about already having some AD587KR, but even so...  Maybe you could dual footprint?

Yes, now that you mention that... you're right.  I could also make the PCB for CERDIP, avoid the milling around  the 587, an mount the SOIC as Kleinstein suggested (with wires).

[Muxr]

As I said above I like the LP2951 with a 40V 1A schottky diode in series.

I like the BAT86 Schottkys (easy name to remember).

[David Hess]

I attach the schematic and the PCB...

Wouldn't it be better to have the reference and sensitive parts on a little peninsula even if that was in the middle of the board with a U "moat" routed around it instead of those opposed corner attachments?  Your layout reminds me of a shear beam load cell.

[Muxr]

Thanks to the great advice in this thread I designed a reference board. Since I live in the tropics I wanted something to have fairly good moisture and humidity resistance. I used Conrad Hofman's MML ltc1027 reference as a basis with some additional protection and the temp sensor, also the LTC1027 noise reduction mylar cap. Decided to skip the NTC temp compensation for now, until I can characterise it and see how it goes.

All critical components are hardened against humidity, ltc1027 LS8 package for instance, capacitors and resistors are moisture resistant. The only thing that isn't is the trimpot, well and non critical power regulation components. I kept the costs fairly low as well. Most expensive being a few wirewound resistors and the reference itself. Haven't done the math on it yet but I think in total around $50 worth of components (I usually order in multiples).



The weird J connectors on the edges are for the copper bridges for easy hookup and measurement with grabber clips. That weird orange cap it's just an aluminum electrolytic.. couldn't find the right footprint for some reason, but the holes match. Done in KiCad.

Anyways thanks to: Conrad Hoffman, pitagoras, lars, Kleinstein, David Hess, Andreas and others I forget to mention in this thread!

[Andreas]

All critical components are hardened against humidity, ltc1027 LS8 package for instance,

Hello,

you forget that the PCB itself is not hardened against humidity.
So this will have some influence.
see also comparison of a soldered and a dead bug style mounted LT1027 in LS8 package.

mounting:
https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1003827/#msg1003827

resulting hysteresis:
https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1011371/#msg1011371

with best regards

Andreas

[Muxr]

Hello,

you forget that the PCB itself is not hardened against humidity.
So this will have some influence.
see also comparison of a soldered and a dead bug style mounted LT1027 in LS8 package.

mounting:
https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1003827/#msg1003827

resulting hysteresis:
https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1011371/#msg1011371

with best regards

Andreas

Hi Andreas, thank you! I did in fact think about doing the dead bug style. However I wanted to have a baseline for comparison before I started doing anything extra. So that I can compare say, NTC compensation and maybe dead bug style.

Have you tried conformal coating in your experiments by any chance? Something like this for instance: https://www.amazon.com/MG-Chemicals-Silicone-Conformal-Coating/dp/B004SPJOKK

[pitagoras]

Thanks Muxr for sharing.
I see you used a grounding technique different as mine; and I was not sure in my case if having a layer with the full ground was a good idea.
Can you comment in your strategy?

[Muxr]

Thanks Muxr for sharing.
I see you used a grounding technique different as mine; and I was not sure in my case if having a layer with the full ground was a good idea.
Can you comment in your strategy?

Sure, I only used the ground plane on the linear regulator side as you can see (both top and bottom layers are the same in this regard). This is to help dissipate the regulator heat. It's only present on that section of the PCB.. the rest of the PCB has no grounding plane. I even went as far to make a cut out right next to the regulator to force the heat to the top and bottom sides of the board away from the critical reference components.

I don't know if any of this will matter in practice, but I wanted to minimise the influence of different voltage (higher voltage more heat generated by the regulator) being supplied to the reference board.

[Andreas]

I don't know if any of this will matter in practice, but I wanted to minimise the influence of different voltage (higher voltage more heat generated by the regulator) being supplied to the reference board.

Hello,

this will matter.
Even on my LTZ1000 board you can measure the influence of the voltage regulator.
A slot improved this by a factor of 2.
So in the re-design I will even put the voltage regulator on the other side of the battery pack so that the distance between regulator and reference increases from 3 to about 8 cm.

Have you tried conformal coating in your experiments by any chance? Something like this for instance:

No I did not try this. Silicones are not vapor tight.
I saw photos with water between PCB and silicone after damp tests.
So the maximum what you can expect from conformal coating is a increased time constant for humidity influences.

with best regards

Andreas

[Muxr]

Finished my LTZ1027 reference (inspired by Conrad Hoffman's MML reference and this thread). For the first go around I just used some decent thin film 10ppm resistors (non matched).

One of the trimmer resistors was not the right value so I just bodged it for now with 3x 50K resistors wired in parallel (of 15ppm variety which I have tons of for now). I didn't even solder them on, they are just plugged into the PCB for now, until I get the right value resistor.



Aside from the above mentioned caveats, the aging process begins. I am actually quite pleased with what I am seeing so far (although probably too early to tell due to all the hysterisis going on):

[Andreas]

Hello,

Mhm, there should be also a 5 V output to distinguish ageing of the reference and ageing of the rest of the cirquit.
I guess that that drift what you see is not the ageing of the reference.

With best regards

Andreas

[Muxr]

Hello,

Mhm, there should be also a 5 V output to distinguish ageing of the reference and ageing of the rest of the cirquit.
I guess that that drift what you see is not the ageing of the reference.

With best regards

Andreas

Good point, aging of the reference [board] as a whole, not the actual chip, thanks.

[0.01C]

In the bbs.38hot.net forum , there are some people diy 10V reference kit for sell (price $100-$250 only), I bought three SZA263 10V(@$200 full set) and one LTZ pwm 10V (free from my friend),
very good 

http://bbs.38hot.net/search.php?mod=forum&searchid=396&orderby=lastpost&ascdesc=desc&searchsubmit=yes&kw=10V+%BB%F9%D7%BC

http://bbs.38hot.net/forum.php?mod=viewthread&tid=174007&extra=&page=1

http://bbs.38hot.net/forum.php?mod=viewthread&tid=147708

http://bbs.38hot.net/forum.php?mod=viewthread&tid=111481

http://bbs.38hot.net/forum.php?mod=viewthread&tid=191238&highlight=10V%2B%BB%F9%D7%BC

http://bbs.38hot.net/forum.php?mod=viewthread&tid=133516&extra=&highlight=10V%2B%BB%F9%D7%BC&page=1

http://bbs.38hot.net/forum.php?mod=viewthread&tid=108021&highlight=10V%2B%BB%F9%D7%BC

update : I just contact with my friend ,he can offer eevblog member at ¥1000 for a LTZ PWM 10V kit ,but only 2 pcs .
PS : If this is not permit here ,please let me know ,Thanks

[agaelema]

Hi lars, could you explain the operation of the trim circuit using the NTC. I developed one reference using the REF102A (not the best) with other 4 multiple voltages (5, 2.5, 1.25, 0.625) using some INA2133. I think that circuit can be implemented in the REF102, I'm correct?

Can you indicate some texts about this TC circuit?

Thanks! 

What I have preferred the last years are actually the AD587 JQ as the LQ isn´t available. That the KQ and LQ isn´t available and that the UQ seems to be trimmed in another way is one reason I think it reasonable to believe a lot of JQ’s will be within 5ppm/C. And of course, I was lucky to get all my AD587JQ within +-5ppm/C. I have nine AD587JQ’s of date codes 1028, 1247, 1517 and 1609 bought from Digikey and RS components.

The datasheet for the AD587JQ says 20ppm/C temperature coefficient and that with the box method. So, at 25C you can have even worse TC. As I, Andreas, Joe Geller and a report from the LISA-project shows the AD587UQ has more like 7-11ppm/C at 25C but the box specification is 5ppm/C.

Joe used the LQ and I think all had the date code 0045. Most of the LQ Joe had were around +2ppm/C at 25C so the simple add-on with a NTC+ resistor worked well.

 I have temperature compensated many 10V references at home (never in a design at work) with NTC’s but mostly used other designs than the SVR-T design. The SVR-T design was just because the trimpot value was already fixed. Enclose a sketch for what I tried to describe earlier as an example for a more general design. It will give an adjustment range to compensate -8ppm/C to +12ppm/C. In this design the sensitivity to drift of all trim components are quite low compared to the SVR-T design. If I know the AD587 have a slight positive TC I can calculate a larger value instead of 1.5M to get less range and some second order compensation. Most AD587 seems to have a slight negative second order component of about -0.03ppm / (°C * °C) around 25C so a larger resistor gives some compensation for this. 1.5M together with 1.36M NTC gives a quite linear compensation, so no second order compensation, but useful for a broad range. I myself has set up an excel sheet for calculation of the NTC compensation.

That I say I prefer the AD587JQ is only true from a hobbyist point and to get a good “transfer standard” to compare and maintain 10V. In my professional work as a design engineer I haven’t designed in an AD587JQ and probably will not. Long long ago I designed in AD688AQ and BQ as they were good choices for +-10V refs with reasonable accuracy in data acquisition systems. As the AD588BQ are similar I guess it is a good choice for 15EUR/USD from eBay if they are not fake. My experiences with AD688xQ compared to AD587xQ are that the 587 is slightly more stable over time and draw much less power (10x) and don´t need dual supplies, so if you don’t need +-10v I prefer the AD587 in all aspects. Having just 2mA current draw is a plus when using say two 9V batteries to power your reference. Today most volt refs I design in are 2.5-5V in SMD of course.

Why 10V even for a hobbyist? I should say because of the traceability chain and uncertainties. If you, like me, after some years comes below 10ppm uncertainty the extra step from say 5 or 7.xV gives extra uncertainty. Not all have a super linear 8 ½ digit DMM or a KVD. My best DMMs are only 6 ½ digits and I never paid more than 100USD for a DMM yet. If you go for a couple of 10V references adjusted close to 10V it is also easy to make a simple x100 amplifier and with just a 3 ½ digit DMM to get 0.1ppm resolution when you regularly compare your references. Of course, after a while you will find it is difficult to measure the absolute long-term drift without repeated calibrations with known uncertainties. But even before that you have learned a lot about drifts between your samples. And temperature coefficients can be checked and adjusted with simple means. My first NTC temperature compensated REF102CM’s I adjusted with just a lamp above the DUT and a 3 ½ digit DMM + amplifier. Three out of four are below 0.1ppm/C at 23C. The fourth I must have made some mistake as it is +0.4ppm/C but as I have a second 10K NTC connected to banana jacks on the front of the box it is easy to compensate afterwards. Some other of my references aren’t compensated but I only compensate afterwards. I prefer the 10V adjusted to within say +-100ppm but some of my better reference have up to 400ppm offset (and 2ppm/C).

As Andreas pointed out you cannot temperature compensate away all TC on the IC. My estimate is maybe 10-20 times for a reference with about 5ppm/C from my experiences with an NTC very close to the IC (and of course mounted in a box, so outside gradients will not affect so much).

As the thread started with suggestion for 10v for MML I would say: DIY with AD587JQ and/or VREF10-003 from voltagestandard.com if you can afford 53USD for it. With the VREF10-003 you get 30ppm for 6 months and as far as I know it is a serious offer compared to many other Vref boards or boxes on eBay. I have only one VREF10-003 but at arrival it was very close and now after more than 1 year it is still just +12ppm. Of course it is humidity sensitive as all plastic refs. It has a cut-out on three sides of the IC but I am not so sure it helps. I think the humidity sensitivity comes from the epoxy in the DIP8 package. Enclose my measurements of it. The result is compensated for temperature (1.3ppm/C).

Having at least three 10V refs is nice as you can compare them. I try to compare my 10V refs each second or third month as that give multiple points over a year, which is nice to see seasonal variations.

Lars

A note: I usually say just “uncertainty” but more mean “expanded uncertainty” as it often is written in calibration protocols with k=2 or a confidence of 95%.

[Andreas]

Hello,

Lars cirquit is optimized for the AD587:

The trim range on AD587 is not symmetrically (-1 .. +3 %) thats why it is a advantage (for the T.C. adjustment) to put the trimmers in series.

For the REF102 the trim range is symmetrically (+/-3%) so it might be better to switch the trimmers in parallel.

Have you measured the tempco-curve of your reference?

But since the REF102 is only available in plastic package the stability will be slightly worse than on the AD587 in CERDIP package.
Attached a picture with ageing drift of a REF102 aginst some AD586LQ. (the small 5V brother of the AD587).
50 deg C constant temperature during measurement.

With best regards

Andreas

[agaelema]

Hi Andreas, thanks for the answer.

Have you measured the tempco-curve of your reference?

- No, how I can do this?

Reading the topics related to precision reference I come to the conclusion that I have much to learn.  Hehe.

Analysing your TC circuit (in the same topic) and doing some simulation I think I understood behaviour.
- the increase in the temperature will decrease the NTC value and this decrease will decrease the voltage applied to the Trim pin, I'm correct?

What is the correct way to calibrate the circuit and select the value of R3 (TC adjust)?

[Andreas]

Hello,

you can either calculate this from datasheet values.
(Trim-range, internal resistors and measured T.C. without adjustment).

But since the values are with large tolerances there is always some trial + error.
Start e.g. with 1 Meg. determine change in T.C.
Double or half the resistor and determine T.C. again.
From this you can calculate the influence more exact and calculate the final R3.

Lars cirquit has the charme that it needs lower power
and you do not have to solder the adjustment resistor.

with best regards

Andreas

[ocw]

I have tested a lot of different voltage references and put together a chart detailing their specifications along with their output voltage as measured on my 34401A.

I tried to list the specification limit, not the "typical" deviation figure.  I didn't want to compare one model's typical as compared to another's maximum.  Occasionally I had to make an educated guess as to its maximum value.  Case-in-point: the beloved LTZ1000.  Linear likes to play up its 0.05ppm/C temperature drift specification without defining it.  It appears to be a typical value.  I guessed that its maximum might be 0.2ppm/C (although it could be 1ppm/C like some other models).  The LTZ1000CH is the only voltage reference detailed on my attachment which I didn't have on hand to measure its actual output voltage.

I colored coded my opinion of the specification.  For example, The LTZ1000 and LM399 have red accuracy specifications, but bright green temperature and long term stability ratings.  I tried to be fair in those ratings and based them on their relative figures compared to the others listed.

The LM4128AMF-3.0 which I tested had a high noise level and poor voltage stability.  It was by far the worst one tested in those regards.

I included a temperature compensated 1N4573A-1 zener in my tests for comparison.  It had good voltage stability and a noise level comparable to the average voltage reference.  The economy BZX79-B6V8,143 zener that I also tested for comparison had both a high noise level and poor voltage stability.

Most of the voltage references tested were purchased either from Mouser, Digi-Key or Arrow.  The two AD584's tested were inside the eBay Chinese boxes which are available.  The AD688BQ was recently obtained from http://www.ebay.com/itm/182537476611 for $18.91 + $4.00 shipping.  It meets its 0.02% voltage accuracy specification for its +/-10V output.  I'll probably add some trimmers to improved that accuracy further.  Its voltage stability has been good in its initial tests.  It's nice not to have to play games to alter the voltage to obtain 10 volts.  Having a similarly accurate 20 volts output can double the precision of tests of some equipment.

Only testing one of most voltage references makes my test of them be not that meaningful.  The LM399AH was the only one where I tested two.  I wish that the tested ISL21009BFB's wouldn't have been discontinued.  The specs of their replacements aren't as good.

Hopefully the attachment is useful for some.  I tried to provide a fair comparison of the different models.  Hopefully I didn't let any typical values slip through and that my approximations of a few maximum specifications aren't too far off the manufacturer's limitations.  Several additional models were just added 6July2017 and 6August2017.  At the bottom of the attached chart I just added the average measured accuracy of all of the voltage references shown which had a specification sheet accuracy of better than 0.10%.  That average accuracy of 0.000486% might be one way of verifying that my Agilent 34401A meter which was used meets its 0.0015% specification sheet accuracy.  35 different voltage references were used in calculating that average.

[ocw]

I originally thought that my measurement of the actual output voltages from the references was important.  Because of that I didn't include the additional voltage references which I had ordered but not yet received or tested.  With the evaluation of generally a single unit not always meaningful, I just added those which I previously deleted.  I just replaced the chart on my prior message with the larger updated one.  Some of the new references added include the AD588BQ, AD587UQ, 1N829A and LT1236ACS8-10.

I also corrected a few errors that I found.  The price color ratings are subjective.  A quality product merits a higher price, although I felt that the noisy LM4128AMF that I tested was over priced at about $2.80.

While I also make semi-regular purchases from Newark, since I'm more likely to make a purchase from the others I didn't include their prices.  The Newark prices aren't generally far off of the others.  I see that there is now a Newark/Arrow connection...

[zhtoor]

I have tested a lot of different voltage references and put together a chart detailing their specifications along with their output voltage as measured on my 34401A.

I tried to list the specification limit, not the "typical" deviation figure.  I didn't want to compare one model's typical as compared to another's maximum.  Occasionally I had to make an educated guess as to its maximum value.  Case-in-point: the beloved LTZ1000.  Linear likes to play up its 0.05ppm/C temperature drift specification without defining it.  It appears to be a typical value.  I guessed that its maximum might be 0.2ppm/C.  The LTZ1000CH is the only voltage reference detailed on my attachment which I didn't have on hand to measure its actual output voltage.

I colored coded my opinion of the specification.  For example, The LTZ1000 and LM399 have red accuracy specifications, but bright green temperature and long term stability ratings.  I tried to be fair in those ratings and based them on their relative figures compared to the others listed.

The LM4128AMF-3.0 which I tested had a high noise level and poor voltage stability.  It was by far the worst one tested in those regards.

I included a temperature compensated 1N4573A-1 zener in my tests for comparison.  It had good voltage stability and a noise level comparable to the average voltage reference.  The economy BZX79-B6V8,143 zener that I also tested for comparison had both a high noise level and poor voltage stability.

Most of the voltage references tested were purchased either from Mouser, Digi-Key or Arrow.  The two AD584's tested were inside the eBay Chinese boxes which are available.  The AD688BQ was recently obtained from http://www.ebay.com/itm/182537476611 for $18.91 + $4.00 shipping.  It meets its 0.02% voltage accuracy specification for its +/-10V output.  I'll probably add some trimmers to improved that accuracy further.  Its voltage stability has been good in its initial tests.  It's nice not to have to play games to alter the voltage to obtain 10 volts.  Having a similarly accurate 20 volts output can double the precision of tests of some equipment.

Only testing one of most voltage references makes my test of them be not that meaningful.  The LM399AH was the only one where I tested two.  I wish that the tested ISL21009BFB's wouldn't have been discontinued.  The specs of their replacements aren't as good.

Hopefully the attachment is useful for some.  I tried to provide a fair comparison of the different models.  Hopefully I didn't let any typical values slip through and that my approximations of a few maximum specifications aren't too far off the manufacturer's limitations.  Several additional models were just added 6July2017.

thanks.

how about LH0070/LT1031B (metal cans)? reportedly pretty good tcr ca. 5ppm and good initial accuracy (+- 3mv for LT1031BMH).

regards.

[Andreas]

I colored coded my opinion of the specification.  For example, The LTZ1000 and LM399 have red accuracy specifications, but bright green temperature and long term stability ratings.

Hello,

be carefully with the long term stability specs:

Some are specified per 1000 hours
others per month. (LT1027)
And some for the 2nd 1000 hours only. (or after pre-conditioning)

Usually they are valid only for the best package of the reference.
(even if the metal can package or the CERDIP package is already obsolete).

with best regards

Andreas

[ocw]

how about LH0070/LT1031B (metal cans)? reportedly pretty good tcr ca. 5ppm and good initial accuracy (+- 3mv for LT1031BMH).

Other than when one is in front of me for testing, I have tried to stay away from listing the obsolete models.  Yes, I know they will probably be available from China as long as there is still demand for them...

Speaking of which, attached are some pictures of my AD688BQ recently received from China.

I have tried to keep all specs to the same standards.  One of my corrections involved changing a uV figure into a PPM number like the others.  Hopefully I have caught all of those.

[GigaJoe]

Need suggestion:

I found nearby AD587LQ -$14 and AD587LN - $2.5

in dilemma: 2-LQ vs 12-LN.
I have a hope , an acrylic conformal coating , a few layers, and metal can fillies with silica where full board will fit in ( thinking of thermos:) ; or sealed metal can ... )  will able to fight with humidity effect for a long term.
adding that with 12 pcs of chip i may have better selective process or do parallel a few

is it a wise, to go with plastic, and adding some humidity countermeasures ?

can someone has opinion to immerse board in some electronic liquid ?

[The Soulman]

If 500ppm (0,05%) of initial accuracy is good enough for you than I'd wouldn't worry about humidity sensitivity to much.
On the other hand if you intend your reference(s) to be measured with lower uncertainty in the future a sealed component would make sense.
 
I have a couple of lt1021DMH10 from Arrow: https://www.arrow.com/en/products/lt1021dmh-10/linear-technology
Poorer initial accuracy but should be pretty stable with time, and not that expensive.

More is always better, but doesn't necessarily help with initial accuracy. 

[Andreas]

is it a wise, to go with plastic, and adding some humidity countermeasures ?

Do you really want to build a humidity sensor with a long (some days) response time?

can someone has opinion to immerse board in some electronic liquid ?

Is that really cheaper (if it works) than using a hermetically package?

And even with hermetically packages you will have to select the best ones for a good reference.

with best regards

Andreas

[GigaJoe]

trying to make something for a long term.
I got LM399 5pcs,   all of them drifting slowly , and 10V amplifier and resistors ...  such a pain .... ( it seems a form of meditation , build something, waiting 5Y, measure , wait another 5  ....)

how big a difference in ppm I may expect from AD587   in cerdip VS plastic if just a humidity factor count, simply count a diff like:   rainy summer (95%) VS sunny winter (30%) ?   


I'm still thinking ... need a case anyway, would be make sense to use 10 in plastic for averaging, and the whole board in a metal container.  like an empty container from the propane can ( or spray) ; filling by silica, the front cover would be PCB with contacts output and power, glued soldered .. inside an inch or 2in of silicone filler. I think it will work ....  I may even consider heater inside,  where ref temp will be LM399.   I'm tend to use a plastic in this construction ...

Thanks for a tip: lt1021DMH10 - interesting option ...

in my understanding a "selection process" from 2 items really sound funny, it seems i ll go to the plastic one; get  it 20-50,  selected items run  in parallel; then all board sealed by some case\process ....

[Andreas]

its just like in the diagram above.

https://www.eevblog.com/forum/metrology/best-out-of-the-box-10v-reference/?action=dlattach;attach=327184;image

The Ref102A is in plastic package : this device has around 5ppm seasonal drift when measured ad 50 deg C.
Rainy summer here is about 60-65% rH
Dry winter about 40% rH.
X-Axis is in day
Y-Axis is in ppm

I have devices up to 0.5ppm / %rH at room temperature in plastic.

with best regards

Andreas

[GigaJoe]

Andreas, thank a lot ...

Seems i toward to the plastic encapsulation , mostly due to quantity i can purchase and selection process,  and trying to mitigate  humidity effect ...

does lt1021 comparable with AD567 for a long term?

Silly question:  wonder, if "soft start" ( V supply rising during seconds may help with accidental Vref shift ? )

[Gyro]

It's probably worth mentioning that $14 is a very good price for the, more or less unobtainable, AD587LQ these days (assuming that they haven't come via China that is).

The TC of the LQ version tends to have a better profile for thermistor compensation around room temperature that the military spec UQ version.

[GigaJoe]

Not sure, if any one here have  interest in AD587LQ version,  I'm not sure a quantity left (it old stock) and seems initial price almost $16.  ( it including sale tax). So PM me if anyone interesting ,  then I will dig further .....

[The Soulman]

does lt1021 comparable with AD567 for a long term?

Silly question:  wonder, if "soft start" ( V supply rising during seconds may help with accidental Vref shift ? )

Both have similar specs.

Soft start only helps if there are capacitors on the output, it will reduce the output current while charging the capacitors.

[eurofox]

My most stable 10V reference at this time is an original Geller SVRT, about 5 years old and was turned ON most of the time.
The picture here shows a recent measurement over night with "cheap normal cables" and around 2 °C change over night.

Actually I am surprised how stable the SVRT really is.
When it is turned off for a few days it needs about 30 h to stabilize again.

Glad that "my" voltage standard got a nice and useful home 

[Insatman]

Greetings,
Attached are graphs of data on a variety of voltage reference ICs I've been testing.  Some have over 1000hrs of data, some less.  Testing is ongoing with a few more types of references to test. 
Note that the VRE305AD was pulled from the test because it clearly was out of spec and replaced by APEX when I sent them the data.  Nice especially since it's a $75 chip. 
One especially nice combination is a AD587UQ with a PTC termistor thermally bonded to it.  The termistor keeps the 587 at about 45C.  The whole mess is mounted in a small ABS box which requires +18VDC at around 30mA.  I plan on building an LTZ1000 down the road but that's a completely new adventure.

[PartialDischarge]

Ok, so a few week ago I finished my heated 10V reference based on the AD588KQ using an enclosure from a broken power supply, this little thing costs 40€ at mouser. It is a handmade prototype with a heated enclosure and I'm still doing modifications but I'm quite happy with it. Here are some results,

-Vertical scale is not the absolute value, I prefer to plot the difference from the mean, in this case in microvolts so really good.
-There is not much difference from the NPLC10 to the NPLC100 and that tells me that noise is not too bad.
-The oscillations are due to the heater turning ON/OFF. I keep it around 40º, but the range is not very tight so still room for improvement here. I'm using a TMP01 so very simple control
-In the NPLC100 measurement the 3458A was still heating so ACAL in the middle clearly did affect the "mean"

Oh, and some details I forgot to mention. The AD588KQ is inside a small hermetic copper can I made for it. On the outside it has a resistive network for adjusting means. This module is contained inside some layers of calcium-magnesium silicate thermal insulation sheet.

[Andreas]

Note that the VRE305AD was pulled from the test because it clearly was out of spec

Hello,

What do you mean with clearly out of spec?
The initial output voltage +1mV against 0.5mV max. (on production, before soldering).
 or the -40 ppm drift over 600 hours against typical 6 ppm/kHr?

Did you also measure 0.1 .. 10 Hz noise?

with best regards

Andreas

[Insatman]

Note that the VRE305AD was pulled from the test because it clearly was out of spec

Hello,

What do you mean with clearly out of spec?
The initial output voltage +1mV against 0.5mV max. (on production, before soldering).
 or the -40 ppm drift over 600 hours against typical 6 ppm/kHr?

Did you also measure 0.1 .. 10 Hz noise?
with best regards

Andreas

It was returned because of initial voltage, but it was also clear the drift was bad as well.  My replacement device was within the initial voltage spec but is drifting way more than specification despite being housed in as ideal environment as I can create for it.  I plan on posting the data in the next month.   I have procured an original Thaler "new old stock" version of the IC recently.  It is currently getting it's initial 200hr burn-in.  I plan on replacing my Apex VRE305 with the Thaler VRE305 in my voltage reference unit. 

A friend who has a lot of experience with these particular references tells me the new Apex devices do not typically meet their drift specifications.  Mine certainly has not.  He says the older Thaler units were quite good, so I'm going to try one I bought from Ebay.  The Apex units short-term tempco appears to be quite low however based on limited data...too bad the long term drift is horrible.  The Apex unit has drifted +150uV since January 3rd of this year.   It's on a very stable clean supply and the trim pot is one of those expensive metal foil type from Vishay.  It is enclosed in an insulated cast aluminum enclosure.   

I am hoping the Thaler unit will provide better performance.  I will know in 6 months or so.   Look for the data on the Apex VRE305 as well as updates on my other references in the next month or so when I get around to updating my Excel speadsheet.

I did not measure the low frequency noise but will do so on my Apex device and Thaler devices for you sometime.  My resolution is limited however as my best meter is an HP34401A. I also have an HP3457A which can average to 7.5digits but I suspect is drifts a bit more than the 34401.   The best standard deviation I can get on the 3457 with any source is around 6.5uV (>1hr measurement window). 

I have recently built a x100 amplifier to measure noise but it has not been well tested as yet.

[Andreas]

Hello,

for measuring 1/f (0.1 .. 10 Hz) noise on references you will need typically a bandwidth limited low noise amplifier with 10000 fold amplification.
(3uVpp give 30 mV on your scope).

I have bought 2 VRE3050AS from DigiKey in 2011. (So short after Thaler was bought by APEX).

One of the main issues that I had was the high noise (3 uVpp typical in data sheet) and 9uVpp in average on measurements.
(Screen shot with 2uV/Div + 1s/Div)

with best regards

Andreas

[Insatman]

Hello,

for measuring 1/f (0.1 .. 10 Hz) noise on references you will need typically a bandwidth limited low noise amplifier with 10000 fold amplification.
(3uVpp give 30 mV on your scope).

I have bought 2 VRE3050AS from DigiKey in 2011. (So short after Thaler was bought by APEX).

One of the main issues that I had was the high noise (3 uVpp typical in data sheet) and 9uVpp in average on measurements.
(Screen shot with 2uV/Div + 1s/Div)

with best regards

Andreas

Andreas,   Any chance you could share the circuit for that 10,000x Amplifier?   I would like to build one.

[cellularmitosis]

@Instaman, there are some good designs in this thread: https://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/

If you are going to DIY, you need an input capacitor with very low leakage.  pmcouto found some rubycons which will work well: https://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg1214172/#msg1214172

You can also purchase an LNA from user Pipelie on this forum, if he still has some in stock.

[Andreas]

Andreas,   Any chance you could share the circuit for that 10,000x Amplifier?   I would like to build one.

already did this in one of the LNA threads:
e.g. starting from here
https://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg1148974/#msg1148974
https://www.eevblog.com/forum/projects/low-frequency-very-low-level-dc-biased-noise-measurements/msg658078/#msg658078
one user here (mimmus78 ?) has built a more professional PCB from the full cirquit diagram (PDF) than my hand etched single layer design.

with best regards

Andreas

[Insatman]

Andreas,   Any chance you could share the circuit for that 10,000x Amplifier?   I would like to build one.

already did this in one of the LNA threads:
e.g. starting from here
https://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg1148974/#msg1148974
https://www.eevblog.com/forum/projects/low-frequency-very-low-level-dc-biased-noise-measurements/msg658078/#msg658078
one user here (mimmus78 ?) has built a more professional PCB from the full cirquit diagram (PDF) than my hand etched single layer design.

with best regards

Andreas

I followed that thread and it contains several spice simulations and a few schematics,  but I never found the schematic or PDF of your amplifier.   I was also not able to find mimmus78.   Lots of discussion of building out of discrete components but the photo shows two LT op amps.   Am I right in that the 10K:1 gain is done in a single stage followed by a buffer amplifier?    I would have guessed multiple stages but I'm not an analog expert.   Any help would be appreciated.

Thanks....Insatman

[cellularmitosis]

It is attached as FILT1105w.pdf here: https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg834013/#msg834013

Google translate actually does a pretty good job with mikrocontroller.net now!

https://www.mikrocontroller.net/topic/250656#2679335

https://www.mikrocontroller.net/topic/207061?goto=2060389#2060389

https://www.mikrocontroller.net/topic/207061?goto=2060260#2060260

[mimmus78]

Hello,

for measuring 1/f (0.1 .. 10 Hz) noise on references you will need typically a bandwidth limited low noise amplifier with 10000 fold amplification.
(3uVpp give 30 mV on your scope).

I have bought 2 VRE3050AS from DigiKey in 2011. (So short after Thaler was bought by APEX).

One of the main issues that I had was the high noise (3 uVpp typical in data sheet) and 9uVpp in average on measurements.
(Screen shot with 2uV/Div + 1s/Div)

with best regards

Andreas

Andreas,   Any chance you could share the circuit for that 10,000x Amplifier?   I would like to build one.

I have few more unpopulated PCB ... If you want one, just pm me.

Inviato dal mio ONEPLUS A5010 utilizzando Tapatalk

[3roomlab]

i had a little play time with the original spice model, and tried some numbers on it. it seems to work (the original file parameter TC = 20e-6)
but does anyone actually use it to determine compensation values?

but realistically speaking, if the VREF does have the TC on the edge of 20ppm/C, and the simulation seem to come up with a compensation for a certain range, would you still do it? (@ about 9th iteration, i found some values which give a "flat-ish" plateau even for a 20 ppm? how real can this be?  ) ** just to be sure, for the plot with the "flat", i added extra 2 points, the curve now appears to be a "S". a very interesting "compensation"