Lowest drift, lowest noise voltage reference (ADR1000AHZ)

[Dr. Frank]

I managed to repair my Fluke 5442A.
It was quite a profane root cause: A current sensing resistor, carbon composite, inside the LM723 based +5V regulator circuit, drifted towards 3 times its nominal value. 
That's reasonable after 40 years, I guess.
Anyhow, the 5442A is now warming up, and I will redo my comparison measurements, and then send my LTZ #7 to branadic, for Volt comparison.
Frank

Well, that 0.15 Ohm resistor now measured 0.418 Ohm, so the ~ 1.7A, 5V supply was clamped down, and the processor board failed with under-voltage..
I wonder, how long the board was running on lower voltage, hopefully the calibration last year was not affected, i.e. the storage of the calibration constants.

I show the original test tips of my 121GW, from the early adopters, as these inhibited a direct identification of the fault.
The red tip is defect, i.e. it's making bad contact inside the tip, so I have varying Ohm readings on the order of several 100mOhms.
So I could not directly identify this increase in resistance. 

[The Soulman]

I managed to repair my Fluke 5442A.
It was quite a profane root cause: A current sensing resistor, carbon composite, inside the LM723 based +5V regulator circuit, drifted towards 3 times its nominal value. 
That's reasonable after 40 years, I guess.
Anyhow, the 5442A is now warming up, and I will redo my comparison measurements, and then send my LTZ #7 to branadic, for Volt comparison.
Frank

Good news it was a "easy" fix, I've seen drifting carbon resistors before on different equipment, lm723's (v-ref zener) do seem very stable over the years.

[Andreas]

Hello,

I now passed 16 kHrs:

the 10V/7V divider on ADR#01 drift has stabilized

The 7V outputs of ADR#01 and ADR#02 now drift ~1.3 ppm over the last year.

Detailed explanation of the diagrams see post of 14 kHrs
https://www.eevblog.com/forum/metrology/lowest-drift-lowest-noise-voltage-reference/msg5138967/#msg5138967

with best regards

Andreas

[Dr. Frank]

The ADR 1000 #3 has recovered to its previous value, after the 6 months of been powered down.
It's the raw reference output, 6.668V, and it's still not yet properly assembled into a tuner box, unlike the other references.



I plan to put it in my 3458A first, to check how the noise of the 3458A would improve.

Frank

[Andreas]

Hmm,

so I fear you may want to operate your HP3458A 24/7 to maintain stability in future instead of saving power.
How about a backup supply in case of power outage?

with best regards

Andreas

[Dr. Frank]

Hmm,

so I fear you may want to operate your HP3458A 24/7 to maintain stability in future instead of saving power.
How about a backup supply in case of power outage?

with best regards

Andreas

Hello Andreas,
definitely, never would I let my 3458A run constantly.
That would reduce its life time, due to deterioration of the fast comparators EL2018, and it would bust my energy bill 

Anyhow, the 3458A just serves as a stable and ultra linear comparator and transfer instrument, but not as a voltage reference.

No,  I just want to assemble the ADR1000 PCB temporarily into the 3458A, just for repetition of the noise figure measurements on different NPLC numbers, as we did during the big DMM noise campaign, which TiN once had initiated.

The ADR1000 is very probably not suitable at all for permanent use in DMMs.
Therefore, it will not replace the LTZ1000, nor the LTFLU in these applications, unless somebody (in England? @ CERN?) finally finds a stable bake-in method.

I still think, that a 50% less noisy reference might give some noise improvement on long scale DMMs.
So maybe Philipp can test that on his Fluke 8588A as well? 

Frank

[Andreas]

I still think, that a 50% less noisy reference might give some noise improvement on long scale DMMs.

Hello,

I think the effect will be more at a input voltage near full scale.
At zero volts I think that mainly the input cirquit and comparator are the main noise source.
But we will see ...

with best regards

Andreas

[Kleinstein]

The reference noise and ADC noise are in principle different parts. Ideally the reference noise only matter proportional to the input voltage.  Many of the multislope and similar ADCs react to some reference noise even with zero input: they probaly missed that higher frequency noise in the range of the feedback modulation (e.g. 10-100 kHz) can also contribute, even with zero input. This part is easy to filter and does not need a lower noise reference. Often there is at least partial filtering - probably for a different reason.

It is the low frequency ref. noise that may want a lower noise reference.  A good ADC may reach the 100-150 nV/sqrt(Hz) range for a 10 V range. At low frequencies (slow conversions and possible additional averaging) the references will be higher than this. The relevant frequencies are often lower than the standard 0.1 - 10 Hz band.  A lower noise ref. could help a 8 digit DMM, not just near full scale, but mainly there. The longer the integration the more relevant the ref. noise also with less than full scale.  For > 1 Hz even the LTZ1000 is not that bad.

Beside the ref. in the DMM there are also only few external sources to measure that are lower noise - the easier part is to use the new ref. as an external reference / source.

For the 3458 ADC the largest noise source at the ADC are the resistors at the integrator. After that there are several similar sized contributions (e.g. OP-amps, jitter, other resistors, maybe the higher frequency ref noise). The comparators should not contribute much, even for fast conversions and even less at 1 PLC and above.

The strong effect of powering down the reference makes the ADR1000 less practical for a normal DMM.  A new DMM may get away with a lot less power than the 3458 and may not run too hot to stay on 24/7.

[branadic]

Given the results shown so far, how do you guys interprete the numbers and figures given in the datasheet?

Long-term drift
   200 h (early life drift) 8.9 ppm
1,000 h 7.7 ppm
2,000 h 6.6 ppm
3,000 h 6.2 ppm

Are these numbers really long-term drift or is it drift rate instead and if one was about to cumulate the numbers and plot a graph we are looking at a more realistic inverse exponential chart?

   200 h (early life drift) 8.9 ppm
1,000 h 16.6 ppm
2,000 h 23.2 ppm
3,000 h 29.4 ppm

-branadic-

[Andreas]

Hello,

Of course we are all hoping the 0.5 ppm/year after 3000 hours.
But obviously we are all not clever enough to follow the recommended cirquit and layout ?

with best regards

Andreas

[The Soulman]

Second option is how I read it. The first wouldn't make much sense.

Edit: the first option does fit the graph Andreas shared. Makes you wonder if those "typical" numbers in the datasheet are based on just these four samples?

[The Soulman]

And of course these are "typical" numbers only, actual performance could be much worse and you wouldn't have right to complain.

[KT88]

measured specs would mean that the drift would have been tested for example for 1k hours - that's not viable. The only way to know for sure is a burn-in. And even then the further LTD would follow the random walk scheme.

[iMo]

I remember the video workshop (I think organized by branadic) with the ADR1000 creators where it was said they did the LTD measurements in an oil bath, with not good results as the oil contained water residuals. They then replaced the oil with a better oil. Double-check that in that video. Perhaps it is why they get such nice LTD.
PS: ..and perhaps they do it with a naked package - the zener only..

[miro123]

Hello,
I fully understand why they use oil bath for LTD measurements.
However I don't understand how do they do it at ovenized references.
My experience is that there is no single manufacture who specify the test conditions.
Datasheet numbers itself  does not tel a lot if you don't know the test conditions.
In this case I the customer starts guessing. e.g. Branadic just guess something. I can guess something else too. Lets see who will win the lottery. :-)
My guess - Maybe the oil bath is heated to T=45..50C. The ADR oven is switched off on such way they eliminated the influence of ADR precision temperature control. It tests only the performance of the zenner.

[miro123]

it was said they did the LTD measurements in an oil bath, with not good results as the oil contained water residuals. They then replaced the oil with a better oil.

I use silicone oil for my resitors references. It performs better is terms of mix with humidity

[iMo]

Hello,
I fully understand why they use oil bath for LTD measurements.
However I don't understand how do they do it at ovenized references..

Imagine you are the R&D engineer at ADI. How would you test say 10 randomly chosen ADR1000 samples for 1000-3000hrs?
For example you may create a 10x10cm large pcb, with 10 pads for the ADR1k packages, solder the chips in, and wire the zeners only out through a flat cable (like 40 wires). Put the pcb into the oil bath, the oil temperature at say 70C. Leave it there during the measurements. You may even sweep from -40 to 125 when the oil allows.
You would definitely not build 10 Vrefs boards with all the resistors and opamps on it, as the ADI is not going to guarantee a performance on someone's fully populated pcb with "random components and design" either..

[Kleinstein]

For a ref. amplifier like the ADR1000 or LTZ1000, there is no good way to only use the zener. One could get away without the temperature regulation, but would still need the external loop for the voltage to get at least a reasonable low TC.   So they may have to build really complete ref. PCBs of some kind.  So the drift should indeed include a set of resistors. To be on the safe side, chances are they used quite good resistors, at least for a few sets. For tests with more parts (e.g. 50 pieces) they may also have lower cost versions.

A problem with the drift tests is that the treatment (e.g. soldering and maybe a bake before soldering, storrage before use) can also effect the drift. With the ADR1000 another parameter is the set temperature. This can make quite some difference in aging. The example cuves are for a rather high set temperature (75 C).

[iMo]

This is what I ment with the test jig (the pcb with packages only) for ie. 10 samples and 40 wires kept in the XXdegC oil bath.
On the other side of the flat cable (off the oil bath) is one opamp with two (70k and 120ohm) resistors and one diode, but only 1x, and there are 10 (x4 contacts) relays or switches or whatever which switch the particular sample in during the measurement..

[branadic]

Hello,

Of course we are all hoping the 0.5 ppm/year after 3000 hours.
But obviously we are all not clever enough to follow the recommended cirquit and layout ?

with best regards

Andreas

Andreas,

if we assume the values given in the datasheet are drift rates instead of long-term drift and cumulate them, we get the blue curve. If we also assume the elapsed time has to be cumulated we get the red curve, which looks way more like an exponential shape and thus more realistic to our observations of time needed for the reference to stabilize, don't you agree?

-branadic-

[Andreas]

Hello branadic,

usually it is more complicated.
My best guess:
- the 0.5 ppm/Year is that what they promised to their first customer who wanted a improved LTZ1000A. -> datasheet (target) spec.
- In reality they have forgotten (the LT-engineers left due to retirement) how to produce a reference with LTZ1000 specs.

has anyone seen a "errata sheet"?

By the way there is a PCN available: (they are changeing the die attach - of course without any changes on electrical specs)
https://www.analog.com/media/en/PCN/ADI_PCN_23_0197_Rev_-_Form.pdf

with best regards

Andreas

[Dr. Frank]

Hello branadic,

usually it is more complicated.
My best guess:
- the 0.5 ppm/Year is that what they promised to their first customer who wanted a improved LTZ1000A. -> datasheet (target) spec.
- In reality they have forgotten (the LT-engineers left due to retirement) how to produce a reference with LTZ1000 specs.

has anyone seen a "errata sheet"?

By the way there is a PCN available: (they are changeing the die attach - of course without any changes on electrical specs)
https://www.analog.com/media/en/PCN/ADI_PCN_23_0197_Rev_-_Form.pdf

with best regards

Andreas

Anyhow, the drift diagram diverges from all of our observations, that the (initial) drift is negative in reality in most/all cases.

After my observations about the hysteresis after months of power down, I'm even more convinced, that most of the initial drift, and all of the hysteresis is related to the adhesive material.

That PCN shows, that they use a special epoxy material, for electrical isolation. It's  LOCTITE ABLESTIK QMI536.
For most semiconductors, you want to use epoxy or a joint with high electrical and thermal conductivity.
As the Ablestik stuff has quite a high thermal conductivity, I assume that they use a similar method for thermal isolation (> 200K/W) like in the LTZ1000A.

Btw.: You'll find an Automotive classification of this change @ ZVEI.org, search for "PCN", then 'More information and Downloads', and then download the ZVEI Delta Qualification Matrix.
This is case SEM-PA-07, Change of die attach material, i.e. a C-level change, which usually requires no further electrical tests.
'No change in form, fit, function' is a buzzword you'll often find in PCNs, but that is the pre-requisite for any PCN, which always implies no change of the specified and non-specified parameters of the component.
The electrical characteristics will not be changed, that is evidently true, but those drift and hysteresis parameters, which belong to the non-specified (or not fully specified) characteristics, might be affected.

Unfortunately, the attached test reporton the ADI product page does not reveal further details, but I guess that Henkel takes care that the thermal and elastic parameters of their epoxy does not change much.

Would be interesting to know the replacement material and download its datasheet, or Henkel will only change the recipe w/o changing the product designator.

In the LTZ they might have used a similar epoxy, maybe there is another PCN on this component, where we can learn more.
Indeed, there's a PCN about the glass beads, they are using for the A version.
 
The LTZ1000A might behave differently concerning hysteresis, when transported cold at lower temperatures, as now.
Maybe I should send my LTZ #7 to branadic right now?

Frank   

[branadic]

Maybe I should send my LTZ #7 to branadic right now?

That would be great.

-branadic-

[Dr. Frank]

Maybe I should send my LTZ #7 to branadic right now?

That would be great.

-branadic-

I will send you the stuff on Monday, as in Stuttgart there is night frost predicted for the weekend, but none for next week.

Congratulations for your successful defense, Dr. branadic!     🥳🍾

[aronake]

https://www.analog.com/en/products/adr1000.html

pricing now avilable on analog webpage.

1.000 pieces for 57.93 USD each
500 pieces for 66.63 USD each

Selling option only "Distribution Only" what that now means, but looks like things are moving forward.