Lowest drift, lowest noise voltage reference (ADR1000AHZ)

[Grandchuck]

Tried it.

Some info here:

https://xdevs.com/article/adr1000order/ 

I guess you need an account.

[RandallMcRee]

Tried it.

 

No, that is the normal search bar, not the shopping cart search bar. The shopping cart search bar is visible in my screenshot; its near the bottom of the page.

Yes, thanks for the link: it does describe the correct procedure. I believe the backorder time to be accurate....nothing until June.

Edit: it allowed me to place an order. Will update y'all in June. 
Edit again: Order confirmation says "In stock - est delivery Jan. 10". So yay!

[Grandchuck]

They really want to sell these?

[Andreas]

One could get such a setup with a bridge with 2 diodes and 2 resistors on opposite sides to replace the 2 diodes in series. The main parts is shown in the attachment.

Hello,

after playing with the suggestion of Kleinstein here the results:
- the T.C. trimming is now symmetrically to a center value near 25 deg C.
- I had to fine trim the suggested 22K resistors which where already a good estimate to 18K4 to achieve this.
- what I had not expected is that the sensitivity is about halved. So I had to reduce R10 from 1 Meg to 470K.

with best regards

Andreas

[Andreas]

Hello,

news from my ADR1000A#01 (which is built on one of branadics PCBs. See first post).

I now changed the Zero-TC-temperature from around 48 deg C to 55.4 deg. (using method a)
(refer to https://www.eevblog.com/forum/metrology/lowest-drift-lowest-noise-voltage-reference/msg3810545/#msg3810545)
The plan was a zero TC temperature between 52-55 deg C for up to 40 deg C environment temperature.
But since I had no suitable resistors below 15 ppm/K for trimming I finally got 55.4 deg C.

Also the setpoint temperature was changed to the corresponding 524.7 mV at the heater sensing transistor. (got 524.5 mV actually)
With this modifications another T.C. test showed +166 ppb/K temperature slope for the zener voltage.
Zener voltage is now increased by ~2.4 mV.

The next step will be trimming the T.C. by around -178 ppb/K (+166ppb for the zener and half of the 10V output stage T.C. of +20-24 ppb/K).

with best regards

Andreas

[Andreas]

Hello,

ADR1000A#01 after T.C. adjustment:
I got near linear T.C. values

6.6V Zener voltage ~     -6 ppb/K
10V Output voltage ~ +20 ppb/K
the 10V output voltage is still too low (~9993 mV) and needs further adjustment (after some ageing drift measurements).

with best regards

Andreas

[Andreas]

Update on Burn-In of ADR#02 after 1kHr total.

compare with:
https://www.eevblog.com/forum/metrology/lowest-drift-lowest-noise-voltage-reference/msg3903542/#msg3903542

just after last shown measurement the ageing changed the slope.

At about the same time the output voltage got a much larger hysteresis, which partly explains the larger stray of the drift values.

Slope is now around -44 ppm/sqrt(kHr) (from initial -62 ppm/kHr over -200 ppm/kHr)

next step is to go down again to 110-130 deg C cycling to see if ageing also here has decreased.

with best regards

Andreas

[branadic]

Thanks for keeping us updated Andreas.
I almost hit 4000 h on my sample, with the setup untouched for the whole time. Attached is an updated graph.

-branadic-

[Andreas]

Hello branadic,

it would be good if we get the exact receipe of burn in for the ADR1000 at Analog Devices.
Is it really the 1 week at  constant 150 deg C under bias?

I now could tweak my burn in cirquit for slightly higher temperatures as I just got a
different overtemperature switch with ~170 deg C.
https://www.conrad.de/de/p/thermorex-tk24-t01-mg01-oe170-s160-bimetallschalter-250-v-16-a-oeffnungstemperatur-5-c-170-c-schliess-temperatur-160-1678193.html

Btw. I have a 2nd temperature sensor measuring the PCB-temperature which is around 30 deg C lower than the heat spreader temperature.
So I can use "normal FR4" for the burn in PCB.

with best regards

Andreas

[branadic]

Is it really the 1 week at  constant 150 deg C under bias?

That's at least what Eric Modica said at MM2021.

Question: At which temperature did you make the burn-in?

I believe it's a 150 °C.

Question: So simply powering the zener and have the ambient at 150°C and then - what was it? - 168 h or something like that?

Yeah, you know and we haven't tested the bounce of that, we basically just copied the bake and burn process of the LTZ1000, which is a 168h both bake and burn. We have at least small sample size evidence that you maybe able to get away with just doing an unpowered bake and that's good enough to disipate the charge, but I have to say I haven't looked at it very closely.

Question: Simply elevated temperature or cycling during burn-in? Because, some rumor says cycling around glass transition temperature might improve the aging.

Our burn-in process doesn't utilize any cycling.

-branadic-

[Andreas]

I almost hit 4000 h on my sample, with the setup untouched for the whole time. Attached is an updated graph.

So to summarize.
you have around -4ppm in 4000 hours / -2 ppm in 1000 hours
so -2 ppm/sqrt(khr) without burn in and @55 deg C heater temperature (assuming you use zero T.C. temperature as set point).
Extrapolating for the next year (13 kHrs in total) we will have -7.2 ppm total drift or -3.2 ppm for the next year.

The data sheet value of 0.5ppm/year after the first 3000 hours with a setpoint temperature of 75 deg C seems to be somewhat optimistic.
There is no burn in mentioned in data sheet for the ageing drift.

When I take my -44 ppm/kHr now at 124 deg C average for ADR#02 and translate it to 55 deg C setpoint temperature.
(factor 2^7 = 128 when doubling all 10 deg C)  = -0.34 ppm/kHr at 55 deg C.
This gives still -1 ppm/year.

with best regards

Andreas

[branadic]

I guess you have to think different. The datasheet states 0.5 ppm/year after 3000 h, not sure if this is with or without burn in, but running the oven at 75°C. It excludes the first 3000 h of stabilizing, that's at least how I interprete things.
So going from here the drift of my sample was far less than 1ppm within the last 1000 hours and from here we need to observe how the drift goes on for the next 7760 h (~11 months). And yes, I'm running my oven at 55°C which is the zero t.c. point of this sample. This means, stabilizing will probably take a bit longer than 3000 h @ 75°C oven temperature, but lets assume it reached its stable point already. We will know more after the next 1000 h, I guess.

-branadic-

[Kleinstein]

The doubling every 10 K is only a rough approximation. There are some effects (especially slow ones) with a stronger temperature dependence.

Similar for the drift over time there are several possible functions: exponential for simple relaxation processes and square root time for more random walk type processes (or superposition of many uncorrelated random effects) are just 2 examples. Another possible form would be a stretched exponential. So extrapolation is somewhat tricky.

For the high temperature part, I see mainly 3 effects to happen: fully finish the chemicial curing of die attach and possible stabilization (e.g. build up of a surface film, surface "reconstruction" ) on the chip. There may be a tiny bit of diffusion at the contacts, but hardly in the silicon itself.
The stress between the die and case should relax as the die attach would be relatively soft.
A later step would than include relaxation of thermal stress from the different thermal expansion of the silicon and the metal case. This would be near the final use temerature as the stress only comes back on cooling. The die attach can also have some structural relaxation when well below a glass temperature. The 150 C are more like too high for this, more like near the glass temperature where the structure is more like in a local equilibrium.

If I anderstand right the data for the high temperature anneal are for the simplified circuit, and this may show additional drift from the resistors and the FET.

The exponential fit to the data from brandic should give around 0.5 ppm/kh (5 µV for 1000 h) after 3000-4000 hours. Part of the drift could also be from the 6.6 to 10 V stage.  So this would not be so far off from the DS numbers.

[Neo2199]

Hi all, I am planning on building my first voltage reference. I have hard time getting standard LTZ1000 but I managed to order one ADR1000 from Analog Devices site which should perform similarly. It is quite hard to figure out good resistor values and set the operating point right where good long term stability is. But based on the info in this and other similar threads I think I have puzzled everything together. My target is 60C, 5mA zener current at 100uA collector current. I came up with R4 12k, R5 1k, R2 61.9k, R3 61.9k, R1 100ohm. Did I get this right, or should I change something?
I am also planning on designing my PCB, but I cannot find too much information about layout recommendation. It is interesting to me to see so many differences in layout of KX LTZ1000, reference Analog Design board and HP3458A LTZ1000 board for example.

[Andreas]

I have hard time getting standard LTZ1000 but I managed to order one ADR1000 from Analog Devices site which should perform similarly.

I came up with R4 12k, R5 1k, R2 61.9k, R3 61.9k, R1 100ohm. Did I get this right, or should I change something?

I am also planning on designing my PCB, but I cannot find too much information about layout recommendation.

Hello,

Each single device behaves different regarding ageing.
So you might want to have at least 3-4 stable references to sort out those with the largest ageing.
Whether ADR1000 behaves similar to the LTZ1000 still has to be proven in practice.
At least it seems that the ADR1000 needs longer time to settle than a typical LTZ1000.

The resistor values are those recommended from the data sheet so it should be ok to go with them.
Others try to set the temperature setpoint to the zero T.C. temperature of the ADR1000 to reduce influence from set poin divider resistor drift.
(this is not possible with LTZ1000 due to the higher zener voltage).

For layout it is essential to do star point wiring and  especially kelvin sensing of the zener voltage.
(no ground currents from heater or OP-Amps across the ground sensing line).
When you have to integrate the zener in a device like 3458A it might be partially easier to work with ground current compensation.
So the constraints (dual/single supply) and pre-ageing and selecting on device / module level may give different board layouts.

with best regards

Andreas

[Andreas]

Update on Burn-In of ADR#02 after ~1.2 kHr total.

compare with:
https://www.eevblog.com/forum/metrology/lowest-drift-lowest-noise-voltage-reference/msg3942472/#msg3942472

After last reporting I reverted back to 110-130 deg C cycling hoping that the ageing slope would flatten.
But it increased instead from -44 ppm/sqrt(kHr) to -84 ppm/sqrt(khr) calculated for 120 deg C.

I wanted further reduce the temperature cycling to 100-120 deg and further steps down to see wether there is some hysteresis.
But unfortunately the controlling PC stopped working so I have to stop the bun in heating for a while.

with best regards

Andreas

[branadic]

Others try to set the temperature setpoint to the zero T.C. temperature of the ADR1000 to reduce influence from set poin divider resistor drift.
(this is not possible with LTZ1000 due to the higher zener voltage).

Why you think this is not possible for the LTZ1000? I did it here.

-branadic-

[Kleinstein]

The extra resistor in sereis with the zener diode mainliy shifts the problem to different resistors. One gets a reduced temperature effect, but quite some effect of the new resistor and also the 100-120 ohms resistor to set the current gets more important.
It can still help, as it reduces external effects on the oven part.

With the ADR1000 the zero TC point is much more accessible, but chances are the exact point depends on individual units. Here it is accessible by adjusting the zener current, set temperature and transistor currents with reasonable values.
The LTZ1000 would need the series resistor or very low currents for the Zener and/or transistors.

There is no need to get the unheated TC very low, but it helps to start in the +-5 ppm/K range instead of the 50 ppm/K like with the LTZ1000.

I don't think the ADR1000 circuit would need the 400 K (R9) resistor for compensation of residual TC (with heaster).

[branadic]

I for myself prefer real experience than guesswork for LTZ but also ADR.

@ Andreas
I can only encourage you to go on. Attached is the drift of my ADR reference, now after half a year and without any burn-in or pre-aging prior to the build and lower oven temperature (55 °C) compared to the datasheet (75 °C), but still at the reference's zero t.c. point. As you can extract from the diagram the reference has really stabilzed, which took a little longer than 3000 h, more like 3500 h, but again I'm running it at a lower oven temperature.

-branadic-

[Andreas]

After having adjusted the T.C. on my ADR#01 (which has no burn-in),
I started ageing drift measurement against 3 instruments (HP34401A, K2000 and ADC27).
From time to time I also do a comparison of the instruments against my most stable reference LTZ#4.

Unfortunately the K2000 shows nearly a +1 ppm Jump between day 8-10 of the measurement due to popcorn noise.
(this jump shows also up on the LTZ#4 measurement).

And also ADR#1 had a -0.6 ppm jump on day 5.
After 500 hours we have a ~-1.5 ppm drift compared to ADC27 and HP34401A.

with best regards

Andreas

[Neo2199]

So I have started with my schematic for my ADR1000 reference. I am wondering if my design will work as expected. Any recommendations?
For the Vishay precision resistors I went with what is available on digikey at the moment.
The opamps are temporary, but I am thinking of using ADA4522.

[Dr. Frank]

Sorry to say so, but these are too many bugs, and an unreadable schematic.

Why don't you simply use any of the working schematics/designs/layouts from the LTZ1000 thread?

Frank

[Neo2199]

Hmm, that's not great. I can tune schematic for readability, it is my first draft, but I won't learn much if I just reuse someone else schematic. Point of this project for me is to learn how to do good/decent design.
It is also true I was basing my design on xDevs design coupled with Marco Reps LTZmu rev 0.2 WIP schematic.

[Kleinstein]

The AZ OPs likely do more harm than good with a LTZ1000 / ADR1000 refrence due to the current spikes and possibly also the startup behaviour. There is nothing wrong with the LT1013.  The ADA4084 suggested in the ADR1000 datasheet may be a bit questionable.

Otherwise the circuit looks like the standard circuit (except for C2 that may be a bit on the high side).

There is no need to have the exact values for the 62 K collector resistors - some 50 K should be OK as well. Using 2 high value resistors in parallel is not such a good idea, as the higher values resistors tend to drift a little more.

The ADR1000 is even less sensitive to the resistor quality than the LTZ1000. So there is no real need to use highest grade resistors at all places.

For the ouput amplifier one should have some fltering action between the reference and OP if an AZ OP is used.  The resistor series strings with multiple 0 Ohms may not be a good idea here, as more junctions can add thermal EMF. Here a parallel trim for fine adjustment may be the better choice. For a 2:1 resistor ratio, I would consider a resistor network (e.g. LT5400 or ORN ), at least as on option.

[Neo2199]

Thank you for the advice. All makes sense to me. I will work on updates.
This design is very hard mainly because of all the parasitics that will make all the difference.