Lowest drift, lowest noise voltage reference (ADR1000AHZ)

[Andreas]

Update on Burn-In of ADR#02 after ~1.5 kHr total.

compare with:
https://www.eevblog.com/forum/metrology/lowest-drift-lowest-noise-voltage-reference/msg3976202/#msg3976202

After last reporting I reduced cycling to 100-120 deg C to see what happens.
After a interuption due to measurement PC failure the ageing inverted from
-84 ppm/sqrt(khr) to +63 ppm/sqrt(khr)

now I think that the "drift" measurements during burn-in with my simplified burin-in cirquit have nothing to do with a real ageing drift.
Perhaps I measured something like a introduced hysteresis?

I am now cycling down in 10 deg C steps to remove hysteresis
and will see on a proper cirquit if ageing drift / noise is lower than ADR#01.

with best regards

Andreas

[ap]

If I read your numbers and chart correct (and I have not studied each and every page of the whole thread thourougly, so correct me if I am wrong please), your graph shows a drift of 80ppm downwards within 1khr. This is supposedly after the first upward drift within the first couple hundred hours as shown in figure 4 of the ADR datasheet. So direction seems ok. To do a very rough and ballpark figure sanity check, at an average assumed say 125°C and assuming an Arrhenius Q10 factor of 2 (and there may of course be other drift contributors as well such as the circuitry resistor drifts, die attach...; so this is really a very rough check), and related to the 75°C Tset of the data sheet, that would translate to a drift rate at 75C of about 2,5ppm/khr. Assuming Q10 of 2,5, it would translate to below 1ppm. So the initial drift you have seen so far may actually be correct and not related to "hysteresis" as you state.
The drift direction change is a different story, it will be interesting to see how this develops and what your cycling results are.

[Andreas]

Hello,

it is slightly > 1200 hrs for the first -80 ppm drift at high temperatures. (note the x-axis is sqrt(kHrs) so a value 1.1 translates to 1200 hrs).
I am now in the downcycling phase (every 2nd day 10 deg C less) so currently a slope measurement does not make sense.
But up to now the 2nd day measurement is always higher than the 1st day after temperature change. (so still upwards drift).

I plan to set the zero-TC and temperature setpoint to ~55 deg C like on sample #1.
And then we will see (after a few khrs) in comparison what the burn in has done a pre-ageing.

with best regards

Andreas

[CurtisSeizert]

I have built up a couple boards that are similar to the HPM7177 schematic (the main difference from the datasheet schematic is QHTR is on the low side and the U2 feedback network is different, also there is a 20R resistor on the zener cathode) and have done some experimenting. I am trying to get a handle on the best strategy for tempco compensation, and I am hoping someone can correct me if I am mistaken.  Referring to the ADR1000 datasheet schematic component references:

-Adding resistance between the zener cathode and the zener+ sense output in the decreases the tempco by weighting the Vbe tempco of Q1 by a factor of (R1 + R_cathode)/(R1)
-Increasing the collector current of Q1 (by reducing the value of R2) increases the tempco by increasing Vbe and decreasing the absolute value of its tempco (see p.5 of http://web.mit.edu/klund/www/Dphysics.pdf for Vbe tempco dependence on Vbe)

I've attached a noise spectrum for one reference that was burned in at 130 for 1 week using the internal heater.  Zener current is 5.5 mA (90.9R for R1), 2.5 mA Ic for Q from 2.5k R2 (!). R_cathode is 20R.  Tempco ended up being around +12ppm/K for the heater setpoint.  I overshot tempco by dropping R2 because it crossed zero between resistor values that I had; it would have been easier with lower R_cathode, but that would have been a PITA to mess with given the design, so I didn't.  Gain stage is ADA4523-1 with feedback from two SMNZ 2k/1k arrays with trim.

Curtis

[branadic]

Here is an update of my ADR1000 after now 5000 h.

-branadic-

[Andreas]

Update on ageing of ADR#01 after T.C. adjustment and with no burn in.

compare:
https://www.eevblog.com/forum/metrology/lowest-drift-lowest-noise-voltage-reference/msg3995003/#msg3995003

Again ageing drift measurement against 3 instruments (HP34401A, K2000 and ADC27). Now at 1 kHrs
Unfortunately the K2000 shows a lot of popcorn noise so it is the most unstable reading here.

As comparison also from time to time readings on my most stable LTZ#4 reference. (-1 .. -2 ppm/year)
At the end of day 42 it shows that K2000 reading is about 1.7 ppm too high and 34401A reading about 0.5 ppm too high whereas ADC27 is about right.

So the total drift of ADR#01 is -2.2ppm/kHr for the first kHr on the 6.6 V output.

with best regards

Andreas

[Andreas]

Hello,

first measurements on burn in of ADR1000A#03

I decided to do a cycling of 110-130 deg C on this device for minimum 2 weeks.
Up to now (250 hrs) or sqrt(250hrs/1kHr) = 0.5 the average drift is  around -27 ppm/sqrt(kHr).

with best regards

Andreas

[Andreas]

Update on ADR1000A#02

Downcycling finished with no surprise: every 2nd day the output value was still higher as on the 1st day with a -10 deg C step.

I now have set the zero-T.C. of the zener to ~55 deg C to have enough headroom up to 40 deg C environment.
The heater setpoint is also set to ~55 deg C.

Now I have done the first T.C. measurement vs environment temperature with a temperature sensor attached to the top of a metal case.
Average T.C. of the 6.6V buffered output is -30 ppb/K unadjusted on my LTZ1047B PCB.
This is much better than on ADR#01 (-166 ppb) with a different PCB.

On ADR#02 I plan to adjust the T.C. with the (nominal 400K) resistor R9. First guess is 1.5 Meg to achieve ~30ppb/K increase.

with best regards

Andreas

[branadic]

Andreas, read the hints I provided by mail on how to compensate for the remaining t.c., there is no need for this dubious 400 k resistor.

-branadic-

[branadic]

Andreas, I'm a bit confused, here you wrote

6.6V Zener voltage ~     -6 ppb/K
10V Output voltage ~ +20 ppb/K

Now you say:

This is much better than on ADR#01 (-166 ppb) with a different PCB.

So what result is true for your ADR1000A#01? Can you clearify on that?

-branadic-

[Andreas]

Hello branadic,

There are 3 steps in adjustment of the reference:

Step 1: Adjust the zero T.C. of the Ref-Amp of zener and transistor (to 55 deg C in my case). By either changeing current through zener or through collector.
Step 2: Adjust the heater temperature setpoint to the zero T.C. temperature. By influencing the R4/R5 ratio.
Step 3: Adjust the final T.C. of the whole reference cirquit against environment temperature. By either R9 or a copper resistor in series to R1 or shortening the legs.

With shortening the legs I am still unshure. On the A-Version of the LTZ I always got the "right" direction (increasing T.C. by shortening the legs.
 But on the non-A Version of LTZ it went the wrong direction.
The ADR seems to be somewhere in between (at least from thermal resistance).

On ADR#2 I am now after step 2.
So you will have to compare it to the Step 2 results of ADR#1 (and not with the final Step 3 results).

So please compare:
https://www.eevblog.com/forum/metrology/lowest-drift-lowest-noise-voltage-reference/msg3921830/#msg3921830
with:
https://www.eevblog.com/forum/metrology/lowest-drift-lowest-noise-voltage-reference/msg4050361/#msg4050361

now you can speculate about the reason.
Accidently (only 2 different samples).
Or result of the different amplification in the heater control cirquit. (one cirquit with transistor used as diode, so transistor amplification is missing).

with best regards

Andreas

[Kleinstein]

Even without the final TC trim in step 3 the TC is quite small. The 30 ppb/K translate to 0.2 µV/K. For the voltage loop OP this would correspond some 40 µV/K at it's input. The extra gain in the voltage loop should have little effect, as the load to the OP/transistor is essentially constant.
There are many other details that can be different between the 2 ref. versions: resistors TC, details in thermal layout, amount of solder at joints, quality of the adjustments and just random variations in the ref chips.

The residual dift after step 2 looks rather linear and they have different sign. The trim with R9 only works in one direction and it is proportional to the heater current and thus the square root of the heater power - so a nonlinear function, especially in the lower power range that is likely more relevent.

[Andreas]

there is no need for this dubious 400 k resistor.

I fear Kleinstein is right. R9 is useful to compensate upwards.
The copper resistor to compensate downwards.
With 30 ppb/K the resulting non-linearity through R9 is low enough.
But with up to 15 deg C temperature difference in my "lab"
between winter and summer I need to have the T.C. as low as possible.

with best regards

Andreas

[branadic]

Andreas, it just depends were you put the copper resistor in

-branadic-

[Andreas]

Hello,

R9 is already planned on my LTZ1047B PCB design.
So more easily populated instead of scratching traces.

First try with 1.5 Meg showed large overcompensation of the negative T.C. So my formula for LT1000A with 1 Meg compensating 40 ppb does not work here.
I would have tried 3.0 Meg but I had only 3.3 Meg in the drawer.
With 3.3 Meg I get -4.5 ppb/K average resulting T.C.
The large hysteresis is due to the fact that the environment temperature sensing NTC is on the top of the aluminium housing so getting the temperature with some time lag.

And a view to the PCB with the 3 trimming resistors across the low T.C. resistors. (R9 is on the right side of the ADR1000A).

with best regards

Andreas

[Kleinstein]

The effectiveness of the R9 compensation depends on the thermal setup.  No cap on the ref chip means more heater power needed and thus a stronger effect of R9. The LTZ  A version needs less heater power and should thus have less effect of R9.

The curve with the 3.3 M resistor does not look so bad: It looks like the slope is larger with low temperature and the range below some 15 C is likely of little interest in later use.

From the picture it looks like there is quite some air space obove the circuit inside the shielding box. This may lead to turbulent convection and resulting low frequency noise. Some filler at the to lid may help.

[Andreas]

Hello Kleinstein,

for the photo I have removed all the thermal isolation stuff.
Of course the ADR is thermally isolated on both sides of the PCB.
The metal housing is only to equalize the environment temperature a little bit.
And of course the measurements are all made with the final thermal isolation installed.

with best regards

Andreas

[Andreas]

Hello,

further measurements on burn in of ADR1000A#03 now at 500 hrs (sqrt(0.5kHrs))

Cycling is unchanged 110-130 deg C
the average drift increased from  around -27 ppm/sqrt(kHr) to -38 ppm/sqrt(kHr)

with best regards

Andreas

[chuckb]

Rev B of ADR Data Sheet on the ADR1000 page at ADI. Has there been a product page before?

[Andreas]

Hello,

update on burn in of ADR1000A#03 now at 1000 hrs (sqrt(1 kHrs))

Cycling is unchanged 110-130 deg C

the average drift from 500-1000 hrs is now -15.5 ppm/sqrt(khr) in blue
against -38 ppm/sqrt(kHr) for the first 500 hrs in green.

so I think the minimum duration for burn in around 120 deg C is ~500 hrs.

with best regards

Andreas

[branadic]

I've passed the 6000 h mark. Some 2700 h ago I've also added the F7000T to the rear terminals of R6581D, this measurement starts directly after the measurement of ADR1000 connected to the front terminals of R6581D.
Unfortunately, I've messed with the setup about 500 h ago, hence why there is a jump of 1.5 µV visible.

-branadic-

[dietert1]

In a previous post you mentioned an exponential relaxation fit to your log.
Can you give an estimate of the time constant? At which temperature are you running the ADR1000?

Regards, Dieter

[branadic]

At some point in time the model stopped fitting the measurement data and I haven't yet played with other models to get a working prediction model again.
The reference is operating at ~55 °C oven temperature, see also the first post of this thread.

-branadic-

[dietert1]

Yes, your measurements seem to exhibit a linear drift recently.
I think your first mention of 55 °C is on page ten of this thread. I missed that. At which temperature are you running your 7 to 10 V amplifier?

[branadic]

You are right, I mentioned it later directly, but had expected that it is clear where I supposed to operate the oven given the t.c. diagram with the zero t.c. visible at ~55°C.

At which temperature are you running your 7 to 10 V amplifier?

Not sure I got this question correct, but the amplifier is at ambient temperature, no additional heating there.

-branadic-