Electronics > Metrology

Lowest drift, lowest noise voltage reference (ADR1000AHZ)

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Andreas:

--- Quote from: branadic on January 16, 2022, 08:41:41 am ---I almost hit 4000 h on my sample, with the setup untouched for the whole time. Attached is an updated graph.

--- End quote ---

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:

--- Quote from: Neo2199 on January 30, 2022, 04:23:01 am ---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.

--- End quote ---

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

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