Lowest drift, lowest noise voltage reference (ADR1000AHZ)

[branadic]

If I got Eric right he said:
"...First I believe Jim Williams used a similar arrangement to make the LT1088 rms to dc converter, but also when you start to look at the stability of any thermal feedback system the transit delay from the heater to the sensor plays a big role in determining the effective loop phase margin of that loop. So in theory a trade-off exists between the thermal gradient and the thermal stability, that is the closer the rings are to the core the less phase delay is introduced in the feedback, but that would cause a larger gradient between Q2, the zener and Q1. So the fact that this was a kit part makes me think that both, Jim and Carl were using this for the wrong purpose, Jim was using it to create an rms to dc converter, so he just wanted the thing to run fast, so he wasn't using the zener, he was using the temp sensor and the heater. However Carl had to be very concerned with this phase delay effect and also gradients. If you move the heater infinitely far from the sensor what you get is basically a bang bang controller. The heater heats up, it takes to long for the sensor to pick it up and consequently the control loop bangs back and forth inbetween the rails..."

What I hear out of that "kit part" is, both Jim and Carl basically used the same set of masks to create their individual solutions, Jim his thermal converter and Carl his heated zener reference. So that could explain why there are these additional heater rings, we again find in the LT1088.

Answer 2 seems wrong. In the zener mode the tempco would move into the negative area, wouldn´t it?

It's always a mixture of Zener and Avalanche effect (breakdown voltage <4V is kind of a pure zener effect, while with breakdown voltage > 6V the Avalanche is dominant). So decreasing the breakdown voltage by accident, as Eric said by "pure luck", its a little more of a Zener than Avalanche effect.

-branadic-

[wutieru]

Test board：

[wutieru]

Test whole night and datas:

[wutieru]

1000 sample data

[Noopy]

It looks like the light is more uniform than in the LTZ1000. Perhaps here we see one reason for the lower noise figure? A more uniform structure in the buried zener?

In the session, I posed the question to Eric Modia, how they achieved these 50% noise level compared to the LTZ.
His answers were
1. "Pure luck"
2. "Due to the lower reference voltage, the zener structure of the ADR1000 operates in the Zener effect mode, compared to the LTZ which operates in the more noisy Avalanche mode"

Answer 2 seems wrong. In the zener mode the tempco would move into the negative area, wouldn´t it?

Hi Noopy,
please carefully read my question. Here I reproduced his answer concerning the noise, but not about the T.C.!
Or what's your point?

The T.C. of the zener diode is indeed depending on its absolute value, or in other words, if it's in either zener or Avalanche mode, I guess.
The trick with LTFLU, SZA263 and ADR1000 (unintentionally?) is, to inversely match its T.C. with the -2mV/K of the transistor BE diode.
Therefore you have a limited choice of zener voltages which will do the job.

You asked him about noise and for that question his answer is ok. Avalanche breakdown is a rude process.
I was wondering if that wouldn´t destroy the T.C. compensation. The transistor gives you the negative T.C. and the zener has to supply you with a positive T.C. otherwise you won´t get near zero T.C.
Moving the zener diode from avalanche to zener breakdown moves its T.C. from positive to negative. Because of that I was wondering if that is even possible. But probably branadic´s point is important: In this range it´s a mixture of avalanche and zener and probably the lower voltage still gives positive T.C.
Of course in the LTZ1000 and the ADR1000 the T.C. compensation of zener and transistor isn´t as important as in other references but in my view it´s not irrelevant.

If I got Eric right he said:
"...First I believe Jim Williams used a similar arrangement to make the LT1088 rms to dc converter, but also when you start to look at the stability of any thermal feedback system the transit delay from the heater to the sensor plays a big role in determining the effective loop phase margin of that loop. So in theory a trade-off exists between the thermal gradient and the thermal stability, that is the closer the rings are to the core the less phase delay is introduced in the feedback, but that would cause a larger gradient between Q2, the zener and Q1. So the fact that this was a kit part makes me think that both, Jim and Carl were using this for the wrong purpose, Jim was using it to create an rms to dc converter, so he just wanted the thing to run fast, so he wasn't using the zener, he was using the temp sensor and the heater. However Carl had to be very concerned with this phase delay effect and also gradients. If you move the heater infinitely far from the sensor what you get is basically a bang bang controller. The heater heats up, it takes to long for the sensor to pick it up and consequently the control loop bangs back and forth inbetween the rails..."

What I hear out of that "kit part" is, both Jim and Carl basically used the same set of masks to create their individual solutions, Jim his thermal converter and Carl his heated zener reference. So that could explain why there are these additional heater rings, we again find in the LT1088.

OK, it´s just the LT1088 thing.
But that´s an interesting background story. 


Thanks Dr. Frank!
Thanks branadic!

[miro123]

1000 sample data

Thanks for sharing. Do you have some pictures of full assembly?

[Kleinstein]

Reading  an external reference directly with a DMM will give the combined noise / drift of the DUT and the meter internal reference. With the ADR1000 based reference chances are most of the noise and a significant part of the drift would be from the meter.

[branadic]

With the ADR1000 based reference chances are most of the noise and a significant part of the drift would be from the meter.

I disagree to some extend. I'm observing my ADR1000 for 2 months now by directly reading it with a R6581D, differential measurement with a K2182A against my F7000-1 and differential measurement of the F7000-1 against F7000-2 with a second K2182A. Consequently, I take readings for an hour each morning. You can clearly see that the DMM can detect the very same amount of drift that is also visible in the differential measurement.
The ADR reference is not pre-aged, neither by an 168 h bake&burn @150 °C that was mentioned by Eric Modica nor a cycling receipe as proposed by Cern for the LTZ.

-branadic-

[Andreas]

Test whole night and datas:

Hello,

- which (model of) instrument did you use?
- do you have any clue where the negative -3 ppm spikes are from?

with best regards

Andreas

[syau]

Test board：

Did you leave any gap between the ADR1000A and the PCB ? Also, what are the value of those film resistors (1K/13K, 120, 70K x 2) ?

I am think of  making up a test board based on Dr. Frank’s PCB using ww resistors.

[wutieru]

I use HP 3458A for testing.
and the -3 ppm spike maybe come from Vref Pin current ，I made a mistake when wiring.

[wutieru]

I use 1K:12.5K / 70K /70K/ 100R for testing.

[iMo]

OT: @wutieru: I downloaded the MeterKnife Lite (as in your picture above) - is there an english version available too?

[wutieru]

not only the software but also a hardware named "metercare" you needed. this is a DIY GPIB device by funs.

[syau]

I use 1K:12.5K / 70K /70K/ 100R for testing.

May I know if you leave any gap between the PCB and the ADR1000A ? If so, what is the distance between the PCB and the ADR1000A ?

[syau]

My version:

ADR1000AHZ



LTZ1000ACH



Those WW resistors were result from a group buy in this forum. Will burn in for few days before taking measurement.

[Noopy]

I found a ADR1000 with datecode 2108. 

Wanna see what´s inside of this batch? 

[wutieru]

Quick start to stable state，only need 30 seconds.

[Andreas]

Quick start to stable state，only need 30 seconds.

Hello,

you should zoom more in (into the ppm level)
The heater voltage on my scope still changes after 7-8 minutes.

with best regards

Andreas

[Kleinstein]

The heater power can change slowly even if the temperature is stable. It is normal that this takes longer to stabilize as the whole PCB heats up a little and not just the refrence chip.  The point to look for the actual temperature would be the voltage over R1 (the resistor setting the Zener current).
The chip is small and the temperatur regultion can thus be quite fast.

The longer part is settling in the reference, e.g. thermal stress between the die an the case. This what can cause hysteresis and slow settling over weeks and more. Epoxy glue and similar polymeres can also show internal relaxation in the structure. Above or near the glass temperature the internal structure reaches a kind of equilibrium (temperature dependent) fast (e.g. a few seconds at the glass temperature). At lower temperature this gets slower (like double the time every 3 to 10 K). So with more than some 50-100 K below the glass temperature the structure tends to lag behind the equilibrium.

[Noopy]

I stripped the layers of the ADR1000.
Here you can see how I proceeded: https://www.eevblog.com/forum/projects/decapping-and-chip-documentation-howto/msg3733501/#msg3733501




The Linear Technology Application Note 83 shows how a buried zener can look like.
Breakdown occurs between the n+ emitter doping and the p+ isolation sinker.
In my view the picture is not ideal. Under the n+ doping there is no p- but a p+ due to the addition of the isolation sinker doping and the base doping. Furthermore the doping decreases to the edges of the sinker. Only because of that the breakdown occurs in the middle of the bottom of the n+ emitter doping.




In my view the ADR1000 is built like this:
The blue temperature sensing transistors are easy to analyse. There is a buried n+ collector connector. In the outer area there is a sinker connecting the buried collector connector. On top of the collector there is a base area in which you can spot the emitter area. 
The collector of the green inner transistor is connected by four long buried n+ collector connectors. The emitter area is the circle in the middle of the die. The base is a wide p doped circle between the collector and the emitter.
The base circle is connected to the anode of the zener too. Different to the AN the anode is smaller than the cathode. We can be sure about that because the wider ring is connected to the metal layer that carries the cathode potential.






Cutting through the die I assume the structures look like this.


https://www.richis-lab.de/REF19.htm

 

[magic]

Mostly the same kind of structure I came up with.
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg3078768/#msg3078768

One difference: if the connection from pin 4 to the anode/base element were made at the surface, there would be no need to join this element with the P isolation wall and the substrate. I speculate that there is a gap between the anode/base and the P isolation wall, and that the deep anode diffusion is not isolated from the substrate except for those four buried Q1 collector links and that's where the connection to pin 4 lies.

The Linear Technology Application Note 83 shows how a buried zener can look like.
Breakdown occurs between the n+ emitter doping and the p+ isolation sinker.
In my view the picture is not ideal. Under the n+ doping there is no p- but a p+ due to the addition of the isolation sinker doping and the base doping. Furthermore the doping decreases to the edges of the sinker. Only because of that the breakdown occurs in the middle of the bottom of the n+ emitter doping.

I agree, that drawing seems inaccurate.

[Noopy]

if the connection from pin 4 to the anode/base element were made at the surface, there would be no need to join this element with the P isolation wall and the substrate. I speculate that there is a gap between the anode/base and the P isolation wall, and that the deep anode diffusion is not isolated from the substrate except for those four buried Q1 collector links and that's where the connection to pin 4 lies.

I agree with you that this configuration would be possible and would provide us with a Kelvin connection but I don´t see a gap... 

[Noopy]

With the input of magic and a second look at the pictures I would like to change the picture a litte.

[Noopy]

i am guessing i got the pin numbers right?
pads A and B, any guesses what are the uses? a special ADR with 10 pins?
there is a square with 25 dots, is that some kind of via?

Let´s take a closer look at the circuit. There are two additional bondpads leading to the reference circuit.








With the two metal layers the circuit is a little harder to read...




The active parts are the same as in the LTZ1000 but here we have the possibility of a 4-wire-connection! VREF and GND are connected a second time directly at the zener-transistor-combo so the bias current and the traces conducting the current don´t interfere with the reference voltage. The sense traces are connected to the two unused bondpads.