Electronics > Metrology

Lowest drift, lowest noise voltage reference (ADR1000AHZ)

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chekhov:
One more observation. In order to proceed with TC finding step in the article, my first step was to measure the Vbe of Q2 versus temperature in order to get an idea of how exactly it changes, maybe starting point at 0 or 25C varies from chip to chip - this way I wanted to confirm values in datasheet vs temperature. This makes sense since datasheet obviously has misprints like that (so who know what else is wrong here ...):

However, the obvious proper way of measuring it is to have only temperature sensing transistor to be turned on, otherwise zener without heater still has quite major self-heating (around ~5C, comparing to ~10C for LTZ1000A). Here is my Vbe sweep in thermal chamber. Tiny PT1000 was sitting on the top of ADR1000A and heating and then cooling was very slow (so that I haven't finished cooling to 25C):

According to my graph, at 25C I have ~598.8 mV, and at 70C I have ~496.1 mV, almost 12mV higher than 484mV for 70C from datasheet (gives me -0.00228(2) V/C coefficient for Vbe). I wonder whether they were measuring them with zener turned on or zener turned off.
Additionally, all my sweeps that I posted previously always had time for self-heating to settle, and usual Vbe value is around 574-576 mV, which from will give around 35C internal temperature for ~25C outside.
(In my case chip is socketed with original long legs, so that helps a little bit with self-heating as well)

chuckb:
Nice test! How was Q2 configured during the test? Was the Base current similar to nominal, 300na?

In the LTZ datasheet there is the statement -
"Production variations in emitter-base voltage will typically cause about ±10°C variation."
I expect it would be the same for the ADR1000. That is why I use another technique to set the chip temperature.

The other way is to operate the chip in it's final configuration and note the heater voltage. I use cheap resistors (1% MF) initially to set the temperature and then note the heater voltage on pin 1 of the Zener chip. For the ADR1000, 4V on pin 1 (0.7v on pin 2) indicates a 10 deg C heater operating margin. With 5V on Pin 1 you would have a 16C operating margin. Select R5 for your desired operating margin. Then order and install the good reisitors. I really don't care about chip temperature, I care about heater operating margin in operation. I try to minimize the margin for best drift charactoristics.

You can decide your own operating margin vs drift rate tradeoff.

chekhov:

--- Quote from: chuckb on May 20, 2024, 07:54:58 pm ---Nice test! How was Q2 configured during the test? Was the Base current similar to nominal, 300na?

--- End quote ---
Base and collector of Q2 were connected together and fed 6.62 V through 61.9K resistor (just from PSU, so there was little variation, but upfront I saw ~0.2 mV Vbe change for 0.05V change on PSU), so looks like 1/3 of this 'nominal'.

Dr. Frank:
Here's the drift update of the ADR1000's raw reference voltage.



After 2 1/2 years, it has stabilized to about -1ppm/year.
 
I made the power interruptions more visible in this graph.
I still have not measured / analysed, how much time it takes to reach the previous reference value, within 0.5ppm. 
Seems to be on the order of 2 weeks, depending on the off time.

Frank

The Soulman:

--- Quote from: Dr. Frank on May 26, 2024, 09:36:27 am ---Here's the drift update of the ADR1000's raw reference voltage.



After 2 1/2 years, it has stabilized to about -1ppm/year.
 
I made the power interruptions more visible in this graph.
I still have not measured / analysed, how much time it takes to reach the previous reference value, within 0.5ppm. 
Seems to be on the order of 2 weeks, depending on the off time.

Frank

--- End quote ---

Do you have a explanation for the long time ( 24+ hours ) it took to settle back close to its original value on 31-11-23?
Is the zener current circuit on your board somehow sensitive to moisture? Or the ADR1000 has a leak?  :-//
Or simply temperature related?

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