Author Topic: MAX6226 voltage reference  (Read 11608 times)

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Online Kleinstein

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Re: MAX6226 voltage reference
« Reply #50 on: August 24, 2021, 07:56:19 am »
There is quite some correlation between the references. Unless there is a prblem with the supply, chances are this could be an effect of the meter or maybe thermal EMF from the cables. What meter is used for the measurement ?  Looks like it has a good ref. with low popcorn noise.

The dead bug mounted refs look good.
 
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Offline exe

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Re: MAX6226 voltage reference
« Reply #51 on: August 24, 2021, 07:49:52 pm »
The meter is K2000 which was probably re-capped by previous owner(s). K2000's manual says it has 1ppm/C coefficient on 10V .
I too noticed correlation, and I don't know if it because of the meter, because all references exhibit similar tempco :). Or may be leads are not very good.

As of IC1 and IC2, those were subject to freezing well below zero (I used a freeze spray), then hotair, then I physically knocked it with quite some force (I wanted to see how sturdy is ceramic carrier), they were also resoldered at least once... May be that's why IC2 is not doing well :).

I kept the lab with closed window over night, so  there is not much temperature and humidity variation. Tomorrow I'll probably leave the window open to see how that will affect measurements. The downside is, this can create drafts.

After about a week or so I plan to load one or two references with a 2.4K resistor (~1.04mA current) to see load regulation.
 

Offline Andreas

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Re: MAX6226 voltage reference
« Reply #52 on: August 24, 2021, 08:01:58 pm »
K2000's manual says it has 1ppm/C coefficient on 10V .

Hello,
where exactly do you read this?

I read:
for 18-28 deg C the tempco is within uncertainity specs.
outside of 18-28 deg C the TC is 2 ppm of reading/K (2,5V in your case = 5 uV/K) + 1 ppm of range/K (10V = 10uV/K) or a total of up to 15 uV/K for this measurement.
I fear you will have to do some T.C. measurements of the instrument first.
See:
https://www.eevblog.com/forum/metrology/project-pimp-a-keithley-2000/msg1106829/#msg1106829

with best regards

Andreas
 

Offline exe

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Re: MAX6226 voltage reference
« Reply #53 on: August 25, 2021, 07:07:08 pm »
2Andreas: oh, right. There is no spec for it. Well, then I can't really say anything about precision of this instrument. Even if I find a cal lab (probably I'll have to send it to Germany as local labs turned me away because I'm not a company), I guess they charge me extra for climate chamber.

 

Offline exe

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Re: MAX6226 voltage reference
« Reply #54 on: August 30, 2021, 07:58:16 pm »
Current state of references.
 

Offline exe

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Re: MAX6226 voltage reference
« Reply #55 on: September 17, 2021, 09:55:41 am »
Currently logged data. I averaged all values with a sliding window of 1000 measurements. It's still hard to see individual voltages, so I'll re-plot each reference individually.

On Sept 6 I loaded ic1 ic3 ic6 and ic8 with 2.4k resistors (~1.04mA). Vertical black dashed line indicates that moment.

On Sep  1 05:37:45 PM (UTC) there was a 30s power outage in my lab, but I don't see any impact on measurements.

IC2 is a wreck :).
 

Offline exe

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Re: MAX6226 voltage reference
« Reply #56 on: October 17, 2021, 09:01:55 am »
I'm attaching latest data (extremely averaged so the trend is clear). There is a 5-day gap because script crashed.

From what I see, long-term drift is in spec ("max 11ppm/1000hr Long-Term Drift"). Tempco looks fine too.

I'd say it's good-enough for 4.5digit or may be even 5.5digit portable instruments if environment conditions are stable.

What should I do with it next? Shall I switch it off for one day and then on again to see if there is any hysteresis?
 

Offline Andreas

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Re: MAX6226 voltage reference
« Reply #57 on: October 17, 2021, 06:45:07 pm »
What should I do with it next? Shall I switch it off for one day and then on again to see if there is any hysteresis?
Hello,

if it is related to humidity one day might be not sufficient.
(Its more like 1-2 weeks off time needed here).

I had one reference (ISL21009) which had after a short power interruption (minutes) a nearly 300 uV offset which needed several days to settle again.

with best regards

Andreas
« Last Edit: October 17, 2021, 06:46:46 pm by Andreas »
 
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Offline exe

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Re: MAX6226 voltage reference
« Reply #58 on: November 10, 2021, 08:36:40 pm »
I unplugged my board for two weeks, then plugged again. The DMM was off as well, but I let it warm up for two days before starting measurements.

I don't see any significant drift except ic2.

One thing I was curious is how bad my cables are. So, when the board was unpowered, it was showing 0.47-0.48mV voltage on output. Touching/moving cables would cause -100uV change, with recovery to within 10uV or so within some tens of second. Not sure if this observation means anything other than my cables suck. 10uV is pretty large value comparing to drift of references themselves.
 

Offline exe

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Re: MAX6226 voltage reference
« Reply #59 on: November 24, 2021, 06:30:54 pm »
Latest data. Periods with less disturbance are when I was absent. I don't know what happened to ic2. It seems it recovered and doesn't suffer excessive drift anymore  :-// .
 

Offline dietert1

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Re: MAX6226 voltage reference
« Reply #60 on: November 24, 2021, 07:12:18 pm »
After three months of operation one could try correlation plots of each channel agains temperature in order to derive a TC, e.g. for the last data taking period. The resulting TC values will contain a common contribution from the DVM you use. If you then plot the residuals after numerically subtracting the average TC effect for each reference, unexpected changes will be much easier to pin down. I'd guess all your references together can make a sub-ppm voltage standard.

Regards, Dieter
 
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Offline exe

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Re: MAX6226 voltage reference
« Reply #61 on: November 24, 2021, 09:42:06 pm »
My friends, on the latest plot I missed that ic1 went nuts. This is weird. I only see the jump correlates with drop of humidity. May be hermetic sealing is broken or something... Idk, 8 ppm sudden change is very suspicions, considering other references don't behave like that.
 

Offline b_force

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Re: MAX6226 voltage reference
« Reply #62 on: April 13, 2022, 12:22:12 pm »
Bit of an older topic, but I was just looking for some kind of affordable voltage reference myself.
There doesn't seem to be a whole lot of information shared about these things unfortunately. :(

I would still like to add some important points to the discussion, especially on the first couple of comments.
It all depends if you're looking for the best possible vs worst case scenario.

Unless I can really specify something, I would go for the last one.
Looking at the long term stability, this will eventually lead to say something like 15-20ppm or so.
Which is an equivalent of 0.002%.
Some devices seem to have an asymptote here, while others just seem to drift forever?

So instead of trying to bring that number down, one can also just use that number as a starting point.
Everything less will be just better in performance.
Obviously it depends on your application, but 0.022% total error is pretty good or very good in most cases.

Although there are two questions from my side;

1 - It seems a little unclear what they sometimes mean by the max accuracy?

According to this article; https://e2e.ti.com/blogs_/archives/b/precisionhub/posts/ic-long-term-stability-the-only-constant-is-change
They see it the rated accuracy as the max accuracy after x many years (see end of the article). While others sometimes seem to use this term as the initial value?
So in the case of the MAX6226, this 0.02% is worst case after many years.

2 - I assume that the long-term stability rating counts for when the device is powered on? Or is there also some natural drifting, even when the device is not being used at all? Certain epoxies and other bonds age over time as well for example.
« Last Edit: April 13, 2022, 12:25:56 pm by b_force »
 

Offline exe

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Re: MAX6226 voltage reference
« Reply #63 on: April 18, 2022, 09:47:06 am »
Hi b_force!

Designing around worst-case scenario is a safe bet, but more often than not leads to unsatisfying guaranteed performance. This is why I made this experiment - to check what I can get in practice. As of ageing, its a good question. I contacted their support for that.
 

Offline Andreas

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Re: MAX6226 voltage reference
« Reply #64 on: April 18, 2022, 01:50:03 pm »
As of ageing, its a good question. I contacted their support for that.

Hello,

please let us know if you get a better/more detailed answer than the typical 11 ppm/1000hrs from the data sheet.

But keep in mind: this ageing drift is done under lab conditions (25 deg C constant over time).
So practically you might have different conditions.
An in my ageing experiment I see sometimes sudden drift steps which I cannot explain.

with best regards

Andreas
 

Offline b_force

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Re: MAX6226 voltage reference
« Reply #65 on: April 20, 2022, 01:22:03 pm »
Hi b_force!

As of ageing, its a good question. I contacted their support for that.
Extremely curious about this, please keep us updated!

Offline b_force

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Re: MAX6226 voltage reference
« Reply #66 on: April 20, 2022, 01:24:30 pm »
As of ageing, its a good question. I contacted their support for that.

Hello,

please let us know if you get a better/more detailed answer than the typical 11 ppm/1000hrs from the data sheet.

But keep in mind: this ageing drift is done under lab conditions (25 deg C constant over time).
So practically you might have different conditions.
An in my ageing experiment I see sometimes sudden drift steps which I cannot explain.

with best regards

Andreas
I was also thinking more in the practical sense of aging.

What happens when those IC's are in storage for a long time, do they need to be kept at constant temperature/humidity level etc etc?
I can't find any information about it with actual data, only anecdotal and personal experiences.

Offline b_force

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Re: MAX6226 voltage reference
« Reply #67 on: April 25, 2022, 11:47:11 am »
Well, an update from my side, since I had to ask some other things anyway'

Quote
1) 0.02% initial accuracy is the worst case scenario one can expect.
 
2) Yes, the stability counts when the device is powered On
 
3) No the device does not need to be stored in a certain way


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