LTC6655B long term drift

[alex-sh]

You clearly can see the difference between a plastic device and the hermetically ones
even when the box is heated up every day for about 1.5 hours.

This pretty much sums it up.
OK, TO/8-CLCC package is now on. 

[SvanGool]

...
Sadly, LT1021 in H package (metal can) has been discontinued - this is what digikey is telling me.
...

The LT1021CMH-10 is available, years already, at a French ebay supplier, eBay auction: #ci-LT1021CMH-10-883-LT-1021-CM-H-10-883-LT1021CMH-10-F10/191123710347.
Alex Nikitin did quite some experiments with these in the past, look for his postings: https://www.eevblog.com/forum/metrology/lt1021cmh-based-mini-reference/msg1125519/#msg1125519

[alex-sh]

2nd best LT1236DCLS8-5

The only one LT1236 available in LS8 package is LT1236AILS8-5#PBF at digikey.
You mentioned  LT1236DCLS8-5. What's the difference between the two? I cannot find any reference to LT1236D in the datasheet.

Another question is that I have a power supply of +5V (USB or battery) and I cannot change that. So it looks like I have to either stick to 4.096v reference or use a charge pump to operate 5V Reference from 5V Supply. Alternatively, I can have a severate power (2xLiPO = 7.4V) for LT1236AILS8-5 which is not ideal either. What are your thoughts?



Thanks
Alex

[alex-sh]

...
Sadly, LT1021 in H package (metal can) has been discontinued - this is what digikey is telling me.
...

The LT1021CMH-10 is available, years already, at a French ebay supplier, eBay auction: #https://www.ebay.com/itm/ci-LT1021CMH-10-883-lt-1021-cm-H-10-883-LT1021CMH-10-F10/202204283094.
Alex Nikitin did quite some experiments with these in the past, look for his postings: https://www.eevblog.com/forum/metrology/lt1021cmh-based-mini-reference/msg1125519/#msg1125519

Thank you.
This is the corrected link to eBay LT1021CMH-10 - https://www.ebay.com/itm/ci-LT1021CMH-10-883-LT-1021-CM-H-10-883-LT1021CMH-10-F10/191123710347?hash=item2c7fdc618b:g:OHIAAMXQuTNTPBSE

[alex-sh]

I just love these statements in the datasheet:

The only way long-term drift can be determined is to measure it over the time interval of interest.

[wraper]

I just love these statements in the datasheet:

The only way long-term drift can be determined is to measure it over the time interval of interest.

I love how you took it out of context 

Long-Term Drift
Long-term drift cannot be extrapolated from accelerated
high temperature testing. This erroneous technique gives
drift numbers that are wildly optimistic. The only way
long-term drift can be determined is to measure it over
the time interval of interest.

[Andreas]

2nd best LT1236DCLS8-5

The only one LT1236 available in LS8 package is LT1236AILS8-5#PBF at digikey.
You mentioned  LT1236DCLS8-5.

 I can have a severate power (2xLiPO = 7.4V) for LT1236AILS8-5 which is not ideal either. What are your thoughts?

Sorry my fault,
I intermixed the numbering with the LT1027DCLS8-5. (LS8- package of LT1027)
For the LS8-package of the LT1236 I have the  LT1236AILS8-5#PBF.

For the power supply: in any case you need a stabilized power supply voltage to keep self heating constant.
And if I look at the PSRR of my measured sample I would choose the supply minimum at 9V or better at 14-15 V.

with best regards

Andreas

EDIT: USB spec means: voltage can be as low as 4.3V so don't calculate with 5V from a USB port.

[alex-sh]

I love how you took it out of context 

I do not see reading this statement out of context. I do not think anyone would ideally want to spend 3000h to determine the long term drift.

[wraper]

I love how you took it out of context 

I do not see reading this statement out of context. I do not think anyone would ideally want to spend 3000h to determine the long term drift.

That statement is relevant to any other voltage reference. Accelerated tests don't show actual long term drift.

[SilverSolder]

What does the accelerated test actually show -  does it give any useful information at all?

[alex-sh]

2nd best LT1236DCLS8-5

The only one LT1236 available in LS8 package is LT1236AILS8-5#PBF at digikey.
You mentioned  LT1236DCLS8-5.

 I can have a severate power (2xLiPO = 7.4V) for LT1236AILS8-5 which is not ideal either. What are your thoughts?

Sorry my fault,
I intermixed the numbering with the LT1027DCLS8-5. (LS8- package of LT1027)
For the LS8-package of the LT1236 I have the  LT1236AILS8-5#PBF.

For the power supply: in any case you need a stabilized power supply voltage to keep self heating constant.
And if I look at the PSRR of my measured sample I would choose the supply minimum at 9V or better at 14-15 V.

with best regards

Andreas

EDIT: USB spec means: voltage can be as low as 4.3V so don't calculate with 5V from a USB port.

OK, in the nutshell I need min 9V power rail to make this vref to work. 

PS LTC6655 is not powered directly from USB and has a number of decoupling caps so I am not concerned about 4.4V < USB < 5.25V.
As I am programming the Cortex-M4 board via USB, it is easy for me to power the whole project via USB for now, but it will be changed to battery power eventually.

[wraper]

What does the accelerated test actually show -  does it give any useful information at all?

To many things it can be quite useful. For example it's not viable to test electromigration for 10 years before releasing the product. In this particular case manufacturer says that particular accelerated test is not applicable.

[alex-sh]

Another vref similar to LT1236AILS8-5 (20ppm/√kHr LT drift) is LT6654LS8 ( 15ppm/√kHr LT drift)
Any experience with this one?

[daqq]

Could conformal coating be used to limit the influence of humidity on plastic references?

[wraper]

Could conformal coating be used to limit the influence of humidity on plastic references?

I don't think it will make any serious effect. Basically any polymer pass some moisture.

[Andreas]

I do not see reading this statement out of context. I do not think anyone would ideally want to spend 3000h to determine the long term drift.

The interesting part of long term drift begins after 5-10 kHrs when the reference begins to settle (or not) and when you begin to see wether you have seasonal changes.

Another vref similar to LT1236AILS8-5 (20ppm/√kHr LT drift) is LT6654LS8 ( 15ppm/√kHr LT drift)
Any experience with this one?

not from my side but: it is a bandgap based reference so typically factor 3-5 more noise like a buried zener and typically significant more ageing drift.
so I would not call it "similar".

with best regards

Andreas

[alex-sh]

Just one reference to long-term drift of LT1236 DIP8 or any other plastic vs LS8.

[iMo]

Why they produce Vrefs in plastic packages then? It is not an Vref anymore, when packaged in the plastic. Frankly, I doubt the cerdip or metal can is so expensive to produce..

[wraper]

Why they produce Vrefs in plastic packages then? It is not an Vref anymore, when packaged in the plastic. Frankly, I doubt the cerdip or metal can is so expensive to produce..

Price, it does the job just fine in many applications. Ceramic is significantly more expensive, not to say ceramic should meet certain requirements to do the job properly.

[Sumesh579]

I would suggest to run the reference continuously for a few days first, then compare the results.

I have seen this reference drifted quite a bit in the first few days then settled to a few ppm level variation within day (mostly due to room temp change).

An extreme case for me was that a 4.096V reference from Maxim output 5 V the first time it powered on with very high noise, then settles to 4.096V specification and ppm level noise in a few days. Mostly due to my soldering.

Awesome Reply!!

[dmm2018]

I built a basic LTC6655BHMS8-1.25 circuit and found out that output voltage kept drifting...
https://www.eevblog.com/forum/projects/1a-current-source-but-still-not-stable/msg2394411/#msg2394411

[alex-sh]

I built a basic LTC6655BHMS8-1.25 circuit and found out that output voltage kept drifting...
https://www.eevblog.com/forum/projects/1a-current-source-but-still-not-stable/msg2394411/#msg2394411

Have you tested it longer than 1 hour?
My LTC6655 does not drift much when powered up, but does drift when powered down.

[Gyro]

My LTC6655 does not drift much when powered up, but does drift when powered down.

That implies that it suffers from hysteresis - there's no way to know if it's drifting while powered down, only power cycling it.

[Andreas]

Hello,

That implies that it suffers from hysteresis - there's no way to know if it's drifting while powered down, only power cycling it.

Hysteresis can be seen here:

https://www.eevblog.com/forum/metrology/ltc6655b-long-term-drift/msg2355873/#msg2355873

The MSOP8 package is worse than the LS8 package.

Besides this: with 5 mA supply current there is also some heating up.

with best regards

Andreas

[alex-sh]

My LTC6655 does not drift much when powered up, but does drift when powered down.

That implies that it suffers from hysteresis - there's no way to know if it's drifting while powered down, only power cycling it.

Let me put it this way - when I leave it running for several days (> 120h) it does not drift. Only when the temp is changing and this can be seen.
Even if I power cycle it, it does not drift. However, when it stays powered down for a week, the voltage value changes.
I really have to start logging it properly like Andreas, but what I can see right now I do not like it. I need to recalibrate the DUT every time the DUT (and vref) powered down for some time and this is just not right.