semiconductor drift

Eww, those amp supplies are *bouncy*... I'd hate to see what happens when you hit it with a reasonable amplitude step (the resonant impedance is 35 ohms), or a sine wave of about, oh, 45.9kHz. Unless those 0.1's have 35 ohms ESR, which seems even more unlikely, but at least they wouldn't resonate much.

Hey, I thought your favorite was 0.33 anyway?...

Tim

You're trying to ask,

"My car pulls slightly to the right on the highway. To people who understand automobiles: what are the physical mechanisms that could do that?"

You already very well know, there are a hundred things in either case which could individually explain it, all of which depend utterly on the internal construction of the system. The design notes of which are unspecified and unavailable, so it's useless to even ask.

And more than likely, it's not even the fault of the system at all, but "did you notice your left arm is shorter than your right?", or, "no wonder, your right tires are balled". Like the resistors others have mentioned.

Come on John, you should know better than to ask such silly questions to a public group. If you just want to give fodder to JT, why don't you e-mail him and save us the trouble.

Tim

-- Seven Transistor Labs Electrical Engineering Consultation Website:

Come on Tim, You're degenerating into Phyllis-class remarks.

John has a drift that potentially portends a failure mechanism, and he's trying to find it.

Indeed there are many possibilities and John is doing what I often do, ask the question before I've fully thought it out myself... then go back and carefully pound the individual components.

John will find the culprit. ...Jim Thompson

Since its common to all channels, I would be thinking about things like change in leakeage current through bypass or filter caps.

CMOS is not all that stable. I might resort to just letting the device sit there and periodically check it over a month or so. My guess is it won't wander much more. ...Jim Thompson

With the input resistors, you've got something closer to 16.6x noise gain on the first amplifier (not really much different, just clarifying). For lack of a better comparison, the LT1028 (which does plot long term stability) drifts ~ 1uV/mo, and has (typ) 1nV/rtHz noise at 10Hz. The LMH6642 has about 400x worse 1/f noise - so it seems plausible that it could be the cause of your drift.

I don't like the looks of Q2 (0.2% of R13), but that would be a span shift, not a zero shift.

Best regards, Spehro Pefhany

Does "shipped" cover transport as air cargo? X-raying can cause drift effects. Admittedly, this is a bit far-fetched because usually doses in the krad range are required to provoke measurable effects.

Just a thought Klaus

I gather L1 and L2 are a pop option. If not, there's your trouble. ;)

Cheers

Phil Hobbs

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fetime drift and did calibration at beginning of life (even temperature cal ibration).

he IC guys and the response was that when a part was powered up again, at a later time in life, the offset could be anywhere within the specs (I don't know if he was just saying that in order not to disclose too much, but it makes sense that a part with large VOS will have a lot of lifetime drift)

offsets

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yeh, I guess * means not mounted :)

-Lasse

I have "parts" in my PSpice library with assignable values, but labeled "DNF" (do not fill), that show on the schematic, but don't netlist, to handle options.

...Jim Thompson

Den fredag den 5. december 2014 00.21.11 UTC+1 skrev Jim Thompson:

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lifetime drift and did calibration at beginning of life (even temperature calibration).

m the IC guys and the response was that when a part was powered up again, a t a later time in life, the offset could be anywhere within the specs (I do n't know if he was just saying that in order not to disclose too much, but it makes sense that a part with large VOS will have a lot of lifetime drift )

amp offsets

on our schematics it is an attribute, when set it puts a "not mounted" next to the component and removes it from the parts list, placements file etc.

-Lasse

As I stated, I am interested in the mechanisms that can make analog ICs drift. The only silly thing about my question was asking it here, where nobody seems to know much about semiconductor physics.

It means "optional on different dash numbers." We only have one released schematic for various versions.

Good idea. I'll change it to an attribute which will insert * in front of the netlist line. I've done that for test vehicles that i step... don't know why I didn't consider it for DNF. ...Jim Thompson

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ginning of life is going to fix that drift magically over time?

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time drift and did calibration at beginning of life (even temperature calib ration).

IC guys and the response was that when a part was powered up again, at a l ater time in life, the offset could be anywhere within the specs (I don't k now if he was just saying that in order not to disclose too much, but it ma kes sense that a part with large VOS will have a lot of lifetime drift)

ffsets

Nice, is there a different make of resistor somewhere? I had some 0.1%R's that weren't.

George H.

WTF? Didn't you read my post? I explained what can cause semiconductors to drift.

Sheesh. Here it is again.

------------------------ Sodium contamination at the factory can cause drift. Less likely would be heavy metals. Sodium is a handling issue. Metals are due to defective fab equipment. I don't know if it exists on the internet, but if you could see the scribe channel side profile, it would show the amount of work that goes into guarding the edges of a chip from contamination.

Also a large voltage applied at the input of the op amp (diff pair) can leave an offset. That is why they diode clamp the inputs.

-----------------

Man, you get some clown in the fab or wafer test that touches a wafer and all hell breaks loose in the analog world. Or the assembly house DI water isn't really DI because they are in some s*it hole sweat shop in Asia. Besides the wafer saw, there is also the back lap process which needs to be clean. Surface mount devices get extreme backlap.

Semiconductor manufacturing requires countless steps that must be performed precisely if you want a repeatable product. Now people claim you can't "test in quality", but you can sure weed out products where some clown didn't do their part of the process correctly.

I suppose to blather on further, all chips prior to release to production will have to pass burn in. But once the product is qual'd, it will not receive burn in unless you pay for it. Even if you do pay for burn in, it may not be all that dynamic unless the chip is designed with a burn in mode (i.e. factory test mode).

Please remember that drift tends to have a curve like 1-e^kt curves with k being like 1/month to 1/year values.

?-)

Some information on the subject:

Have you tried to do thermal cycling on a part, with a lot of cycles per day, that should accelerate the drift, since on of the stated reasons for drift in VOS is the thermal expansion of the die

Cheers

Klaus