On Thu, 04 Dec 2014 14:29:13 -0600, Jon Elson <jmelson@wustl.edu> wrote:>John Larkin wrote: > > >> Just now, we're experimenting with these two boards. >> We'll bake one to maybe accelerate the aging (and, hopefully, not >> anneal the drift out.) >And that might just reset the drift back to zero, and start the >process all over again. > >JonYeah, that would trash the idea of baking to accelerate aging! -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
semiconductor drift
Started by ●December 3, 2014
Reply by ●December 4, 20142014-12-04
Reply by ●December 4, 20142014-12-04
"John Larkin" <jlarkin@highlandtechnology.com> wrote in message news:b4g18a996fjvmbnsgilrlfs0quapasrfr8@4ax.com...> Here it is: > > https://dl.dropboxusercontent.com/u/53724080/Circuits/VME/22S340B_sh2.pdf > > The overall gain from the DAC diff output, to the final unloaded > output, is about 13, so, if the drift is at the front end, it's maybe > 500 uV. If it's in U8, it's equivalent to about 1 mV. > > We are doing some tests to see who is drifting.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 -- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
Reply by ●December 4, 20142014-12-04
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: http://seventransistorlabs.com "John Larkin" <jlarkin@highlandtechnology.com> wrote in message news:c3fv7al1si4jqvdkvlvihdjjskqf0ptf3p@4ax.com...> > We have two 8-channel waveform generators that were shipped 4 months > ago, and came back because the customer ordered too many or something. > We routinely test anything that comes back, before returning them or > returning to stock. > > What's interesting is that all 16 channels have a negative DC offset. > Each channel is a diff-current-output cmos DAC, an opamp diffamp, a > passive LC filter, and an output amp; the opamps are fast bipolars. We > apply a software cal factor to the DAC data (saved in a cal table) to > get the offsets way below 1 mV when we ship. After 4 months, we're > seeing offsets from -5 to -10 mV. These are not actual failures, but I > don't like or understand the trend. > > We'll be doing some tests to try to isolate the drift to dac, diffamp, > or output amp. I figure we could measure things on one board, bake to > accelerate aging, and re-measure. > > My general question, to people who understand semi physics: what are > the physical mechanisms that could make the DAC, or the opamps, have > this ensemble negative drift vs time? > > Parts are DAC2904, LMH6642, and THS3062. > > THS3062 is known to be buggy, latching up if slewed hard at high > frequency, but this board doesn't stress them up there. > > > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com >
Reply by ●December 4, 20142014-12-04
On Thu, 4 Dec 2014 15:09:23 -0600, "Tim Williams" <tiwill@seventransistorlabs.com> wrote:>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. > >TimCome 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 -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●December 4, 20142014-12-04
On 4/12/2014 12:01 PM, John Larkin wrote:> > We have two 8-channel waveform generators that were shipped 4 months > ago, and came back because the customer ordered too many or something. > We routinely test anything that comes back, before returning them or > returning to stock. > > What's interesting is that all 16 channels have a negative DC offset. > Each channel is a diff-current-output cmos DAC, an opamp diffamp, a > passive LC filter, and an output amp; the opamps are fast bipolars. We > apply a software cal factor to the DAC data (saved in a cal table) to > get the offsets way below 1 mV when we ship. After 4 months, we're > seeing offsets from -5 to -10 mV. These are not actual failures, but I > don't like or understand the trend. > > We'll be doing some tests to try to isolate the drift to dac, diffamp, > or output amp. I figure we could measure things on one board, bake to > accelerate aging, and re-measure. > > My general question, to people who understand semi physics: what are > the physical mechanisms that could make the DAC, or the opamps, have > this ensemble negative drift vs time? > > Parts are DAC2904, LMH6642, and THS3062. > > THS3062 is known to be buggy, latching up if slewed hard at high > frequency, but this board doesn't stress them up there. > > > >Since its common to all channels, I would be thinking about things like change in leakeage current through bypass or filter caps. -- Regards, Adrian Jansen adrianjansen at internode dot on dot net Note reply address is invalid, convert address above to machine form.
Reply by ●December 4, 20142014-12-04
On Thu, 04 Dec 2014 12:18:26 -0800, John Larkin wrote:> On Thu, 4 Dec 2014 19:46:32 +0000 (UTC), Frank Miles > <fpm@u.washington.edu> wrote: > >>On Thu, 04 Dec 2014 11:30:13 -0800, John Larkin wrote: >> >>> On Thu, 4 Dec 2014 06:43:28 -0800 (PST), Klaus Kragelund >>> <klauskvik@hotmail.com> wrote: >>> >>>>On Thursday, December 4, 2014 3:01:11 AM UTC+1, John Larkin wrote: >>>>> We have two 8-channel waveform generators that were shipped 4 months >>>>> ago, and came back because the customer ordered too many or something. >>>>> We routinely test anything that comes back, before returning them or >>>>> returning to stock. >>>>> >>>>> What's interesting is that all 16 channels have a negative DC offset. >>>>> Each channel is a diff-current-output cmos DAC, an opamp diffamp, a >>>>> passive LC filter, and an output amp; the opamps are fast bipolars. We >>>>> apply a software cal factor to the DAC data (saved in a cal table) to >>>>> get the offsets way below 1 mV when we ship. After 4 months, we're >>>>> seeing offsets from -5 to -10 mV. These are not actual failures, but I >>>>> don't like or understand the trend. >>>>> >>>>> We'll be doing some tests to try to isolate the drift to dac, diffamp, >>>>> or output amp. I figure we could measure things on one board, bake to >>>>> accelerate aging, and re-measure. >>>>> >>>>> My general question, to people who understand semi physics: what are >>>>> the physical mechanisms that could make the DAC, or the opamps, have >>>>> this ensemble negative drift vs time? >>>>> >>>>> Parts are DAC2904, LMH6642, and THS3062. >>>>> >>>>> THS3062 is known to be buggy, latching up if slewed hard at high >>>>> frequency, but this board doesn't stress them up there. >>>>> >>>> >>>>Both the LMH6642 and THS3062 has specified up to 5mV input offset voltage >>>> >>>>So with that wide input offset value, do you think a calibration at beginning of life is going to fix that drift magically over time? >>> >>> These opamps are spec'd for offset and for offset tempco, but not for >>> longterm drift. It's rare to have such a spec. My question, for people >>> who actually understand semiconductor physics, is about the possible >>> drift mechanisms. >>> >>> Seems lot a lot of drift, too. >>> >>> >>> >>> >>>> >>>>The parts has defined temperature drift (average), but no lifetime specs >>>> >>>>In a earlier employment we did the same. Just closed our eyes for lifetime drift and did calibration at beginning of life (even temperature calibration). >>>> >>>>I wanted to know more, asked a supplier, got information deep from the 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) >>> >>> Sounds like butt-covering. >> >>How about showing your circuit - or at least telling us how much opamp offsets >>would be required to cause its output to change by 10mV? > > > Here it is: > > https://dl.dropboxusercontent.com/u/53724080/Circuits/VME/22S340B_sh2.pdf > > The overall gain from the DAC diff output, to the final unloaded > output, is about 13, so, if the drift is at the front end, it's maybe > 500 uV. If it's in U8, it's equivalent to about 1 mV. > > We are doing some tests to see who is drifting.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.
Reply by ●December 4, 20142014-12-04
On Fri, 05 Dec 2014 07:37:16 +1000, Adrian Jansen <adrian@qq.vv.net> wrote:>On 4/12/2014 12:01 PM, John Larkin wrote: >> >> We have two 8-channel waveform generators that were shipped 4 months >> ago, and came back because the customer ordered too many or something. >> We routinely test anything that comes back, before returning them or >> returning to stock. >> >> What's interesting is that all 16 channels have a negative DC offset. >> Each channel is a diff-current-output cmos DAC, an opamp diffamp, a >> passive LC filter, and an output amp; the opamps are fast bipolars. We >> apply a software cal factor to the DAC data (saved in a cal table) to >> get the offsets way below 1 mV when we ship. After 4 months, we're >> seeing offsets from -5 to -10 mV. These are not actual failures, but I >> don't like or understand the trend. >> >> We'll be doing some tests to try to isolate the drift to dac, diffamp, >> or output amp. I figure we could measure things on one board, bake to >> accelerate aging, and re-measure. >> >> My general question, to people who understand semi physics: what are >> the physical mechanisms that could make the DAC, or the opamps, have >> this ensemble negative drift vs time? >> >> Parts are DAC2904, LMH6642, and THS3062. >> >> THS3062 is known to be buggy, latching up if slewed hard at high >> frequency, but this board doesn't stress them up there. >> >> >> >> >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 -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●December 4, 20142014-12-04
On Thu, 04 Dec 2014 12:18:26 -0800, John Larkin <jlarkin@highlandtechnology.com> wrote:> >Here it is: > >https://dl.dropboxusercontent.com/u/53724080/Circuits/VME/22S340B_sh2.pdf > >The overall gain from the DAC diff output, to the final unloaded >output, is about 13, so, if the drift is at the front end, it's maybe >500 uV. If it's in U8, it's equivalent to about 1 mV. > >We are doing some tests to see who is drifting. >650nA leaking from the minus supply into the plus input of the LMH6642 would do it. The pins are adjacent. That's a whole whack of currrent, but not inconceivable if the wrong flux was used. 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
Reply by ●December 4, 20142014-12-04
On 04-12-2014 03:01, John Larkin wrote:> > We have two 8-channel waveform generators that were shipped 4 months > ago, and came back because the customer ordered too many or something.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
Reply by ●December 4, 20142014-12-04
On 12/04/2014 03:18 PM, John Larkin wrote:> On Thu, 4 Dec 2014 19:46:32 +0000 (UTC), Frank Miles > <fpm@u.washington.edu> wrote: > >> On Thu, 04 Dec 2014 11:30:13 -0800, John Larkin wrote: >> >>> On Thu, 4 Dec 2014 06:43:28 -0800 (PST), Klaus Kragelund >>> <klauskvik@hotmail.com> wrote: >>> >>>> On Thursday, December 4, 2014 3:01:11 AM UTC+1, John Larkin wrote: >>>>> We have two 8-channel waveform generators that were shipped 4 months >>>>> ago, and came back because the customer ordered too many or something. >>>>> We routinely test anything that comes back, before returning them or >>>>> returning to stock. >>>>> >>>>> What's interesting is that all 16 channels have a negative DC offset. >>>>> Each channel is a diff-current-output cmos DAC, an opamp diffamp, a >>>>> passive LC filter, and an output amp; the opamps are fast bipolars. We >>>>> apply a software cal factor to the DAC data (saved in a cal table) to >>>>> get the offsets way below 1 mV when we ship. After 4 months, we're >>>>> seeing offsets from -5 to -10 mV. These are not actual failures, but I >>>>> don't like or understand the trend. >>>>> >>>>> We'll be doing some tests to try to isolate the drift to dac, diffamp, >>>>> or output amp. I figure we could measure things on one board, bake to >>>>> accelerate aging, and re-measure. >>>>> >>>>> My general question, to people who understand semi physics: what are >>>>> the physical mechanisms that could make the DAC, or the opamps, have >>>>> this ensemble negative drift vs time? >>>>> >>>>> Parts are DAC2904, LMH6642, and THS3062. >>>>> >>>>> THS3062 is known to be buggy, latching up if slewed hard at high >>>>> frequency, but this board doesn't stress them up there. >>>>> >>>> >>>> Both the LMH6642 and THS3062 has specified up to 5mV input offset voltage >>>> >>>> So with that wide input offset value, do you think a calibration at beginning of life is going to fix that drift magically over time? >>> >>> These opamps are spec'd for offset and for offset tempco, but not for >>> longterm drift. It's rare to have such a spec. My question, for people >>> who actually understand semiconductor physics, is about the possible >>> drift mechanisms. >>> >>> Seems lot a lot of drift, too. >>> >>> >>> >>> >>>> >>>> The parts has defined temperature drift (average), but no lifetime specs >>>> >>>> In a earlier employment we did the same. Just closed our eyes for lifetime drift and did calibration at beginning of life (even temperature calibration). >>>> >>>> I wanted to know more, asked a supplier, got information deep from the 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) >>> >>> Sounds like butt-covering. >> >> How about showing your circuit - or at least telling us how much opamp offsets >> would be required to cause its output to change by 10mV? > > > Here it is: > > https://dl.dropboxusercontent.com/u/53724080/Circuits/VME/22S340B_sh2.pdf > > The overall gain from the DAC diff output, to the final unloaded > output, is about 13, so, if the drift is at the front end, it's maybe > 500 uV. If it's in U8, it's equivalent to about 1 mV. > > We are doing some tests to see who is drifting. > > > >I gather L1 and L2 are a pop option. If not, there's your trouble. ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
