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
semiconductor drift
Started by ●December 3, 2014
Reply by ●December 3, 20142014-12-03
On Wed, 03 Dec 2014 18:01:04 -0800, John Larkin <jlarkin@highlandtechnology.com> 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.Could it be moisture related? FR4 can absorb moisture, a certain % of its weight per month. Maybe put the boards under vacuum and draw it out and look at the difference. Do you use any High impedance resistors in there? Low bias current amps? Cheers
Reply by ●December 4, 20142014-12-04
On Wed, 03 Dec 2014 22:13:42 -0500, Martin Riddle <martin_rid@verizon.net> wrote:>On Wed, 03 Dec 2014 18:01:04 -0800, John Larkin ><jlarkin@highlandtechnology.com> 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. > >Could it be moisture related? FR4 can absorb moisture, a certain % of >its weight per month. Maybe put the boards under vacuum and draw it >out and look at the difference. Do you use any High impedance >resistors in there? Low bias current amps? > >CheersEverything is low impedance, so I don't think that PCB leakage is significant. I think one of the ICs is drifting. It must be inherent to that IC type, since all 16 channels on two boards are drifting negative. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●December 4, 20142014-12-04
On Wed, 03 Dec 2014 18:01:04 -0800, 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.Calibrated in July or August, returned in December? Could it just be the difference in room temperature? There, now you have something other than humidity effects to worry about. -- www.wescottdesign.com
Reply by ●December 4, 20142014-12-04
On Wed, 03 Dec 2014 22:30:19 -0600, Tim Wescott <tim@seemywebsite.com> wrote:>On Wed, 03 Dec 2014 18:01:04 -0800, 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. > >Calibrated in July or August, returned in December? Could it just be the >difference in room temperature?Unlikely. The device specs shouldn't allow offsets that big. And all in the same direction! I was curious about what physics can cause drift in linear ICs. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●December 4, 20142014-12-04
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. >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.
Reply by ●December 4, 20142014-12-04
On Wed, 03 Dec 2014 21:56:40 -0800, the renowned John Larkin <jlarkin@highlandtechnology.com> wrote:> >Unlikely. The device specs shouldn't allow offsets that big. And all >in the same direction! > >I was curious about what physics can cause drift in linear ICs. > > >-- > >John Larkin Highland Technology, Inc >picosecond timing laser drivers and controllersCMOS-input op-amps can develop offset voltages if they see a large differential voltage for a long time (due to movement of ionic impurities, IIRC). Could that be happening somewhere? Beta degradation with Vbe breakdown could lead to Vos shift if the bias currents were changed enough. Firmware glitch that cleared out lower bytes of calibration numbers or something like that? Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by ●December 4, 20142014-12-04
On a sunny day (Wed, 03 Dec 2014 20:27:52 -0800) it happened John Larkin <jlarkin@highlandtechnology.com> wrote in <7eov7atsddu4t0h4mueeoakuouve36m3cc@4ax.com>:>Cheers > >Everything is low impedance, so I don't think that PCB leakage is >significant. I think one of the ICs is drifting. It must be inherent >to that IC type, since all 16 channels on two boards are drifting >negative.Supply voltage change?
Reply by ●December 4, 20142014-12-04
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. >=20 > 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. >=20 > 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. >=20 > 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?=20 >=20 > Parts are DAC2904, LMH6642, and THS3062. >=20 > THS3062 is known to be buggy, latching up if slewed hard at high > frequency, but this board doesn't stress them up there. >=20Both 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 beginni= ng of life is going to fix that drift magically over time? 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 calibratio= n).=20 I wanted to know more, asked a supplier, got information deep from the IC g= uys 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 i= f he was just saying that in order not to disclose too much, but it makes s= ense that a part with large VOS will have a lot of lifetime drift) Otherwise, you could just use a LM324 for anything, just calibrate is befor= e use.... Cheers Klaus
Reply by ●December 4, 20142014-12-04
On Thu, 4 Dec 2014 06:43:28 -0800 (PST), Klaus Kragelund <klauskvik@hotmail.com> wrote:>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? > >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) > >Otherwise, you could just use a LM324 for anything, just calibrate is before use.... > >CheersLM324s don't actually drift much with time under benign conditions. They do drift a lot with temperature changes.
