LT1236LS8 drift on power restart - mechanism of action? - Page 1
Forum section: electronics / general

author: Svgeesus
author_rank: Regular Contributor
timestamp: August 10, 2025, 07:15:59 pm
content: Context:I have an LT1236 LS8-based voltage reference board which is used to supply an accurate +5V and -5V to two AD5781A DACs (in this application, accuracy is more important than precision). OPA2192 is used as unity-gain buffer for +5V and gain -1 buffer for -5V. The datasheet trim circuit with 1N4148 compensation diode is used, and after initial burn-in was adjusted to give +5.000 025V.This is a fit it and forget it application (part of a Eurorack MIDI to Pitch CV module) so size and current consumption are critical factors. A heated reference is not feasible here.LT1763 and LT1964-BYP (on a separate PCB) are used to give a secondary regulated ±9.5V supply from the somewhat noisy, switcher based Eurorack ±12 supply.This initial "does it work" board #1 uses a pair of Susumu RG2012V-182-P-T1 1.8k 0.02% 5ppm/°C 0805 resistors for the inverting stage; the second board #2 will use Vishay Precision Group Y16292K50000T9R 2.5k 0.01% 0.2 ppm/°C 0805 foil resistors. However, this post concerns the positive output, so those don't affect the result.Question:Board #1 now has 500 hours on-time, and the positive output had a stable 5.000 015V sd 5μV output. It was then off for two weeks, and I recently measured it again. The power supply and the meter (Keysight 34465A, 10V range, 100PLC) had both been on for over 2 hours. I powered up the reference, discarded the first 10 minutes measurements to allow for vref warmup, then logged for a couple of days. For 12 hours, there was a steady climb from 4.999 97V to 5.000 03V, then it was level for the remaining time.I don't understand the mechanism of action for this long inital climb.- the meter was warmed up and has a heated LM399 reference so surely this is not meter warmup- the vref is not heated, has minimal power draw (the entire thing including secondary regulators, vref and buffer draws 4mA so self-heating can't be it) so surely is warmed up too- room is air conditioned and stable 23C ±1CIs this stress in the the die settling, or something like that?(Yes I know this is not a metrology grade circuit, but I am looking for metrology-informed answers.)

author: iMo
author_rank: Super Contributor
timestamp: August 10, 2025, 07:29:37 pm
content: Mine 34401A needs 5 days to settle, so I doubt 2h is enough. And those LT1236 (even ceramic) may suffer of some hysteresis when powered off/on.. Also the way how the LT is mounted plays a role..

author: lichen8867
author_rank: Newbie
timestamp: August 12, 2025, 01:06:19 am
content: the reason for the drift /long time settling should be the stress mismatch between chip package and PCB, here is a reference from Andreas. if the ls8 chip is not fixed to pcb, the stess change before /after power up might be much smaller.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 12, 2025, 02:23:45 am
content: Hi,You might want to look at the components attached to the TRIM pin. Page 7 & 8 of the datasheet include a different trim circuit for the 'A' version.You could also try removing R2 and D1 and looking at how long it takes to settle.Do you have a ground star point? Currents flowing in the ground traces can produce small offsets.Regards,Jay_Diddy_B

author: Andreas
author_rank: Super Contributor
timestamp: August 12, 2025, 06:53:27 am
content: Quote from: Svgeesus on August 10, 2025, 07:15:59 pmMaybe: I also had strange readings for several days after a 9 day power outage with the LT1236AILS8-5 in my ageing box.note that the references are all mounted with wires into sockets.So I think it is some interaction between the (Epoxy-)cement which is used to mount the die into the package and the die.I had no visible change with AD586LQ devices.with best regardsAndreasEdit: I guess in your case also the PCB (humidity swelling) plays a role.So either you put all onto a ceramic substrate or put the reference on a polyimide (flexible) PCB

author: lichen8867
author_rank: Newbie
timestamp: August 12, 2025, 08:56:57 am
content: thank Andreas.for LS8 package, the main material of body of the package is ceramic， but the cover is metal. these two materials have different thermal coefficient of expansion. this might cause stress change when temperature change.as for AD586LQ, both the main body and the cover are made of ceramic, that's the difference.

author: iMo
author_rank: Super Contributor
timestamp: August 12, 2025, 09:16:59 am
content: O fair silicon, child of heated breath,Thy tender frame swells ‘neath the kindly flame;Yet ceramic walls, cold in steadfast death,Refuse thy plea, nor share thy swelling frame.Now wires of silver strain as captives bound,Their trembling strength bespeaks the coming woe;A crack! — like ice that splits on frozen ground —Proclaims that war ‘twixt heat and cold doth grow.Thus physics writes in bonds both fierce and small,The tale of love where neither side may bend;And so within this package’s fragile wall,Expansion meets resistance — and meets its end.

author: Svgeesus
author_rank: Regular Contributor
timestamp: August 12, 2025, 07:15:30 pm
content: Quote from: Jay_Diddy_B on August 12, 2025, 02:23:45 amTrimming is on page 7 and the circuit there is exactly identical to the one I have, and does not have a different circuit for the A version.Page 8 is about Load Regulation, Air Movement and Long Term Drift.Aha perhaps you are looking at the LT1236 datasheet (not the one for the LT1236LS8) which does have a different trim circuit for LT1236-10 “A” - but I am not using that part. There is no 10V LS8, just a 5V.

author: Svgeesus
author_rank: Regular Contributor
timestamp: August 12, 2025, 07:21:21 pm
content: Quote from: lichen8867 on August 12, 2025, 08:56:57 amThat did strike me as an odd choice, indeed.

author: Svgeesus
author_rank: Regular Contributor
timestamp: August 12, 2025, 07:30:13 pm
content: Quote from: Jay_Diddy_B on August 12, 2025, 02:23:45 amOn the top layer, there is a star ground from the GND terminal on the LS8 and the input capacitors, to the GND pin on the power connection.The bottom layer is a ground plane which connects that star point to the GND on the +5V and -5V outputs and the GND on the trimmer.