Tales from the mouth of a voltage reference ageing box - Page 1
Forum section: electronics / general

author: Andreas
author_rank: Super Contributor
timestamp: August 01, 2021, 06:22:32 pm
content: Hello,as some other volt-nuts I operate some ageing boxes to pre age mainly 5V and 7V voltage reference ICs.From time to time I am harvesting those with the least drift to build a stable ADC or voltage reference cirquit.After a 9 day power outage in my lab I had some (partly) unexpected excursions in my daily measurements.The references in the ageing box are operated 24/7 so the temperature within the box is ~10 deg C warmer than the environment.Every morning the box is heated for ~1.5 hours to 50 deg C (+/- 0.1 deg C) during the measurement of the references.Most of the references are 5V buried zeners in hermetically tight packages (AD586LQ/LT1236AILS8-5) but also one 10V and some 5V devices in plastic packages.A further description of the DIP8 ageing box can be found here:https://www.eevblog.com/forum/metrology/small-oven-controler-for-voltage-reference/msg1655015/#msg1655015with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 01, 2021, 06:25:05 pm
content: Chapter 1: REF102APThis reference (10V) in a plastic DIP8 is a lower grade version of the REF102CP which is used in the infamous 10V reference.Even in the daily heated state (where I would think the references are "dry") I get around 6 ppm seasonal changes which correspond to humidity which is ~40% rH during winter and ~60-70% rH during summer in my area.After power outage (in summer) I can see additional 6 ppm initial drift which needs about 1 week to stabilize again to the value of daily heating.As comparison some AD586LQ in cerdip package which are not affected by seasonal changes and the power outage.So the humidity coefficient (unheated) of my sample of REF102AP is around 0.5 ppm / % rHwith best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 01, 2021, 06:27:06 pm
content: Chapter 2: LT1019CCN8-5other manufacturer this time a 5V bandgap reference with similar story after power outage in plastic DIP8 package.This reference I have choosen to be evaluated because it has a (undocumented in newer datasheets) internal heater whichcould be used to do a oven together with the internal temperature sensor.Unfortunately the heater is connected with common ground and creates large offset shifts when operated.Seasonal changes are already somewhat higher ~23 ppm in dayly heated state.Excursion after power outage additional ~30 ppmAnd again you can see that it takes nearly 2 weeks to get to the old ppm value before power outage.So the humidity coefficient (unheated) is around 2.5 ppm / % rHSome older drift data of the LT1019CCN8-5 is here:https://www.eevblog.com/forum/metrology/ltc6655b-long-term-drift/msg2360088/#msg2360088with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 01, 2021, 06:29:02 pm
content: Chapter 3: MAX6250A + AD586MNZAgain plastic DIP8 packages with 3 samples of MAX6250A and one AD586MNZThe MAX6250A show still a relative high average drift of ~5 ppm/year after 7 years of ageing. (initially ~10 ppm / year)Overlaid by some seasonal changes of ~5ppm due to humidity.Excursion after power outage additional ~5-6 ppm with time constant ~1 week to settle after power outage.So the humidity coefficient (unheated) is around ~0.5 ppm / % rHThe AD586MNZ (DIP8) shows directly the difference to the AD586LQ (CERDIP8)With daily heating the AD586MNZ has settled to <1 ppm/year ageing drift and < 2 ppm seasonal changes.But during power outage up to 5 ppm drift due to humidity.So the humidity coefficient (unheated) is around <0.3 ppm / % rHwith best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 01, 2021, 06:31:04 pm
content: Chapter 4: MAX6350Maxims "best" reference with 1 ppm/K tempco.But again the plastic DIP8 package spoils the usage as precision device.The MAX6350 show still a relative high average drift of ~5 ppm/year after 7 years of ageing. (initially 10 ppm/year)Overlaid by some seasonal changes of ~5ppm due to humidity.Excursion after power outage additional ~3 ppm with time constant 3 days to settle after power outage.So the humidity coefficient (unheated) is around 0.2 ppm / % rHwith best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 01, 2021, 06:33:08 pm
content: Chapter 5: LT1236AILS8-5This one was surprising to meThe devices are in SMD ceramics hermetically sealed package.In my case I have soldered 3 wires to put them into a standard 8 pin precision socket within the ageing box.So no direct influence from soldering the reference to a PCB.But even under this conditions there are some excursions after power outage up to ~9 ppm depending on device.Strange: It should not be humidity related.But what is the reason for the drift during power outage?Has it something to do with the die attach?Anyway when comparing with a AD586LQ the "noise" of a LT1236AILS8-5(standard deviation from day to day) is about double the value.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 02, 2021, 06:13:00 pm
content: Hmm,any opinion on the surprising behaviour of the LT1236AILS8-5 ?with best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: August 02, 2021, 06:57:12 pm
content: For LT1236LS8 this might be an effect of self-heating and stress forming due to the die attach and the equalizing temperature difference between die and package and once turned on everything has to get back to "mechanical stress" equilibrium?-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: August 02, 2021, 07:17:48 pm
content: Die attach with some glue / conductive epoxy could have an effect. With a glue the glass temperature may be considerably lower than for the epoxy in the FR4 material (something like 150 C). I rememeber a 2 K epoxy I used with a glass temperature of 100-120 C. After heating close to the glass temperature the rate off cooling can have an effect on the properties. A glass temperaure close to 50 C would still be unusually low, but may be possible for an intendionally softer glue.In theory the heating to 50 C could have an effect that lasts longer than 1 day. So with a dayly heating cylce one may not see mach change, but letting it sit cold for 9 days may be enough to accumulate. In this case one would still have some daily effect, but it may not be obvious in the noise. Averaging the data from the same time from different days may make such small effects from a daily cycle visible.With a zener based reference, there could also be some electronic memory: hot electrons may cause all kinds of strange excitations, like the low level light emitted from some zener diodes. In theory the burried zener should at least keep the hot electrons away from the surface, but some of the burried zeners still show the glow. Some electronic states, especially in the oxide can have a rather long lifetime. Excitations from radioactive decay are known to accumulate over thousands of years and can be used to date some ceramics.The effect of the power outage could be the missing heating cycles, but it could also be just the time the zener was not active and thus no longer exciting long lived state and this way reduce the number of such excited long lived states.A related effect may be charge accumulation on the oxide surface of the chip. The oxide is a very good isolator, but still not perfect. Just normal lateral electric field could cause such charges to slowly build up when in operation.

author: Andreas
author_rank: Super Contributor
timestamp: August 03, 2021, 06:39:19 pm
content: Quote from: branadic on August 02, 2021, 06:57:12 pmHello,that is similar to what I think. The die attach seems to be not fully hardened.Perhaps I should not place them in a upright position into the ageing box.@KleinsteinI do not believe to a glass temperature below the operating temperature range. (85 deg C for the I-type)And other buried zeners with hermetically housings (AD586LQ, LM399) do not show any change after power outage.with best regardsAndreas

author: DeltaSigmaD
author_rank: Contributor
timestamp: August 04, 2021, 09:17:26 am
content: The temperature-controlled LT1236 circuit (seehttps://www.eevblog.com/forum/metrology/module-with-temperature-stabilised-lt1236-5ls8/msg3082132/#msg3082132) shows similar power-off drift. That's the reason why I dropped the LT1236 and continued with JFET reference tests. These tests are not completed yet, but I have found promising reproducibility in the 1ppm range with some circuits.