LTC6655B long term drift - Page 1
Forum section: electronics / general

author: alex-sh
author_rank: Regular Contributor
timestamp: April 20, 2019, 12:44:22 pm
content: Hello all,I have a small pcb with a voltage vref LTC6655B. Unfortunately, the long-term drift specified only for LS8 is 20ppm/sqr(1000). This means a drift os 81uV. I have MSOP package though. I measured the vref on 11.02.2019 and it was 4.097200v with my 6-1/2 DDM (recently calibrated), which was already outside 0.025% max accuracyRight now, it is 4.097784V. It is hardly been used in between the dates. This means the vref drifted 584uV. This is a huge drift in my view. Any idea why the LTC6655 drifted that much? Any recommendatins? Maybe I should have burn it for several months for long-term stability?ThanksAlex

author: ycui7
author_rank: Contributor
timestamp: April 20, 2019, 12:52:50 pm
content: I would suggest to run the reference continuously for a few days first, then compare the results.I have seen this reference drifted quite a bit in the first few days then settled to a few ppm level variation within day (mostly due to room temp change).An extreme case for me was that a 4.096V reference from Maxim output 5 V the first time it powered on with very high noise, then settles to 4.096V specification and ppm level noise in a few days. Mostly due to my soldering.

author: Andreas
author_rank: Super Contributor
timestamp: April 20, 2019, 01:32:22 pm
content: Quote from: alex-sh on April 20, 2019, 12:44:22 pmHello,it has a reason why the (typical) drift is only specified for the hermetically package.The plastic package absorbs humidity which gives stress to the die and thus changes the output voltage.If you solder the MSOP8 package it gets dry.The following days/weeks the package absorbs humidity from the air -> output voltage changes.After a week or two the package stabilizes (see ADC14).What do you think happens if you burn in the plastic package: after the burn in. -> it is useless.By the way: independant of LS8 package or MSOP package: my samples showed a relative large hysteresis so I do not consider the LTC6655 as a "precision" reference. The LS8 package is better, but only if you mount it dead bug style on the PCB so that there is no influence from the PCB.With best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: April 20, 2019, 01:52:27 pm
content: I would highly recommend you to clean up your PCBs from the soldering flux (with IPA) carefully..

author: Andreas
author_rank: Super Contributor
timestamp: April 20, 2019, 02:04:10 pm
content: The photo was made before soldering the connectors and before cleaning.best regardsAndreas

author: alex-sh
author_rank: Regular Contributor
timestamp: April 20, 2019, 02:13:42 pm
content: Quote from: Andreas on April 20, 2019, 01:32:22 pmSadly, you are correct. I have been running a few tests myself with several LTC6655 and very disappointed with results.It is a very low noise vref, but I would not call it precise either. The long term drift is horrible.Would you recommend any better .096V vref in MSOP package please?

author: alex-sh
author_rank: Regular Contributor
timestamp: April 20, 2019, 02:14:53 pm
content: Quote from: imo on April 20, 2019, 01:52:27 pmNot sure if it was addressed to me or Andreas, I have cleaned up my PCB several times just to rule out this issue.

author: Andreas
author_rank: Super Contributor
timestamp: April 20, 2019, 02:59:30 pm
content: Quote from: alex-sh on April 20, 2019, 02:13:42 pmI see 2 problems in this question.The smaller (less stiff) the plastic package the more instable the device. (see e.g. ADR293 datasheet).So a DIP8 device is more stable than a MSOP8Plastic packages generally have more or less influence from humidity.So if you need really a stable device over time/humidity there is no way out to use a metal can case or as 2nd best alternative a ceramic case (which has less mechanical decoupling from PCB humidity influences).4.096V is usually a bandgap reference. The ageing drift of bandgap references tends to be higher than that of buried zener references. (which need a higher power supply voltage since the internal reference is somewhere between 6-7V).So choosing a optimal reference is always depending on what you really need.with best regardsAndreas

author: alex-sh
author_rank: Regular Contributor
timestamp: April 20, 2019, 03:23:36 pm
content: Quote from: Andreas on April 20, 2019, 02:59:30 pmThe problem is that I have already MSOP8 footprint on the PCB, which I do not want to change.I can do a daughter board and probably accommodate other packages. I have a choice to either1) leave existing 4.096V LTC6655B <- this would require more frequent calibration2) change it for DIP8 / LS8 on the daughter board3) go for LM399 with some LT5400 to order to derive as close as possible 4.096V <- this would probably be the best in terms of long-term.I am building a probrammable voltage reference.ThanksAlex

author: iMo
author_rank: Super Contributor
timestamp: April 20, 2019, 06:00:51 pm
content: What about ref5040? Similar spec..Quote from: Andreas on April 20, 2019, 01:32:22 pmOnce I messed with some expensive 144pin tqfp (epoxy) the storage box contained an instruction how to treat the package such it will not crack upon soldering because of moisture - as I can remember they (ADI) recommended to put the package for maybe 24h into 150C prior soldering..

author: Andreas
author_rank: Super Contributor
timestamp: April 20, 2019, 08:26:21 pm
content: Quote from: alex-sh on April 20, 2019, 03:23:36 pmHello,So you are fixed to the LTC6655 because it has a unique pinning (force + sense)Dont get me wrong: the LTC6655 is not really bad. But you cannot expect 5-6 digits accuracy in MSOP.Its more like 4-5 digits. (as long as you do not apply force to the PCB).By the way the LTC6655 has high current consumption and so heats up rather high.Did you keep air drafts away from it?That does not explain why you need the 4.1V reference.(you do not really want to make a 1mV step calibrator with a 12 bit DAC).The best results (hysteresis + humidity stability) up to now I have with the AD586LQ (5V) and AD587JQ (10V)But for high demands you have to select for tempco and ageing.The next best device for 4.1V could be a MAX6341. (if you have >=8V stabilized supply voltage).But all my MAX6250A and MAX6350 (5V in DIP8) from the same series have around 10 ppm/year ageing drift.And also on many samples I see popcorn noise of 1-2 ppm near certain temperatures.Quote from: imo on April 20, 2019, 06:00:51 pmIf you ask me: I have no experience with this part.(and low interest because it is in a plastic package).And be carefully with "typical" datasheet specs regarding long term drift and hysteresis.In most cases they have been done once with the "best package option" long time ago.(e.g. metal can case which is long obsolete).They are also never updated when the epoxy which is used to fix the part into the packageis changed (e.g. due to RoHS or other change of supplier).So if you really want to have exact data you have to test every single device under your individual operating conditions.with best regardsAndreas

author: razvan784
author_rank: Regular Contributor
timestamp: April 20, 2019, 09:11:14 pm
content: Quote from: alex-sh on April 20, 2019, 12:44:22 pmYes, it is off by 0.029%, assuming your meter is spot-on. I guess the specification applies to the component as delivered, before soldering -- because that's how they are able to test and adjust it.You have to add the hysteresis due to soldering - some plots (typical, not guaranteed!) are given in the applications section of the datasheet.Quote from: alex-sh on April 20, 2019, 12:44:22 pmThat's around 0.014%, or 140 ppm, or around 70 ppm/month. As others have already said, that's not huge for a MSOP package, which is optimized for a small PCB footprint, at the cost of accuracy.Look in the datasheet, Figure 10 - Long-Term Drift for MS8 and LS8. The MS8 graph show 4 traces (not statistically significant!) and one of them shows around 100 ppm drift - so 140 is not at all unreasonable. The hermetic LS8 is not much greater (50 instead of 100 ppm, though more samples shown), and it is explicitly stated that it is also humidity-sensitive by influence of the PCB. I particularly like how they emphasize that "proper board materials and layout are essential", without stating what those materials might beCeramic anyone? (See those Fluke calibrators).So, IMHO you have two options:* if you really want stability, use a LTZ1000, LM399, or a CERDIP like the AD586. Redesign the circuit to not need exactly 4096 mV and calibrate it in software. Do an initial cal and repeat once in a while.* if you really want small size and low power, use the LTC6655, or maybe a LT1461 (very low power, excellent TC), or a TI REF50-something, and calibrate it often.

author: alex-sh
author_rank: Regular Contributor
timestamp: April 20, 2019, 10:27:45 pm
content: QuoteYou are right:Long-term drift cannot be extrapolated from accelerated high temperature testing. This erroneous technique gives drift numbers that are wildly optimistic.The only way long-term drift can be determined is to measure it over the time interval of interest.The LTC6655 long-term drift data was collected on 80 parts that were soldered into printed circuit boards similar to a real world application. The boards were then placed into a constant temperature oven with a TA = 35°C, their outputs were scanned regularly and measured with an 8.5 digit DVM. Typical long-term drift is illustrated in Figure 10a. The hermetic LS8 package provides additional stability as shown in Figure 10b.Sadly, the datasheet is giving an optimictic results (for 2.5V vref only):Quote from: razvan784 on April 20, 2019, 09:11:14 pmThis would be the best option if the calibration was simple. I have 60uV steps and I can calibrate it in 1mV stepts (4096 steps in total).Very tedious if I was to do it on a regular basis sadly.

author: alex-sh
author_rank: Regular Contributor
timestamp: April 21, 2019, 06:20:54 am
content: Quote from: Andreas on April 20, 2019, 08:26:21 pmThank you AndreasMy expectation is not meeting reality I suppose. I was hoping for 5-6 digits accuracy, but with LTC6655 I am disappointed. Good initial results, but very poor long term performance.Yes, the design of the PCB takes in the account that the vref may heat up and it is isolated away from the rest of the components.I am using AD5541LRZ (16 bit DAC) and have 1mV steps from 1mV to 4096mV with ability of 60uV calibration steps. I'll look into this, but I think 4.096V vref is not critical. The problem is that the vref is fed from +5V power rail and this means using 5V vref would not be possible without modifying or adding a new power rail.AD586LQ would be a good one if the design was catering for 10.8V ~ 36V input voltage.I also have MAX6341CPA+-ND in DIP8 - this means I can use it on the daughter board with a few caps. I can probably live with +/- 40uV / year drift.I suppose I should think what to do next.ThanksAlex

author: splin
author_rank: Frequent Contributor
timestamp: April 21, 2019, 01:20:09 pm
content: Quote from: Andreas on April 20, 2019, 01:32:22 pmAndreas, have you noticed any change in the hysterisis characteristics of these, or other (non-hermetic) references with repeated cycling? I can't image ceramic packaged devices changing much but it may be possible for relaxation effects in plastic packaged parts to show reduced hysterisis after many temperature cycles.

author: alex-sh
author_rank: Regular Contributor
timestamp: April 21, 2019, 06:32:23 pm
content: Anyone have had any experience with LT1021BCN8-5 (5v vref)?From the datasheet, I can operate a 5V Reference from 5V Supply - this would be the best way for me to move from bandgap voltage reference to a zener diode one with a very good long-term drift.

author: iMo
author_rank: Super Contributor
timestamp: April 21, 2019, 06:43:19 pm
content: The LT1021-5V needs at least 6.9V based on my measurements.PS: all LT1021s include a 6.2V burried zener plus some electronics, including trimmed divider (-5, -10), the -7 is without the divider and has got the best long term stability.

author: alex-sh
author_rank: Regular Contributor
timestamp: April 21, 2019, 08:03:01 pm
content: Quote from: imo on April 21, 2019, 06:43:19 pmPlease take a look at the attachement (this is from the LT1021 datasheet).I have LT1021BCN8-10 but I hate to do a new power rail + INA105 to drop it to 5V

author: Andreas
author_rank: Super Contributor
timestamp: April 21, 2019, 08:34:22 pm
content: Quote from: splin on April 21, 2019, 01:20:09 pmHello,of course there are differences between repeated cyclings on plastic packages.Mainly dependant on humidity.So you cannot expect them getting better and better in a non humidity controlled environment.Quote from: alex-sh on April 21, 2019, 08:03:01 pmYes but practically you want to operate it above 10V (because of better PSRR).A charge pump introduces additional noise.By the way: the LT1236A is the enhanced version of the LT1021B(+C).(low T.C. and low output voltage tolerance at the same time).So unless you are using the metal can package for the LT1021 does not give any advantage over the LT1236.with best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: April 21, 2019, 09:07:33 pm
content: Quote from: alex-sh on April 21, 2019, 08:03:01 pmThe LT1021B-10 requires at least 12V.LT1021B-5 is not easily trimmable by a pot.The B versions are pretty off without trimming (ie +0.01 to +0.02V in my case) but with lowest tempco.

author: alex-sh
author_rank: Regular Contributor
timestamp: April 22, 2019, 05:19:16 am
content: Quote from: Andreas on April 21, 2019, 08:34:22 pmThat's why I am using 4.096V vref - not to introduce another power rail. I have +5V/-5V/+3.3V and trying to take advantage of any of these.I wonder if I can live with the charge pump noise...Sadly, LT1021 in H package (metal can) has been discontinued - this is what digikey is telling me.LT1236 is available in LS8 hermitically sealed.Did you have any experience with LT1019A?

author: alex-sh
author_rank: Regular Contributor
timestamp: April 22, 2019, 05:47:37 am
content: Quote from: imo on April 21, 2019, 09:07:33 pmI would rather avoid using the pot.If the vref is not initially accurate (+0.01V) I can probably correct it in software.Though the idea is to find the holy grail - excellent initially accuracy, low long term drift, small package and low power.However, looks like no way to have a good long term drift without using TO5-3/TO5-8 package.

author: iMo
author_rank: Super Contributor
timestamp: April 22, 2019, 06:17:43 am
content: Quote from: alex-sh on April 22, 2019, 05:47:37 amWe all do as well..

author: Andreas
author_rank: Super Contributor
timestamp: April 22, 2019, 08:31:23 am
content: Quote from: imo on April 22, 2019, 06:17:43 amHello,I am now trying since 11 years.Best result up to now AD586LQ / selected (for low T.C.) AD587JQ.2nd best LT1236DCLS8-5. There is one Idea that I have not done up to now:If you are not using the trim pin you should be able to use it to measurethe internal chip temperature. (-1.7 mV/K according to data sheet).With a Resolution of 0.01K or 17uV and a high impedant buffer amplifier this systemcould be better than my AD586 solution with a external NTC and 0.1K(10 Bit ADC resolution) for temperature compensation.Under same conditions a AD586 based system delivers a factor 2 lowerstandard deviation over 1000 Hours daily measurements.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: April 22, 2019, 08:55:10 am
content: Quote from: alex-sh on April 22, 2019, 05:19:16 amHello,nearly overlooked the question.Not the A-Version but the C-Version. (which was easier to get for me).here a diagram of the drift in my ageing-Box for the last 1.5 years.The ageing box is heated every day to 50.0 deg C to measure several references.You clearly can see the difference between a plastic device and the hermetically oneseven when the box is heated up every day for about 1.5 hours.with best regardsAndreas

author: alex-sh
author_rank: Regular Contributor
timestamp: April 22, 2019, 11:59:51 am
content: Quote from: Andreas on April 22, 2019, 08:55:10 amThis pretty much sums it up.OK, TO/8-CLCC package is now on.

author: SvanGool
author_rank: Regular Contributor
timestamp: April 22, 2019, 12:04:58 pm
content: Quote from: alex-sh on April 22, 2019, 05:19:16 amThe LT1021CMH-10 is available, years already, at a French ebay supplier,eBay auction: #ci-LT1021CMH-10-883-LT-1021-CM-H-10-883-LT1021CMH-10-F10/191123710347.Alex Nikitin did quite some experiments with these in the past, look for his postings:https://www.eevblog.com/forum/metrology/lt1021cmh-based-mini-reference/msg1125519/#msg1125519

author: alex-sh
author_rank: Regular Contributor
timestamp: April 22, 2019, 12:07:05 pm
content: Quote from: Andreas on April 22, 2019, 08:31:23 amThe only one LT1236 available in LS8 package is LT1236AILS8-5#PBF at digikey.You mentioned LT1236DCLS8-5. What's the difference between the two? I cannot find any reference to LT1236Din the datasheet.Another question is that I have a power supply of +5V (USB or battery) and I cannot change that. So it looks like I have to either stick to 4.096v reference or use a charge pump to operate 5V Reference from 5V Supply. Alternatively, I can have a severate power (2xLiPO = 7.4V) for LT1236AILS8-5 which is not ideal either. What are your thoughts?ThanksAlex

author: alex-sh
author_rank: Regular Contributor
timestamp: April 22, 2019, 12:09:27 pm
content: Quote from: SvanGool on April 22, 2019, 12:04:58 pmThank you.This is the corrected link to eBay LT1021CMH-10 -https://www.ebay.com/itm/ci-LT1021CMH-10-883-LT-1021-CM-H-10-883-LT1021CMH-10-F10/191123710347?hash=item2c7fdc618b:g:OHIAAMXQuTNTPBSE

author: alex-sh
author_rank: Regular Contributor
timestamp: April 22, 2019, 12:12:52 pm
content: I just love these statements in the datasheet:The only way long-term drift can be determined is to measure it over the time interval of interest.

author: wraper
author_rank: Unknown
timestamp: April 22, 2019, 12:34:01 pm
content: Quote from: alex-sh on April 22, 2019, 12:12:52 pmI love how you took it out of contextQuote

author: Andreas
author_rank: Super Contributor
timestamp: April 22, 2019, 02:39:33 pm
content: Quote from: alex-sh on April 22, 2019, 12:07:05 pmSorry my fault,I intermixed the numbering with the LT1027DCLS8-5. (LS8- package of LT1027)For the LS8-package of the LT1236 I have the LT1236AILS8-5#PBF.For the power supply: in any case you need a stabilized power supply voltage to keep self heating constant.And if I look at the PSRR of my measured sample I would choose the supply minimum at 9V or better at 14-15 V.with best regardsAndreasEDIT: USB spec means: voltage can be as low as 4.3V so don't calculate with 5V from a USB port.

author: alex-sh
author_rank: Regular Contributor
timestamp: April 22, 2019, 03:42:48 pm
content: QuoteI do not see reading this statement out of context. I do not think anyone would ideally want to spend 3000h to determine the long term drift.

author: wraper
author_rank: Unknown
timestamp: April 22, 2019, 03:45:14 pm
content: Quote from: alex-sh on April 22, 2019, 03:42:48 pmThat statement is relevant to any other voltage reference. Accelerated tests don't show actual long term drift.

author: SilverSolder
author_rank: Super Contributor
timestamp: April 22, 2019, 03:59:18 pm
content: What does the accelerated test actually show - does it give any useful information at all?

author: alex-sh
author_rank: Regular Contributor
timestamp: April 22, 2019, 04:05:05 pm
content: Quote from: Andreas on April 22, 2019, 02:39:33 pmOK, in the nutshell I need min 9V power rail to make this vref to work.PS LTC6655 is not powered directly from USB and has a number of decoupling caps so I am not concerned about 4.4V < USB < 5.25V.As I am programming the Cortex-M4 board via USB, it is easy for me to power the whole project via USB for now, but it will be changed to battery power eventually.

author: wraper
author_rank: Unknown
timestamp: April 22, 2019, 04:07:35 pm
content: Quote from: SilverSolder on April 22, 2019, 03:59:18 pmTo many things it can be quite useful. For example it's not viable to test electromigration for 10 years before releasing the product. In this particular case manufacturer says that particular accelerated test is not applicable.

author: alex-sh
author_rank: Regular Contributor
timestamp: April 22, 2019, 05:05:43 pm
content: Another vref similar to LT1236AILS8-5 (20ppm/√kHr LT drift) is LT6654LS8 ( 15ppm/√kHr LT drift)Any experience with this one?

author: daqq
author_rank: Super Contributor
timestamp: April 22, 2019, 05:39:19 pm
content: Could conformal coating be used to limit the influence of humidity on plastic references?

author: wraper
author_rank: Unknown
timestamp: April 22, 2019, 05:47:27 pm
content: Quote from: daqq on April 22, 2019, 05:39:19 pmI don't think it will make any serious effect. Basically any polymer pass some moisture.

author: Andreas
author_rank: Super Contributor
timestamp: April 22, 2019, 06:42:39 pm
content: Quote from: alex-sh on April 22, 2019, 03:42:48 pmThe interesting part of long term drift begins after 5-10 kHrs when the reference begins to settle (or not) and when you begin to see wether you have seasonal changes.Quote from: alex-sh on April 22, 2019, 05:05:43 pmnot from my side but: it is a bandgap based reference so typically factor 3-5 more noise like a buried zener and typically significant more ageing drift.so I would not call it "similar".with best regardsAndreas

author: alex-sh
author_rank: Regular Contributor
timestamp: April 28, 2019, 12:06:20 pm
content: Just one reference to long-term drift of LT1236 DIP8 or any other plastic vs LS8.

author: iMo
author_rank: Super Contributor
timestamp: April 28, 2019, 04:38:09 pm
content: Why they produce Vrefs in plastic packages then? It is not an Vref anymore, when packaged in the plastic. Frankly, I doubt the cerdip or metal can is so expensive to produce..

author: wraper
author_rank: Unknown
timestamp: April 28, 2019, 04:54:22 pm
content: Quote from: imo on April 28, 2019, 04:38:09 pmPrice, it does the job just fine in many applications. Ceramic is significantly more expensive, not to say ceramic should meet certain requirements to do the job properly.

author: Sumesh579
author_rank: Newbie
timestamp: April 30, 2019, 08:52:11 am
content: Quote from: ycui7 on April 20, 2019, 12:52:50 pmAwesome Reply!!

author: dmm2018
author_rank: Contributor
timestamp: May 07, 2019, 01:25:34 pm
content: I built a basic LTC6655BHMS8-1.25 circuit and found out that output voltage kept drifting...https://www.eevblog.com/forum/projects/1a-current-source-but-still-not-stable/msg2394411/#msg2394411

author: alex-sh
author_rank: Regular Contributor
timestamp: May 17, 2019, 08:09:52 pm
content: Quote from: dmm2018 on May 07, 2019, 01:25:34 pmHave you tested it longer than 1 hour?My LTC6655 does not drift much when powered up, but does drift when powered down.

author: Gyro
author_rank: Super Contributor
timestamp: May 18, 2019, 05:49:02 pm
content: QuoteThat implies that it suffers from hysteresis - there's no way to know if it's drifting while powered down, only power cycling it.

author: Andreas
author_rank: Super Contributor
timestamp: May 18, 2019, 06:30:39 pm
content: Hello,Quote from: Gyro on May 18, 2019, 05:49:02 pmHysteresis can be seen here:https://www.eevblog.com/forum/metrology/ltc6655b-long-term-drift/msg2355873/#msg2355873The MSOP8 package is worse than the LS8 package.Besides this: with 5 mA supply current there is also some heating up.with best regardsAndreas

author: alex-sh
author_rank: Regular Contributor
timestamp: May 18, 2019, 07:37:22 pm
content: Quote from: Gyro on May 18, 2019, 05:49:02 pmLet me put it this way - when I leave it running for several days (> 120h) it does not drift. Only when the temp is changing and this can be seen.Even if I power cycle it, it does not drift. However, when it stays powered down for a week, the voltage value changes.I really have to start logging it properly like Andreas, but what I can see right now I do not like it. I need to recalibrate the DUT every time the DUT (and vref) powered down for some time and this is just not right.

author: BravoV
author_rank: Super Contributor
timestamp: May 18, 2019, 07:41:51 pm
content: Watch out for humidity change too.

author: alex-sh
author_rank: Regular Contributor
timestamp: May 18, 2019, 07:47:48 pm
content: Quote from: Andreas on May 18, 2019, 06:30:39 pmVery much true.In fact I did not realise that MSOP8 would be that bad in the beginning.I am testing now LT1236LS8 with a proper pcb cut-outs.

author: alex-sh
author_rank: Regular Contributor
timestamp: May 18, 2019, 07:49:25 pm
content: Quote from: BravoV on May 18, 2019, 07:41:51 pmThis I can hardly control.I do not even have a lab with const temp / hum.

author: BravoV
author_rank: Super Contributor
timestamp: May 18, 2019, 07:54:35 pm
content: Quote from: alex-sh on May 18, 2019, 07:49:25 pmAlthough I haven't try it my self, may be one day, as curious what if the chip is sealed inside a glass tube. Of course you need to extend the pins longer like through hole components.Just an example ampoule glass sealing, google for your self, random youtube video to give your the idea.https://youtu.be/GEmINGq8HDQ

author: Gyro
author_rank: Super Contributor
timestamp: May 18, 2019, 08:19:47 pm
content: The problem might be getting sufficiently hermetic sealing of the leadout wires to the glass - wetting, different thermal coefficients of expansion etc.You'd be better with a small soldered copper sheet box construction with ready made hermetic feedthroughs, eg.https://www.ebay.co.uk/sch/i.html?_odkw=hermetic+feedthrough&_osacat=0&_from=R40&_trksid=m570.l1313&_nkw=glass+hermetic+feedthrough&_sacat=0You can also then pass normal copper wires through them to minimise thermocouple effects.