T.C. + Hysteresis measurements on brand new LT1027DCLS8-5 voltage reference - Page 1
Forum section: electronics / general

author: Andreas
author_rank: Super Contributor
timestamp: June 22, 2016, 09:38:02 pm
content: Hello,I have long waiting for the "ideal" 5V voltage reference for my 24 bit ADCs.- low T.C. < 1 ppm/K around 25 deg- low hysteresis < 1 ppm for +/-15 degC- power supply voltage below 12V (from around 10V on) for battery supplied devices.After having measured the LT1027CCN8-5 (DIP8 package)against other references I always wanted to have this chip in a hermetically package.The plastic package showed near ideal behaviour with respect to T.C. + hysteresis.But humidity sensitivity with 0.5ppm/% rH makes the DIP8 package nearly useless.Unfortunately the TO-5 package is no longer manufactured, so unobtanium.But since a few weeks the LS8-package was introduced:So I ordered some for some measurements...Measurement setup:The references are used for the VREF input of a 24 bit ADC which is connected with photocouplers to a PC for evaluation.Near the GND pin of the LT1027 a NTC senses the temperature of the reference. The reference is wired into a DIP socket.The input of the ADC (LT2400) is fed from a 7.2V stable voltage reference (LTZ#5) by a stable 2:1 capacitive divider (LTC1043).So the ADC is reading around 3600 mV. A positive drift of the VREF will give a negative drift on the ADC readings.The ADC + 2:1 divider are put in a peltier cooler together with a small heater foil.The LTZ#5 stable reference is outside the temperature box.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: June 22, 2016, 09:55:26 pm
content: First sample #1Wires soldered at evening before measurement.Measurement is started next morning without any pre-conditioning.I generally start from around room temperature to the minimum temperature.From there to maximum and back to room temperature.My "lab" heats up in the evening so I have problems reaching low temperatures in the evening.19.06.2016: (first run)around 140 uV drift over 25 deg Cgiving around 1.6 ppm/K with respect to the 3600 mV.on the first temperature cycle also a large drift at the beginning of the measurement.But all in all very low hysteresis below +/-2 uV including measurement noise.20.06.2016: (2nd run)again around 140uV drift over 25 deg C -> 1.6ppm/Kthe ageing drift against the first run is around 7uV or 1.9ppm/day initial drift after soldering.Hysteresis is much smaller than on the first run. (when neglecting noise).So this explains why some manufacturers specify the hysteresis being measured on the 2nd temperature cycle.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: June 23, 2016, 05:54:06 am
content: 2nd sample #2again no pre-conditioning after soldering.This time I had the time for cleaning the reference after soldering.21.06.2016 first runTC is relative good on this device 48uV over 25 deg gives 0.53 ppm/K with box method.There is also a "sweet spot" near room temperature with zero TC.Especially when regarding the self heating of the reference of about 2-3 deg C.Unfortunately the hysteresis is relative large (nearly +/-1 ppm) on the first run.But this can be also partly due to the initial ageing drift.22.06.2016 2nd runnoting to complain: the hysteresis has decreased to +/-0.5 ppm.T.C. als slightly decreased to 40uV over 25 deg C = 0.44 ppm/K.Initial drift around 1.7 ppm/day after soldering.So I guess this will even be better after a longer run in phase.This sample is the candidate that I am looking for my ADCs.I guess that I will get a relative good yield as long as LT sells only the "D" grade specced with 3 ppm/K.With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: June 23, 2016, 10:18:44 am
content: Wow, seems to be a really good candidate for the ADC.I today received two pieces of LT1027DILS8-5. The question is how they will compare to LT1027DCLS8-5.

author: branadic
author_rank: Super Contributor
timestamp: June 23, 2016, 06:49:39 pm
content: Sad they used a different pinout for LT1027LS8 compared to LT1236LS8, so a direct replacement is impossible and the ADC pcb needs a redesign. Same is true for LTC6655LS8Why? I thought Linear Technology had some ingenieurs working for them. Unbelievable.

author: Andreas
author_rank: Super Contributor
timestamp: June 23, 2016, 07:27:53 pm
content: Hello,yes it is sad: every LS8 package with different pin-out.Even worse if you put the 1027 in a 1236 design the output of the LT1027LS8 will short the batteryThere is only one advantage on the LT1027LS8: it is easier to make a cutout on the PCB since all relevant pins are on thesame side of the reference.With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: June 23, 2016, 07:54:32 pm
content: Andreas, you used the LT1027 only in 3 pin configuration, which means without noise reduction cap and without additional guard trace. So there is space for a bit improvement in noise as well.

author: Andreas
author_rank: Super Contributor
timestamp: June 23, 2016, 08:04:35 pm
content: Hello,the NR pin reduces mainly wideband noise.The 1/f noise is not affected much.Since I use a integrating (sigma delta) converterI do not worry much about wideband noise.The LTC2400 has about factor 3 more 1/f noise than the reference.With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: June 23, 2016, 08:43:47 pm
content: Well, when reading this: "...This pin is the most sensitive pin on the device. For maximum protection a guard ring is recommended. The ring should be driven from a resistive divider from VOUT set to 4.4V (the opencircuit voltage on the NR pin)..." I would at least add the guard, for maximum protection on that pin. But how to drive this guard? Possibly 3k + 22k works fine?

author: Andreas
author_rank: Super Contributor
timestamp: June 23, 2016, 09:21:47 pm
content: Hello,Even a higher ohmic divider would do the job. (and save current + self heating of the reference).2M2 + 300K would give for a 0.1uA (unrealistic high) leakage a shift of 30mV for the guard ring.If you use a trim pot you could derrive the 0.6V at the upper end by adding a series resistor to the pot.I for my part will isolate the NR pin by air. (or the ceramic package of the reference on the other side).Using my in hole dead bug mounting method.With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: June 26, 2016, 08:29:11 pm
content: Sample #3.1 day preconditioned (running at room temperature).T.C. around 0.8 ppm/K with sweet spot at 23 deg C (obviously for countrys north of my lab).First I thougtht of large hysteresis.But when looking closer to the diagrams it is in fact a ageing drift of around 2ppm/day of the regression curve.The ageing drift obviously takes mostly place (as steps) at higher temperatures.So I think sample #3 needs a relative long time to settle down with ageing drift.With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: June 26, 2016, 08:54:46 pm
content: Hello,I also did some 1/f (low frequency) noise (0.1-10Hz) measurements on the samples.Typical measurements attached below.I use 100 second measurement time.So this will give usually somewhat larger peak-peak values than in datasheet (more outliers).On the other side it is easier to detect popcorn noise with a longer measurement time.Perhaps a better comparison for noise will be the AC rms value (corresponding to standard deviation).LT1027DCLS8-5 #01 2.82uVpp or 354 nVrms 0.1-10 Hz noiseLT1027DCLS8-5 #02 2.23uVpp or 294 nVrmsLT1027DCLS8-5 #03 2.21uVpp or 312 nVrmsAs comparison some values of other references in hermetically LS8-package:LTC6655#1 (ADC20) 3.65uVpp or 430 nVrmsLT1236AILS8-5 #06 3.34uVpp or 414 nVrmsSo all in all the LT1027 samples show slightly lower 1/f noise than the other parts.The higer noise on the LT1236 can be explained by lower power consumption.0.8mA for the LT1236 against 2.2mA for the LT1027 at the same technology (buried zener).For the LTC6655 I always have larger noise values on my samples than the specced 0.25ppmpp (1.25uVpp) of the datasheet.But the measurement cirquit in the datasheet/AN124 has the wrong (too low) bandwidth as we discussed already with Lymex and Alex.https://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg939106/#msg939106So all in all the noise performance / mA (5 mA typical) is not the best for the LTC6655.With best regardsAndreasEdit: corrected attachments

author: Kleinstein
author_rank: Super Contributor
timestamp: June 27, 2016, 03:43:01 pm
content: For the power consumption, one also needs to take the supply voltage into account. The LTC6655 only needs about half the voltage compared to the buried zener references. Still the noise / power performance is not that good, especially for 5 V.With a low (e.g. 3.3 V) supply for a 2.5 V reference performance is not that bad.

author: Andreas
author_rank: Super Contributor
timestamp: June 28, 2016, 07:32:17 pm
content: Hello,measured sample #4.In my eyes it is defective ...I had 3 days preconditioning at room temperature hoping to get initial drift somewhat away.But T.C. and Hysteresis are relative high.- Box TC 200uV over 26 deg C giving 2.1 ppm/K- Hysteresis +/- 1.5 - 2 ppm- And even on the large T.C. a visible "noise" in the (integrating) measurements.The 1/f noise measurements show a catastrophic noise level of 23.1uVpp or 3709nV rms over 100 seconds.So a factor 10 over the normal level.The 10 second measurement shows something similar to popcorn noise.With best regardsAndreas

author: zlymex
author_rank: Frequent Contributor
timestamp: July 03, 2016, 04:09:03 am
content: Hello Andreas,Very nice setup and measurement. It seems to me that temperature curves of LT1027 have quadratic shape.Have you measured the noise of that 3.6V(50% of LTZ1000 by LTC1043)?

author: Andreas
author_rank: Super Contributor
timestamp: July 03, 2016, 08:03:30 am
content: Quote from: DiligentMinds.com on June 28, 2016, 08:23:29 pmHello,if I were LT I would want back all defective items for examination especially on "new" parts.But I will send #4 to Branadic, and hope he can do a photo.Quote from: zlymex on July 03, 2016, 04:09:03 amIf you look at the old 1027 datasheet there are several local minimums and maximums over the temperature range due to the LT1027 internal temperature compensation.So the quadratic shape is only true for a small (30 deg C) temperature range.Up to now I did no noise measurement of the LTC1043 output.The LTC2400 itself has large noise (1.5uVeff or 10uVpp) and I am usually measuring below that in a 10 second time window.So I have to use a 1 minute integration time to get stable values below 1uVpp.But this measurement is a interesting idea.Edit: but I will have to change probably to a LTC2057 amplifier instead of a LTC1050 with 1.6uVpp noise at gain = 1 on my LTC1043 board.With best regardsAndreas

author: zlymex
author_rank: Frequent Contributor
timestamp: July 03, 2016, 10:59:49 am
content: Hello Andreas,What I'd like to know is whether LTC1043 add significant noise when divide voltage by half.The 7.2V output of LTZ1000 has 1.2uVpp noise nominal, if step down to 3.6V by a pair of resistive divider, it will be around 0.6uVpp. If the performance of LTC1043 is ideal, we can expect similar result.

author: Andreas
author_rank: Super Contributor
timestamp: July 03, 2016, 02:17:25 pm
content: Hello,but with a LTC1043 you will need a buffer amplifier with low leakage on the input.Otherwise you will see a 400 Hz ripple (switching frequency) caused by the leakage current on the output.If you use a zero offset (drift) amplifier like LTC1050 as buffer this one will determine the noise level on the output.I guess I will have to built one divider with a LTC2057 with 0.2uVpp so that the LTZ has the highest noise level.with best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: July 05, 2016, 05:21:02 pm
content: QuoteI will take a photo of the die, that's for sureIt's up on the postman right now to bring the LT1027 to me.QuoteI already designed a 2:1 divider board with LT5400 and LTC2057.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 05, 2016, 07:37:47 pm
content: The noise of the reference chips and AZ OPs have a different frequency dependence: at low frequencies, like measured the a LTC2400, the references have mainly 1/f type noise. The AZ OPs and the LTC2400 are mainly white noise and thus the peak-peak value really depends on the upper limit of the frequency band, not so much on the lower limit.So one can not directly compare those numbers - it depends on the amount of averaging used. With the more realistic 0.01 - 1 Hz band, the noise of the AZ OPs will be about 1/3 of the 0.1-10 Hz band usually used in the DS. For the references the 0.01-1 Hz and 0.1-10 Hz band have comparable noise, thus the lower bound is also important.

author: Andreas
author_rank: Super Contributor
timestamp: July 05, 2016, 09:33:00 pm
content: Quote from: branadic on July 05, 2016, 05:21:02 pmSounds like "comparison"What will the integrating behaviour of the capacitive divider (LT1043)(ok it is "out of band" for the 0.1 - 10 Hz noise with 400 Hz chopping)be worth against the pure Johnson noise of a LT5400 resistive divider.With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: July 06, 2016, 12:09:29 pm
content: So here we are. This is a very first impression of the LT1027 die.It's pretty hugh for the former microscope with x5, so I had to use another one with x3.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: July 06, 2016, 05:43:50 pm
content: Andreas,The LT5400 does indeed have 1/f noise like all other resistors with the exception of precision wire wounds, they are the only resistor type that do not have 1/f noise.The LT5400 data sheet does not specify noise specifics, only an excess noise of <-55dB as a total noise within bandwidth, They do not specifically state that there is no 1/f noise in these resistors.

author: Gyro
author_rank: Super Contributor
timestamp: July 06, 2016, 07:02:05 pm
content: Quote from: branadic on July 06, 2016, 12:09:29 pmAn interesting die photo, thanks.I see there are quite a number of trimming fuses and networks associated with the test pads on the left hand side. I assume the burried zener is the round structure at the top edge next to the top left bond wire? Although there seems to be another half way down on the left hand side.It would be interesting to see what an AD587 (or 586) looks like in comparison.

author: branadic
author_rank: Super Contributor
timestamp: July 06, 2016, 07:25:15 pm
content: I will do some further pictures within the next days. Also a comparision of LT1236 und LT1027 in matter of die size.QuoteNo problem, if you want to send me an exemplar preferable already opened I can take some pictures of it

author: Gyro
author_rank: Super Contributor
timestamp: July 06, 2016, 07:28:41 pm
content: Ah, that might be a problem, the only one I have is nestling comfortably in my SVR-T.

author: branadic
author_rank: Super Contributor
timestamp: July 06, 2016, 07:39:35 pm
content: QuoteWell, AD586LQ is available at digikey for 13.24GBP, so if you want to invest this money in this photo it's up to you:http://www.digikey.co.uk/product-search/en?keywords=AD586LQI don't have an AD586 or AD587 that I want to immolate.

author: Andreas
author_rank: Super Contributor
timestamp: July 06, 2016, 07:56:49 pm
content: Quote from: branadic on July 06, 2016, 12:09:29 pmHello branadic,Thanks for the photo.The large die might be an explanation why it took so long to package the LT1027 within the LS8 package.(a LT1236 might be much cheaper in production).For the photo a JQ-version of the AD58x should do the job.But I am shure that I can find a sample within my AD586LQ which has a too large T.C. for my purposes.(Should have thought about that before sending the LT1027#4).With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: July 06, 2016, 08:31:01 pm
content: QuoteYou can send it every time, if you want to. To be honest, I don't have an idea how to crack them open without damaging the die. Maybe a short stroke with a hammer will crack the glas sealing?

author: Gyro
author_rank: Super Contributor
timestamp: July 06, 2016, 08:59:14 pm
content: I cracked open many a CERDIP package as a kid (lots of unmarked IC bargain packs). If you find a corner where the frit seal doesn't extend right into the corner (or scrape some of it away) then inserting and twisting a flat blade screwdriver will cleanly separate the two halves without damaging the die. This used to work 9 times out of 10 anyway. Sometimes some of the legs would come away with the top half, but more often than not the entire leadframe would remain attached to the bottom half with bond wires intact.

author: Andreas
author_rank: Super Contributor
timestamp: July 06, 2016, 09:21:11 pm
content: there are 2 recommendations here:http://semitracks.com/reference-material/failure-and-yield-analysis/failure-analysis-package-level/delid-and-decap.phpor this way:https://www.eevblog.com/forum/projects/experiment-buying-obscure-obsolete-ic-via-chip-broker/msg864675/#msg864675with best regardsAndreas

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: July 06, 2016, 10:31:05 pm
content: Quote from: branadic on July 06, 2016, 07:25:15 pmthanks to the both of you. I would like a pad/pin numbering on the photos for numpties such as myself. That is all the people need paint by numbers to follow what is going on.

author: branadic
author_rank: Super Contributor
timestamp: July 07, 2016, 06:59:28 am
content: So here is a direct comparison of LT1236LS8 (left) and LT1027LS8 (right), with pin 8 showing to the left (NC @ LT1236LS8 and Vin @ LT1027LS8).

author: TiN
author_rank: Super Contributor
timestamp: July 07, 2016, 09:31:10 am
content: Great shots, thank you.

author: Gyro
author_rank: Super Contributor
timestamp: July 07, 2016, 09:46:50 am
content: Quote from: branadic on July 07, 2016, 06:59:28 amThey really had to push those bond wires didn't they!

author: branadic
author_rank: Super Contributor
timestamp: July 07, 2016, 07:29:30 pm
content: Not really, but as you can see, the die is nearly twice the sice of LT1236.And there is still enough space inside the cavity. Maybe LM399-LS8 comes next, followed by LTZ1000-LS8I took also some better die photos, but as the die is pretty hugh and the good microscope has x5 it tooks two pictures to visualize the chip.

author: Gyro
author_rank: Super Contributor
timestamp: July 07, 2016, 07:37:50 pm
content: Wow, impressive! You can certainly see the fuses and resistors now.

author: d-smes
author_rank: Regular Contributor
timestamp: July 08, 2016, 01:41:15 pm
content: Why didn't they shift the die to the left in the LT1027LS8 photo to make the wire bonds easier? Does it have something to do with the black square at the lower left? What is that anyway?

author: TiN
author_rank: Super Contributor
timestamp: July 08, 2016, 02:04:42 pm
content: It's because that patch is unplated ceramic substrate. Die need to be soldered down, and that can be done to metal surface only, hence the gold plating under the chip. Making bond wire 10mm longer is still cheaper than design and manufacturing bit different package for new chip, so they reused same one best they could.It would be interesting to see LM399/LTZ refresh, but that's unlikely to happen due to amounts validation work required and size of that market.

author: branadic
author_rank: Super Contributor
timestamp: July 08, 2016, 04:50:36 pm
content: QuoteI'm pretty sure the die is not soldered into the ceramic, but glued with silver filled adhesive. Soldering requires a backside metallization (gold). Hence backside metallization is an additional process and metallization of the die is normally aluminium metallization with gold is a special-purpose step.The indicated patch is probably for automated pick and place of the die. Squares are easy to identify with a camera as it acts as a marker inside the package.QuoteWell, you can't extend bond wires infinity. At a certain lenght they will sag.And I'm sure they know why they have decided for LS8. LT1027LS8 uses only 5 pins, LT1236LS8 uses only 4 pins and LTC6655LS8 uses all 8 pins but 4 of them are GND (we haven't open one yet to see the inside).So maybe they choose for LS8 5x5mm² 8pin with the idea that many reference chips can be placed inside. And I'm sure there will be a batch of voltage references in LS8 package in the future. Why not LMx99 or LTZ? They only have to make sure they can fullfil the given datasheet of the devices.On the other hand similar packages are already available for acceleration sensors. I vouch for that the package is made by Kyocera.

author: TiN
author_rank: Super Contributor
timestamp: July 08, 2016, 05:27:51 pm
content: Also fixed image for your into single:Can I use on my site for reference die photos collection? Your shots are great, I should invest time to connect my DSLR to microscope to do similar ones.I have 6655LS8, will open one to take some photos once tests done (in some distant future...)...

author: branadic
author_rank: Super Contributor
timestamp: July 09, 2016, 12:25:14 pm
content: QuoteWell done, thank you very much. Hadn't had the time for it yet.QuoteSureQuoteNot a big deal, the microscope with cam is a Leica solution. A trained monkey can do such pictures as well, it's pretty simple. But thanks.

author: David Hess
author_rank: Super Contributor
timestamp: July 09, 2016, 08:34:14 pm
content: Quote from: Andreas on June 23, 2016, 08:04:35 pmReally? The 1/f corner for the LT1027 is not particularly low and I thought delta-sigma converters had flat 1/f noise because of input chopping.Hmm, unfortunately Linear Technology does not include a noise spectrum graph in the LTC2400 datasheet.Air currents can cause excess noise at low frequencies through thermocouple effects considerably larger than low frequency noise in low noise chopped and non-chopped inputs. Jim Williams liked to use Dixie cup enclosures to ameliorate this.

author: Andreas
author_rank: Super Contributor
timestamp: July 09, 2016, 10:08:09 pm
content: Quote from: David Hess on July 09, 2016, 08:34:14 pmHello,the noise spec in data sheet is 0.3ppm.This value is for 5 V reference with a shorted input.And of course the value is a RMS value.So the 1.5uVeff turn out to be 9-10uVpp.Which compares to the 3uVpp of the LT1027.(I searched long time for the "error" in my cirquit having around 10uVppuntil I found the 1.5uV are a RMS value).The LTC2400 is not very low noise due to its low power consumption.But the biggest advantages are the automated zero and full scale adjustments on every conversion.So the LTC2400 itself is very stable over temperature.And in my measurements I usually have the time to integrate over a full minute to reduce the noise.With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: July 10, 2016, 08:36:55 am
content: QuoteWell, a spectrum would for sure be very interesting. Maybe Andreas can provide one, elsewise I will do some measurements in the future.

author: branadic
author_rank: Super Contributor
timestamp: July 10, 2016, 11:02:21 am
content: Can you provide a spectrum?

author: Andreas
author_rank: Super Contributor
timestamp: July 10, 2016, 09:09:15 pm
content: Quote from: EmmanuelFaure on July 10, 2016, 10:19:20 amMhm,and how does this fit to 1.5uV RMS noise for the LTC2400?under which conditions measured?How is the uVpp noise under those conditions.With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: July 10, 2016, 10:05:29 pm
content: Hello,a noise measurement of LTC2400 (with 2:1 LTC1043 input divider) in mV (5V input range referred to input of ADC).1505 measurement values (within 5 minutes). so 5Hz sampling frequency.the amplitude over time shows mostly around 10uVpp with some events (14uVpp).FFT shows around -72dB (mV) from 0.02 to 2.5 Hz.With best regardsAndreas

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2016, 10:28:44 pm
content: Quote from: Andreas on July 10, 2016, 09:09:15 pmIt is hard to tell from the datasheet but the 1µV/sqHz EmmanuelFaure measured looks consistent with the noise specifications in the datasheet when the sinc filter shown in the "Rejection vs Frequency at Vin" figure is considered. I had taken the noise of these converters for granted but as the datasheet points out, this is the output noise and "The output noise includes the contribution of the internal calibration operations."I find it fascinating to consider ways to improve the noise performance. What is the easiest way now to implement a home grown 7 or 8 digit converter these days?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 11, 2016, 06:19:43 pm
content: The default speed of the LTC2400 is about 6 readings per second. So at 1 µV/ sqrt(Hz), a 6 Hz Bandwidth should give about 2.5 µV RMS noise. So my guess is the measured 1 µV/ sqrt(Hz) value includes some extra noise sources, like from the reference (not measuring at 0) or not so good anti aliasing filtering. It is also possible that noise gets higher if not measuring at 0, due the adjustment procedure. The 1.5 µV_rms specs should correspond to about 0.6 µV/Sqrt(Hz). The conversion from RMS to peak - peak values depends on the bandwidth / frequency ratio. As the readings are essentially independent it's usually a factor of 5 to 7.There are other integrated sigma delta ADCs with a lower noise value, but often higher INL. Some if the high resolution (e.g. 20-24 Bit) SAR ADCs from LT could be a option, though possibly with 1/f noise. Single reading noise is high, but with some averaging / integration a line period really low noise levels are possible.Using a dual slope ADC with residual ADC is tricky, as dielectric absorption will severely limit INL, unless an exotic integration cap is used (e.g. Teflon, vacuum). So this would usually need a kind of multi slope variation, e.g. similar to the 34401. This would be more like 20 Bits from the up / down slops and maybe 12 Bits from the residual and some loss due to overlap. It's not just using many slopes, but low loss caps are much easier at 1 nF than at 100 nF.

author: Andreas
author_rank: Super Contributor
timestamp: July 11, 2016, 09:42:27 pm
content: Hello,Quote from: David Hess on July 10, 2016, 10:28:44 pmI can only tell how I do it:Select the best of a batch of 5V references. (T.C. < 1ppm/K and hysteresis < 1 ppm in near room temperature).Combine with a LTC2400 and a LTC1043 divider.Put a uC to the whole for transfer data to a PC and hold calibration constants and measure temperature of the reference.Adjust NTC for temperature measurement (is optional since the true temperature is not essential).Adjust T.C. of VREF with 3rd order calibration to better 1 ppm over a 10-40 deg C temperature range.Adjust linearity to better 1 ppm with simple square approximation of the error curve.Optional adjust Offset over temperature.Adjust full scale (nominal VRef value at 25 deg).Age the whole for 5000-10000 hrs.Eventually repeat the calibrations.What do you get after 2-3 weekends adjustment and the run in phase:A unipolar +0..10V input range (up to now not very high impedant).around 1-2uV resolution with a integration time of 1 minute.(so more a old style Solartron than a modern instrument)Better stability than a typical 6.5 digit instrument.Allan deviation shows around a factor 5 worse than a 3458A.(ok the 3458A is much faster with 100NPLC compared to 1 minute integration time).Standard deviation against a LTZ1000A over 42 days is around 0.25ppm for my best devices.Drift on ADC13 is nearly the same as on my LTZ#1.(around 1 ppm/year + additional 1 ppm seasonal changes between ADC+LTZ1000A due to humidity).With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: July 11, 2016, 10:03:16 pm
content: Hello,its time to go on topic again and show some overview over the measurements:From 10 references I found 3 (eventually 4) usable ones for my purposes. (marked green).(T.C. < 1ppm/K at least in the 18-32 deg C range and hysteresis < 1 ppm).The "hysteresis" on these devices is mostly a large initial drift of 2 ppm/day,where I hope that it gets reduced after some more temperature cycles.I tryed several preconditioning either with running some days at room temperatureor (with asterisk) with a 15 mA load which is switched on 1.5 hrs within a cycle of 2 hrs.I will have to compare the results with my other references especially with the AD586LQ and the LT1027CCN8-5.In every case the LT1027DCLS8-5 (average T.C. around 1.4 ppm/K and Hysteresis around/below 2ppm)is much better than the LT1236AILS8-5 with around 3 ppm/K and larger Hysteresis.But I fear that the LS8 package is not so good compared to a metal can package.Especially when mounted directly on a PCB.So beware of a LM399 or LTZ1000 in a LS8 package.Unfortunately I have no LT1027 in metal can package to compare with.With best regardsAndreas

author: zlymex
author_rank: Frequent Contributor
timestamp: July 12, 2016, 02:40:52 am
content: Speaking of preconditioning, there was one article by V. S. Orlov(from Datron? Attached) describing a method called 'soft' thermal shock.Some one tried this with good result(http://bbs.38hot.net/forum.php?mod=viewthread&tid=25193)

author: Andreas
author_rank: Super Contributor
timestamp: July 17, 2016, 10:03:57 pm
content: Quote from: zlymex on July 03, 2016, 04:09:03 amHello,now I didFirst results:Setup: LTZ1047B#4 with LTC1043 divider followed by a LTC2057 buffer amplifier.The LTC2057 buffer has a 825R series resistor + 820pF at the output. (Improves noise for the LTC2400)When used with the 0.1 .. 10 Hz filter amplifier with a input impedance of 1000R this gives a voltage divider with factor 1.825 which has to be considered for the readings.First picture 1/f (0.1 .. 10 Hz) noise of LTZ#4 (around 1200nVpp) with 2:1 dividerreading 545 nVpp * 1.825 = 994 nVpp at output of unloaded divider.So around factor 1.6 against a noiseless divider. (would be 600nVpp)2nd picture: 1/f noise of input from LTC1043 divider shorted to GND.reading 213 nVpp * 1.825 = 389 nVpp at output of unloaded divider.(noise floor of the filter amplifier alone is < 200nVpp).3rd picture: wideband noise 10Hz ... 100kHz as FFT.Around 350 Hz the switching frequency of the LTC1043 with -117 dbV (1.4 uV)I long searched for the "switchmode power supply" at 80 kHz until I recognized that this is in reality the (100kHz typ.) chopper frequency of the LTC2057 buffer amplifier. Amplitude -67dBV (450uV).with best regardsAndreas

author: zlymex
author_rank: Frequent Contributor
timestamp: July 21, 2016, 05:51:29 am
content: Thanks very much Andreas for the test.It seems to me that LTC1043 added not much noise when divided by 2, a good sign.The question is, how to calculate the noise added.1. by multiplication factor? 994nV/600nV=1.66. However, it cannot be apply for shorted input where 389nVpp/0nVpp has no meaning. Or it should be 389/200=1.945?2. by Root-Sum-Square? sqrt(994^2-600^2)=792nVpp, much higher than sqrt(389^2-0^2)=389nVpp3. by simple addition/substraction? 994-600=394nVpp, 389-0=389nVpp, although agreed, but I suspect that this is the way to calculate.

author: Andreas
author_rank: Super Contributor
timestamp: July 21, 2016, 07:51:05 pm
content: Hello,if the noise sources are independent they should add as root sum squares.But I think there is some charge injection from the LTC1043 switching capacitortowards the buffer op amp of the LTZ1000 reference. So maybe this is non linear.By the way: I am matching the capacitors of the LTC1043 to be within 1% (typical 0.1-0.2%) to minimize settling time.With best regardsAndreas

author: zlymex
author_rank: Frequent Contributor
timestamp: July 22, 2016, 02:25:16 am
content: Hello Andreas,Were you suggesting that the noise generated by LTC1043 depends on the input voltage?That is to say, the noise is smaller when input shorted?

author: Andreas
author_rank: Super Contributor
timestamp: July 22, 2016, 04:26:03 am
content: Hello,no I think more that the noise of the switches of the LTC1043 inputinfluences the buffer amplifier o(LTC2057) f the LTZ1000 output.(will do further measurements on week end).With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: July 28, 2016, 09:49:34 pm
content: Quote from: Andreas on July 22, 2016, 04:26:03 amHello,obviously that is not true.If I measure the noise at the output of the LTZ there is no significant differencebetween idle LTZ and LTC1043 connected to the buffered output. (first row of measurement table).I also removed the 825R series resistor on the LTC1043 buffer to see wether the voltage divider against my 1K input impedancehas a influence. (column 1 vs column 2 of measurement table).Also no significant change. (Except the fact that ~230nVpp are very close to the noise floor of ~120nVpp of my filter amplifier)So all in all the buffered LTC1043 adds around 370-400nVpp on the output signal.With best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: July 29, 2016, 03:12:12 pm
content: For comparing noise measurements, it's often better to use the RMS values instead of the peak to peak values. They are scattering less. For peak to peak measurement more data would be needed to reduce scattering. Also some kind of median / robust mean (drop the larges and smallest quarter and average the rest) would be better than the plain average.For the shown data, there seem to be quite some interaction of the LT1043 stage and the LTZ reference:With a shortet input, the LT1043 stage gives a noise of about 50 nV(RMS), including the output buffer amplifier. With the LTZ as source, the noise increases from an expected 76 nV to about 131 nV. Thus the additional noise would be at about 110 nV (due to adding as squares !). So most of the noise would be from the interaction.One reason would be that the noise of the LT1043 stage depends on the input voltage. It might be interesting to use a different input source to the LT1043 stage instead of the LTZ. For the noise measurement some batteries / NiCd accumulators might be worth a measurement.

author: Andreas
author_rank: Super Contributor
timestamp: August 01, 2016, 09:41:19 pm
content: Hello,Quote from: Kleinstein on July 29, 2016, 03:12:12 pmthats why I give both values. The Vpp for comparison with the data sheet. The RMS value for easier comparison.Quote from: Kleinstein on July 29, 2016, 03:12:12 pmGood Idea.I just checked this with 2, 4 and 6 NiMH cells (2.7V 5.3V and 8.0V).The result shows no significant dependancy from the input voltage.I needed a while to get reproducable results with this measurement.The reason are the different battery holders with springs (most probably steel)and thus thermo voltages up to 3uVpp measured at the output of the voltage divider.And this even within my metal cookies box!Surrounding the battery holder with a microfibre cloth and wiggling the contactsbefore measurement gave reproducable results in the 400nVpp range.So noise measurements also depend much on the mechanics.With best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: August 02, 2016, 07:31:44 am
content: So the LT1043 by itself seems to be not adding extra noise. But there still is the higher noise, when dividing the LTZ signal.The way the Lt1043 divider circuit is build, it is sampling the input only for sort intervals, just before switching. This will make it sensitive to noise at higher frequencies. So noise in the higher frequency range (e.g. around 1 kHz) might get mixed down to the LF band and thus this way increase the noise. So it might be a good idea to have an low noise filter at the input of the LT1043 divider. Also a series resistor to limit the speed how fast the capacitor is recharged could be an important point to limit the effective bandwidth.

author: Andreas
author_rank: Super Contributor
timestamp: August 02, 2016, 06:50:39 pm
content: Quote from: Kleinstein on August 02, 2016, 07:31:44 amThe LTC1043 itself has a large series resistor within the switches (300-700 Ohms).So with the 1uF capacitors there is already a low pass in the range of 300 Hz.Of course there is some charge injection outside the series resistors.With best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: August 02, 2016, 08:03:48 pm
content: The on resistance and the 1 µC capacitance helps to a certain degree, but there is still some higher frequency noise from the LTZ to be mixed down to the LF range. With something like a 300 Hz band limit my estimate is not to expect that much noise, but still a small contribution. It might be still worth to add some filtering (e.g. 1 K and 10 µF) between the LTZ and divider, as the low noise NIMH cells did show the much lower noise level. Good filtering without adding resistor noise or drift / offsets due to capacitor leakage might be tricky and it can also interact with the buffer amplifier.I don't think charge injection should be the problem - this is more like a source of offset and possibly drift.The charge injection current might limit the amount of RC filtering that is useful.The other possible source could be RF noise going back from the LT1043 to the LTZ reference / buffer amplifier and causing trouble there.

author: Andreas
author_rank: Super Contributor
timestamp: August 03, 2016, 09:18:35 pm
content: Hello,did a simple test to check the charge injection: I shorted the 10nF capacitor of the R/C-Oscillator of the LTC1043 to GND.(Stopping the oscillator and thus all switching of the LTC1043).Result: dramatically increasing (factor 2) of the noise.So the only explanation is the current noise of the LTC2057 buffer amplifier.(The input node of the OP gets high ohmic by stopping the oscillator).After removing all shorts I got again the normal values.(ROut = 825 R so correction factor 1.825 is again needed).I also checked a 2nd sample with LTC2057 buffer amplifier (V10_1501#2).This sample had even with working oscillator nearly double the noise of sample #1 (V10_1501#1).The standard cirquit with LTC1050 (sample V10_1102#2) shows 1.75uVpp together with the LTC1043.(Datasheet value with low impedance input is 1.6uVpp for the LTC1050 alone).With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 04, 2016, 06:45:13 am
content: Quote from: DiligentMinds.com on August 03, 2016, 10:33:53 pmHello Ken,I have a unitiy gain cirquit. So the capacitor in my case is a zero ohms resistor.I do not think that I can improve this.Do you have other experiences?With best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: August 04, 2016, 09:29:42 am
content: I won't expect current noise to be a major issue, as the switched capacitor stage is still reasonably low impedance (e.g. < 1 K range). This gets different with the clock stopped or a much lower clock.The difference between the two circuits could be due to different clock speed or on resistance of the switches.To reduce the effect of mixing down noise from the input to the LF range one might consider using both half's of the LTC1043 chip, used with opposite phase. This would sample the input nearly all the time and thus might reduce feed through of noise from the input.

author: Andreas
author_rank: Super Contributor
timestamp: August 05, 2016, 09:27:03 pm
content: Hello,I have to correct my measurement from 02.08.2016:(The difference between LTC2057#1 and #2 sample did not let me sleep).I recognized that on sample #2 of V10_1501 the oscillator did not work.Reason was a defective 10nF capacitor for the oscillator of the LTC1043.So the noise corresponds to the measurement of #1 with shorted 10nF capacitor.After repair the noise is around 430nVpp so very similar to sample #1.Quote from: DiligentMinds.com on August 04, 2016, 05:25:15 pmAttached the cirquit of the buffer stage.the +Input is from a 1uF foil capacitor.When the oscillator of the LTC1043 is stopped by shorting the 10nF capacitor (not shown)then the flying capacitor is in parallel with the 1uF output capacitor.(The switch is shown in the "high" state of the oscillator pin.Edit: in the actual build with LTC2057 C28 is populated as 0 Ohms Resistor and R26 is not populated.Quote from: Kleinstein on August 04, 2016, 09:29:42 amUnfortunately there is only one oscillator pin for both switches.And I do not really know what happens if you exchange input with outputbecause of the charge injection compensation cirquitry.At least the "gain" of the cirquit changes if you do that.with best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2016, 07:39:54 am
content: The charge injection circuit could add a little offset / gain difference. Combining the two parts would than result in a small modulation at the output. But this should not be so much with the relatively large capacitors.The advantage would be a much lower (e.g. a factor of 10-100) sensitivity to noise from the band around the clock frequency.With the rather low clock frequency (10 nF should result in about 300-400 Hz). The clock frequency is about in the cross over range of the filtering through on-resistance and the caps. So it might be worth testing the noise with a faster clock (e.g. 2 nF cap). A higher frequency would also make extra external filtering more practical, without adding to much resistance.What is the reason to choose this low frequency ?

author: Andreas
author_rank: Super Contributor
timestamp: August 06, 2016, 09:54:13 am
content: The low frequency isa) recommendation (10nF) in the data sheet. (typical 400-500 Hz)b) after my experiences the precision parameters (linearity) get worse if you change any of the capacitor values.With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 09, 2016, 09:13:39 pm
content: Hello,to come back onto topic: the rest of my LT1027DCLS8 measurements.From 25 references there are 4 good and 3 "usable" references regarding T.C.Interestingly: the best 4 all show lower readings of LTZ#4 voltage. (so have VREF at the upper end of the distribution).The other way round is not true so a higher output voltage has not necessarily a lower T.C.In average the T.C. is around 2 ppm/K (against 3 ppm/K for the LT1236 in LS8-package).The measured "hysteresis" is mostly a ageing drift which takes place at higher temperatures.I hope that this drift will settle down after a few cycles.I will build 4 ADCs (2 with PCBs from branadic and 2 with my stressless mounting method) to check ageing drift.All in all I am not so lucky with the LS8-package. There seems to be a problem with the die attach which creates those relative large ageing drifts/hysteresis.I will have to compare with my AD586LQ and LT1027CCN8-5 measurements. But from remembering the LT1027 in plastic package had far low "hysteresis".And the AD586LQ had lower T.C. in average.with best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: August 10, 2016, 03:53:58 pm
content: Die attachment with some kind of epoxy glue could cause some drift / aging. Typical epoxy has is amorphous with a glass transition temperature somewhere around 100-150 C. So heating above that temperature (e.g during soldering) can cause a new aging cycle and the speed of cooling can have an influence. Ideally one would expect a more stable behavior when cooling down to about 70 C is slow. After fast cooling there could be quite some structural relaxation in the glue.For the LTZ1043 it could be just charge injection that causes more nonlinearity / errors, when the frequency is higher. Adding resistance to get more filtering would also increase the errors due to change injection. So the way around would be more with larger caps than the 1 µF - but this makes things bigger. Still 2.2 µF or 4.7 µF may be still acceptable as film caps.

author: Andreas
author_rank: Super Contributor
timestamp: August 13, 2016, 09:52:45 pm
content: Hello,just built the first 2 ADCs with the new references.2 different designs.One older from me which I used already with LT1236 (the upper on the overview picture).And one PCB with design from Branadic.The PCBs with differences in detail:Branadics PCB contains already a pre-regulator 10.5V and the USB-Interface on board.The mounting of the LT1027DCLS8-5 (#7 of the measured ones) is done withthe recommended cut outs according to data sheet / application note.My design uses external pre-regulator (9.31V on the picture but will replace with 10.5V).USB-Interface is externally done.Mounting of the LT1027DCLS8-5 (#2 of the measured ones) is done with mystressless in hole mounting method.Software is flashed on both devices.I will have to calibrate the NTC temperature measurement and then the T.C.of the reference with final thermal shielding.And finally the linearity of the ADC needs to be compensated.With best regardsAndreas

author: David Hess
author_rank: Super Contributor
timestamp: August 14, 2016, 04:11:30 am
content: That galvanically isolated USB interface is a work of art. I just about jumped up to cheer when I saw it. \o/

author: Andreas
author_rank: Super Contributor
timestamp: August 19, 2016, 12:32:04 pm
content: Hello,after having made one stability measurement with the first device (ADC21) without T.C. calibrationhttps://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg1004639/#msg1004639I also made a measurement after T.C. calibration of my own design.I hoped that the stability would be below 0.25uV (after 2:1 divider).But obviously there is still a large ageing drift (independant from temperature).From the Allan deviation the standard deviation goes up to 0.5uV (2:1) or 1.0uV(before divider) for measurement times of 100 minutes.I hope that the drift of the LT1027DCLS8-5 will settle downto the values that I am used from my AD586 devices.With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 19, 2016, 12:52:18 pm
content: Hello,just as comparison some measurements from other ADCs (around 7.5 hours):ADC13, ADC15 and ADC16 are based on temperature compensated AD586 voltage reference.Allan deviation stays below 0.25uV (after 2:1 divider) or 0.5uV (before divider = 10V range) even for long integration times.ADC17 carries a LT1236AILS8-5 (also a LS8 SMD ceramic package as LT1027DCLS8-5)Here the Allan deviation goes up to 2 uV (after 2:1 divider) or 4 uV (before divider) above 200 minutes integration time.Finally a K2000 and a HP34401A instrument in differential mode(measuring ~41mV difference of LTZ1-LTZ2 voltage reference in 100mV range).They start with below 0.1uV (absolute) stability for short integration times.But go up to 0.5uV for longer measurement times.So the stability of my AD586 ADCs in 10V absolute range is around thatwhat can be reached with a 6.5 digit multimeter as differential measurement in 100mV range.With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 24, 2016, 09:02:22 pm
content: Hello,and here the T.C. adjustment of the 2nd ADC (ADC#23 with LT1027DCLS8-5#02).This time with reference soldered on a slotted PCB according to application note. (IMG2446w.JPG from above)There is a large hysteresis of around 3 ppm. (Measurement of 20160818)During selection within ADC09 this hysteresis of same reference was much smaller (below 1ppm).(see measurement 20160622)So most probably the datasheet spec for the hysteresis is mainly caused by the influence of the PCB board material.As comparison the "free floating mounted" ADC21 with LT1027DCLS8-5#07. (IMG2447w.JPG from above)After soldering to the final ADC the hysteresis is still around 1 ppm.(measurement 20160814).Similar to the measurement (with wires) during selection in ADC09 (20160703)and also similar to the selection measurement of reference #2 within ADC09 (20160622)With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: August 27, 2016, 08:20:49 pm
content: QuoteThis is with the guard trace connected to ground right? Could this be a result of your configuration? Is there a chance to test your setup with the populated resistor divider for the guard?

author: Andreas
author_rank: Super Contributor
timestamp: August 28, 2016, 05:21:33 am
content: Hello,on ADC23 the guard trace is accidently shorted to the outputwhich has nearly the same voltage than the NR pin so theinfluence should be nearly zero .(you can see this on the photo IMG2446w.JPG)I already tried to remove the short during solderinbut I would have to do a complete reworkwhich again introduces drift to the device.What I cannot guarantee is that there is no flux residueunder the part with my (mostly hand) soldering method.I am tinning the device first (with solder wire and enough flux)and then do a hot air reflow soldering.Perhaps you have more luck if using special solder paste.I am more and more convinced that the LS8 package does it notmake easy to get stabilities in the 1 ppm or sub 1 ppm range.With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 28, 2016, 09:51:10 am
content: Hello,another hysteresis measurement from the 2nd slotted PCB (ADC24).This time 5.6 ppm hysteresis (including ageing drift) at 25 deg.But also this is significantly larger than during wire hook up on ADC9 of the LT1027#17 during reference selection.(around 2ppm mostly drift).So for me the influence of the PCB on hysteresis gets systematic.With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 28, 2016, 10:03:44 am
content: Last but not leastHysteresis from ADC22: the 2nd dead bug (in hole) mounted LT1027.Here the hysteresis is below 0.5ppm at 25 deg C.During selection of LT1027#03 within ADC09 there was also a large ageing drift.(so drift + hysteresis around 2 ppm).With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: August 31, 2016, 07:41:33 pm
content: How can you be sure that desoldering and resoldering hasn't change the conditions of the LT1027LS8?

author: Andreas
author_rank: Super Contributor
timestamp: August 31, 2016, 08:00:20 pm
content: Hello branadic,I have here the fact that on 2 samples (with dead bug style mounting)the hysteresis has not changed (even with some soldering).Of course I have some drift of the output voltage between the measurements before and after.On the 2 samples with slotted PCB the hysteresis is increased significantly.So I see a good correlation (of course no proof) between mounting and hysteresis.Do you have different experiences?With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: September 02, 2016, 04:29:04 pm
content: QuoteNo, I'm just thinking loud

author: Andreas
author_rank: Super Contributor
timestamp: September 03, 2016, 02:35:03 pm
content: Hello branadic,it´s often good too ask some questions.There is a risk for getting blind for the things I do as a habit with my ADC´s.This week someoneasked me about the discrepancy of the decoupling capacitor for the LTC2400 and the LT1027.In LTC2400 datasheet a 100nF ceramics and a 10uF Ta is recommended.In LT1027 datasheet either a 4.7 uF or no decoupling capacitor at the VREF output is recommended.The LT1027 datasheet tells that with a 100nF capacitor the output ringing will be a maximum.In my cirquits I use 100nF and 10uF as a habit (for critical debouncings).The intention here is that the 10uF Ta with its ESR of around 2 Ohms acts similar to a snubber network (Boucherot)to dampen any oscillations created by a ceramics capacitor (with wire inductivity).But after the question I was not shure: will this work in any case and also with the LT1027?-> I had to check this.I tested 3 different configurations together with ADC24 (slotted PCB).VREF1: 100nF X7R + 10uF Ta Size A (initial configuration)VREF2: 10uF Ta aloneVREF3: 100nF X7RVREF4: same as VREF1 (after all changes)The 10uF Ta is a RS-components 684-4443http://de.rs-online.com/web/p/tantal-elektrolytkondensator/6844443/I measured VREF with a 200 MHz BW (+10:1 probe with 250 MHz BW)oscilloscope with short connection to the D-Sub connector (see photo)to see any oscillations of the reference.In parallel I measured LTZ#4 via 2:1 divider with the ADC.First surprise: there are large spikes (200-300 mVpp) with risetimes in the 2-3 ns region(near the BW-limit of the scope = 2 ns) from the internal 153 kHz clock of the LTC2400.This is something that I never expected from a analog cirquit.To the scope pictures:Overview shows the first 5 ms of the conversion.There is a large first spike (start of conversion) followed by some smaller spikeson each edge of the 153 kHz clock (mostly switching noise of VREF input).Zoom 500x10 (x = 500 y = 10) shows the conversion clock.VREF3 (100nF alone) shows large oscillations between the clock.The other configurations do not differ much.Zoom 10k (x = 10000) shows a single edge of the conversion clock.The ringing is for the 10uF and 10uF + 100nF configurationshorter than the 460ns datasheet spec.So from the scope pictures there is not much difference between the10uF and the 10uF + 100nF configuration.The maximum spike amplitudes seem to be 3-5% smaller with thecombination of the capacitors.On the 100nF alone configuration the ringing is clearly visible.(see also table).The comparison of the ADC24 readings shows a factor 2-3 increased uVpp noise for the 100nF alone capacitor.The readings of the LTZ#3 are also increased by 170 uV (=47ppm) in VREF3 against the VREF1 configuration.But also the VREF2 configuration (10 uF alone) has a increased reading of 7.5 ppm against VREF1 configuration.So which one is nearer on the truth?This gets clearer when measuring the own 5V reference through the 2:1 divider on the ADC input.In this case the theoretical reading should be exactly 2500 mV.In practice there is some loss in the 2:1 divider (which can be seen by the HP34401A high impedance measurement).The actual value is around 20uV or 7.6 ppm lower than the half value.So the nominal reading in this case should be 2499.981 mV.VREF2 is +9.7 ppm too highVREF3 +30.5 ppm andVREF4 is -3.5ppm (low)I not really thought that the decoupling capacitor influences the readings (INL)that much regarding the dynamic resistance of the VREF in the mOhm range.So there is shurely room for some improvement by the right placement and value of the decoupling capacitors.with best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: September 03, 2016, 03:42:16 pm
content: This is really quite some spikes on the reference voltage. The spikes are also really fast - so the layout might be important too. The LTC2400 does not have many pins, so there could be also some interaction with the decoupling of the supply and ground, not only the reference input.I am also surprised to see such fast signals with a supposedly slow and low power ADC. So we have to expect even worse trouble with something like the LTC2440 - though at least they have separate differential inputs and not everything to one poor GND pin.I like the idea of damping through the ESR of the Ta cap, but I think the ESR is to low for this - the impedance of lines is more in the 30-150 Ohms range.

author: branadic
author_rank: Super Contributor
timestamp: September 03, 2016, 08:30:24 pm
content: What about setting up a pi-filter using a resistor or inductance such as a ferrit bead?

author: Andreas
author_rank: Super Contributor
timestamp: September 03, 2016, 09:25:41 pm
content: Hello,@branadic: how exactly? every Ohm between reference output and VREF pin on the ADC will degrade INL according to data sheet when having a large (>10nF) capacitor on the ADC VREF pin side.Perhaps the cleanest solution would be to make a voltage tracker (TLE4250G?) for the VDD pin and connect VREF directly with either a small (1nF) capacitor directly or via a resistor to the reference.Other thoughts are to have perhaps a additional 1uF foil capacitor to deliver more energy to the ADC and reduce the amplitude of the spikes.@Kleinstein: I do not think that the line (2-3 cm) is long enough to have a line impedance.It is more a small inductivity which generates a resonance with the high Q ceramics capacitor.With best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: September 04, 2016, 09:17:51 am
content: In the pictures above I could see the caps at the reference voltage, but not the caps at the supply. So troublesome spikes could also depend on the supply decoupling. I would expect similar spikes there too, maybe even more.From the scope pics is looks like ringing is at something like 20 MHz. With a 100 nF cap this would be a resonance in the 1 nH range, which is rather low for something like 3-4 mm. So it could be there is a different part resonating, not the 100 nF cap and a short piece of wire.If it is the 100nF and an inductance resonating, impedance would be in the 1 Ohms range (Q / C/ omega). Q here is the quality factor of the ringing. So the 3 Ohms ESR of the Ta cap might be not that bad.At this speed a 1 µF foil cap could be tricky, as it is large and thus can not be that close to the chip.On the positive side, a foil cap would be less temperature dependent. As long as the ringing is constant, it might not be that bad. The big trouble could happen if it changes a lot with temperature.

author: branadic
author_rank: Super Contributor
timestamp: September 04, 2016, 12:04:45 pm
content: First of all, I guess you are refering to Figure 25. INL Error vs RVREF (Large C) with your combination of 100nF and 10µF? I have a few problems interpreting the diagram shown. It seems that there is no difference between 1µF and 10µF?So LTC2400 should work with 4.7µF without problems in INL? On the other hand the datasheet say's:"...If the external capacitance is large (CVREF > 0.01µF), the linearity will be degraded by 0.15ppm/? independent of capacitance at VREF, see Figure 25..."So what is the truth?On the other hand a 4.7µF to ground for the LT1027LS8 followed by a ferrit bead with low series resistance for decouling followed by the combination of 10µF and 100nF for the LTC2400 could do the job?

author: Andreas
author_rank: Super Contributor
timestamp: September 04, 2016, 02:56:36 pm
content: Hello,Quote from: Kleinstein on September 04, 2016, 09:17:51 amin this case the VREF and supply are tied together like in many application notes (the LTC2400 needs nearly no current).Quote from: Kleinstein on September 04, 2016, 09:17:51 amThe ringing that I measure is in the 8 ns-range period time. (120 MHz).(pictures 10k zoom are 50 ns / div) (500us/div / zoom = 10000).This makes it even more mysterious.On the other side: Inductivity also depends on the loop area between the wires and can go down to 0.2 nH/mm in a practical 2-layer PCB layout.For the 1uF: there should be room for a WIMA SMD capacitor (PPS-series) on the other side of the PCB.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: September 04, 2016, 03:47:41 pm
content: Quote from: branadic on September 04, 2016, 12:04:45 pmHello branadic,yes I refer to the diagram 25.And also there is no difference between 1uF and 10uF except of the ESR if using Ta.When I started my first design the 10uF Ta caps had 6 Ohms.The latest Ta size A now have 1.7 Ohms.And at those high frequencies which are generated by the LTC2400 theLT1027 might be too far away to contribute to the current spikes.The truth might be anywhere in between.Fact is that I measure a difference in INL (2:1 own reference) dependingon the capacitors populated while having the same PCB layout and LT1027.with best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: September 04, 2016, 05:31:35 pm
content: With such a high frequency, it is possibly that even the small loop at the probe could pick up the ringing. At least the amplitude and waveform could be different.The supply current is low, but there are current spikes at both the reference input and at the supply pin. So separate supply and ref. decoupling might be a good idea. However there is only one GND pin - so they are coupled anyway. From the shown internal circuit, I would not expect a really large spike at the reference input - a large spike from the supply input in more what one expects. So my best guess would be to have separate decoupling for the reference and supply, with a filter (e.g. ferrite) for the supply. However this could be tricky to impossible with the existing PCB.The INL is influenced by the combination of resistance and capacitance. A high capacitance is a problem, when there is a resistance at the reference. With reference and supply tied together, there could be an additional error from the supply current - not much, but it also does not need a lot to drop the supply a few ppm. So in this case a very low impedance for V_ref = V_supply is needed. I am not sure about the frequency of the impedance that is relevant. Like a linear voltage regulator, the output impedance of the LT1027 reference should be rather inductive over a wide range. This should be more of a problem than the few cm in between.

author: Andreas
author_rank: Super Contributor
timestamp: September 28, 2016, 09:39:24 pm
content: Hello,a comparison of some measurements on ADC21 (LT1027DCLS8-5 reference dead bug in hole mounted)and ADC24 (LT1027DCLS8-5 mounted in slotted PCB) after all adjustments are done:Both devices hooked up in parallel on LTZ#4.on 14.09.2016 and 17.09.2016 T.C. measurement of the resulting T.C. after 3rd order correction(and to show the resulting drift of the temperature cycles).ADC21 deviation is below 1 ppm (although there is some ageing drift during cold phase on 17.09.)ADC24 deviation is around 6 ppm (20 uV / 3.6V) most of it is hysteresis influence due to the PCB.Both devices show a rather large ageing drift within the 3 days.I hope that this will reduce during the next few kHrs.on 15.09.2016 offset temperature drift of the ADCs (input shorted).This checks mainly the LTC2400 drift and the buffer op-amp of the 2:1 capacitive divider.ADC21 has LTC1050 and 16 nV/K (0.5 uV after 2:1 divider from 15 .. 40 deg C)ADC24 has LTC2057 and -41 nV/K (1uV after 2:1 divider from 15 .. 40 deg C)on 16.09.2016 2:1 divider input shorted to own VREF.this checks gain drift of the capacitive divider at half input range (VREF drift cancels out).ADC21 -28 nV/KADC24 -61 nV/Kwith best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: September 28, 2016, 10:01:21 pm
content: So what's the conclusion?I guess there is a need to find and investigate different assembly technologies such as flexible circuit boards made of polyimid or semiflexible circuit boards made of thin FR4 to build up such sensitive devices, as long as dead bug technics isn't the way you want to go, even in a commercial product.Also a ceramic substrate with a single sided lead frame could be a solution, carrying all necessary passives.Maybe we can find further ways of connecting such a device to a board?

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: September 28, 2016, 11:28:44 pm
content: Andreas, by any chance have you measured at least one LT1027's drift since you began to test them?

author: acbern
author_rank: Frequent Contributor
timestamp: September 29, 2016, 01:43:02 pm
content: This reference is rated appr. 30ppm/1000h for the H package (hermetic, but no longer available), so the standard plastic package drift will be even more. This is not a low drift reference, working on dead bug mounting analysis or so is not worth it (except as for an excercise), the drift fluctuations peces to pice will probably be more. For low drift optimization, a burried zener in an hermetic case is inevitable.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: September 29, 2016, 03:44:20 pm
content: Quote from: acbern on September 29, 2016, 01:43:02 pmHmm, the LT1027DCLS8-5 is a buried zener reference in a hermetic case.CheersAlex

author: acbern
author_rank: Frequent Contributor
timestamp: September 29, 2016, 04:49:20 pm
content: True, wrong device, my mistake. And it is only 12ppm/18ppm at 1/3khr.

author: Andreas
author_rank: Super Contributor
timestamp: September 29, 2016, 08:15:07 pm
content: Hello,from my measurements with plastic DIP devices I know that the LT1027 chip itself has very good features.The only problem with PDIP package is humidity sensitivity.When I calibrate it out (with a filtered time constant of around 3-5 days) I get relative stable measurements.Of course the rH cannot be completely canceled out since the behaviour is probably non-linear.On 2 well pre-aged devices (ADC4 + ADC8) I measured around 0.5ppm/%rH and a yearly (average) drift of 1-2 ppm per year.So the values in the older datasheets are probably very conservative.@branadic: it is too early to get a final conclusion.PCB-mounting is critical but perhaps the hysteresis gets smaller with a smaller temperature gradient.The 0.12 deg C/minute or 7.2 deg C per hour is somewhat high even in my "lab".I think 2-3 deg C per hour is more realistic.Some day I will do a further test with reduced ramp speed.@Emanuel: I have stored all measurement values. But partly with different LTZ references and with different ADCs between selection and final build (different gain and INL). And between the steps soldering of wires to the reference, removing wires and again soldering to the PCB. So each step has its own ageing drifts. I dont know if it makes really sense to calculate any drift with all these treatments.At the moment I do some drift measurements of all 4 ADCs in parallel and without temperature cycling (except for changes in room temperature).Perhaps I will see more in a few hundreds of hours.with best regardsAndreasEdit: have forgotten to add the drift chart of ADC4 + ADC8 (LT1027 in PDIP package).In the chart are the original measurement values LTZ#1 (thin lines) on the left side (with 2:1 divider)together with filtered RH-value (thin line green) scale on the right side(unfortunately in rH instead of rH % so a factor 100 in between)on the X-Axis: days with zero = last calibration.The dotted points are the corrected values (with rH correction).The imperfect correction shows that the modeling of rH soaking with a averaging filter over 5 days is not perfect.

author: Andreas
author_rank: Super Contributor
timestamp: October 03, 2016, 08:40:48 pm
content: Hello,and the other verification charts from the remaining 2 ADCs (ADC22 dead bug + ADC23 slotted PCB) see also:https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1037076/#msg1037076Both devices hooked up in parallel on LTZ#4.on 19.09.2016 and 22.09.2016 T.C. measurement of the resulting T.C. after 3rd order correction(and to show the resulting drift of the temperature cycles).ADC22 (dead bug) shows large ageing drift on the first cycle(Temperature correction on the 2nd cycle is still not optimal perhaps due to drift during calibration).ADC23 (slotted) has large hysteresis on both cycles.And also the results of the offset and gain drift.The exact values are in the attached comparison table.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: October 07, 2016, 09:37:32 pm
content: First ageing results:all devices have been calibrated to my ADC13 = 3597.846 mV after 2:1 divider on LTZ#4.Calibration was between day 1-5.Day 9-12 temperature cycling + T.C. verification of ADC21+ADC24Day 14-18 temperature cycling + T.C. verification of ADC22+ADC23From day 19 on all ADCs at room temperature running 24/7Drift through 14 days (day 19-33) is between 2.7 and 5.4 ppmOr 0.2 ... 0.4 ppm/dayor (extrapolated) 8 ... 16.3 ppm/khrQuite a lot and up to now the drift looks more linear than square root.With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: October 18, 2016, 09:05:51 pm
content: Hello,now being at 1000 hrs from first calibration.5 to 17 ppm/kHr.too much for volt nuttery.With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: October 18, 2016, 09:29:26 pm
content: QuoteHowever, this is well within the given typical value of 12ppm/kHr. So the typical 18ppm/3kHr will also match, I guess. Well, the LS8 package differs more than expected from the former hermetic TO package, what ever the reason might be, glue or package itself.

author: Andreas
author_rank: Super Contributor
timestamp: October 24, 2016, 09:11:40 pm
content: Hello,as already announced a hysteresis test with a much smaller temperature slope.Again on ADC22 + ADC23 like on 22.09.2016 (see above).Reduced to 0.04K/minute or 2.4 degC / hour.So this is a more realistic value for my "lab" temperature change.The hope was that this would reduce the hysteresis.But all what I can see is a large drift during temperature cycling.Around 3.1 ppm @25 deg C for ADC22 (against previous drift direction)and 3.9 ppm @ 25 deg C for ADC23With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: November 01, 2016, 12:17:12 pm
content: Hello,this weekend I did a slow ramp of 0.04 K / deg on the remaining 2 ADCs(ADC21 and ADC24).ADC21 (dead bug) shows a excellent behaviour: less than 1 ppm change over 30 deg C temperature change.ADC24 (slotted PCB) shows the well known hysteresis curve of around 4 ppm @ 25 deg C and 6 ppm change over 30 deg C temperature range.So ADC21 (the only one of the 4 ADCs with selected LT1027DCLS8-5) seems to have nearly stabilized now.Stability evaluation of the 36 hrs measurement shows a stability of 0.4uV (after 2:1 divider) corresponding to 0.8uV at 0..10V input. (= 0.08 ppm! StdDev)And this over the whole 30 deg C temperature range.With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: November 29, 2016, 10:32:18 pm
content: 2kHrs drift data:first diagram LTZ#4 (with 2:1 divider) in mV in total view since start2nd diagram calculated VREF-drift of LT1027 references zoomed in for 2nd kHr.Still up to 8 ppm drift for the slotted PCB´s.the jump on ADC22 by -3 ppm is after temperature cycling (see above).Now I am doing 15 mA Loads on ADC#22+#23 over night.I hope this will give accelerated ageing.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: January 12, 2017, 09:40:35 pm
content: 3kHrs drift data:as promised ADC22+ADC23 with 15 mA load over (nearly) every night for accelerated ageing.ADC22 shows a direction change + large increase of drift to 8ppm/kHr.ADC23 is nearly unimpressed by the treatment.ADC21 shows drift below 1 ppm during 2-3kHrs.with best regardsAndreas

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: January 13, 2017, 12:46:27 am
content: What if you were to average the four references? Have you considered that?Eyeballing the graph seems to suggest an overall very good result. Half the noise, too.Randy

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 13, 2017, 04:25:07 am
content: A couple thoughts on humidity, one unlikely, the other not so much-1) I've heard that bee's wax has remarkably good moisture properties. Perhaps bake the board and dip coat it in molten bee's wax, maybe a couple coats. It worked for resistors and such in the old days.2) My HP vector impedance analyzer runs the tuning cap in an oil bath to stabilize it against atmospheric changes, or so I believe. Bake the water out of some mineral oil or silicone oil and run the board in an oil bath. Certainly thermal benefits and maybe humidity protection.

author: branadic
author_rank: Super Contributor
timestamp: January 13, 2017, 07:21:06 pm
content: QuoteWell, ADC22 is dead bug mounted and ADC23 is the slotted pcb. Up to now I was believing dead bug is better, but this latest test now shows a different picture. Any ideas of interpretation of this results available?Are you planing a similar test with ADC21 (dead bug) and ADC24 (slotted pcb) as a cross comparison?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 13, 2017, 08:08:33 pm
content: The dead bug type of mounting could result in higher thermal resistance and thus higher temperature from the 15 mA burn in procedure. Especially the jumps in the curve don't look that good.The way one solders the chips also has an influence on the starting point. Soldering SMD chips is well enough to reach the 120 C range where epoxy starts to soften / changes structure.

author: Andreas
author_rank: Super Contributor
timestamp: January 13, 2017, 09:31:39 pm
content: Hello,Quote from: RandallMcRee on January 13, 2017, 12:46:27 amno definitively not. I´ll keep only the best for precision measurements.Quote from: Conrad Hoffman on January 13, 2017, 04:25:07 amThe LS8 package is insensitive to humidity.And at least ADC21 + ADC22 (dead bug mounted) have no influence from PCB epoxy.Quote from: branadic on January 13, 2017, 07:21:06 pmat the moment the humidity (in winter) is near constantly low at 50% rF here.So what I see as drift has nothing to do with mounting method.ADC22 has probably a wrongly mounted chip within package.(simply behaving strange).So for deciding about the mounting method we would have to wait until the annual drift is below 1-2ppm/year and then see the rF changes between summer / winter.What I plan is to build some ADCs with (new) AD586 to compare.But I can´t remember that any of my AD586 ever had drifts in the 8ppm/kHr range.Quote from: Kleinstein on January 13, 2017, 08:08:33 pmI did not consider the higher thermal resistance up to now. But this should make less than a factor of 2 in effects.As already said: Those jumps are most probably something strange with the (individual) chip itself or the mounting within LS8-package.I am convinced that the LS8-package is not a good replacement for the CERDIP nor the metal can package.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: February 02, 2017, 09:15:02 am
content: Hello,Further drift data after having removed the ageing loading from ADC22 + ADC23ADC22 (green) again changes directionand also ADC23 shows (a small) influence.So I will go back to AD586LQ for my purposes.Even if they need a higher power supply voltage for proper operation.For my part I will order some fresh AD586LQ from DigiKey and do a comparison.I do not remember that they are such drifty as the LT1027.But who knows if the newer parts also suffer from such drifts(perhaps due to a different die attach due to RoHS)With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: June 04, 2017, 10:11:38 am
content: Hello,just for comparison I ordered 10 "new" AD586 (datecode 1606) and measured to select for new ADCs.This time I got a rather mediocre batch of AD586 regarding noise.Usually I have the AD586 with 2-3 uVpp 1/f noise.This time I have many above 3 uVpp (up to 9.2 uVpp) and many with popcorn noise (up to 5uVpp).So there is only one really excellent reference (#42) within the 10.My selection criterias are:TC below 1 ppm/K best with a "sweet spot = zero TC" near room temperatureHysteresis below 1 ppm over 30 deg C span.noise below 3uVppno (<1uV) popcorn noise.I selected the 2 best of the 10 AD586 to built 2 new ADCs which will be calibrated and then monitored for ageing.AD586#42 is obviously the best.AD586#38 was mainly selected because it has a "sweet spot" near room temperature.Stressless mounting with only 1 pin soldered directly to the PCB was used to compare with the "dead bug mounting" of the LT1027DCLS8-5. The other connections are made by thin "VERO wire".with best regardsAndreas

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: July 26, 2017, 05:36:29 pm
content: Just FYI these are "on-sale" at Newark/Element 14 (not sure if this is US only, or UK/Europe as well):http://www.newark.com/w/c/semiconductors-ics/power-management-ics-pmic/voltage-references?reference-voltage=5v&voltage-reference-case-style=lcc&st=semiconductors+CbargainsSeems like Andreas did not find these outstanding but in case someone wants to experiment this seems like a good deal.Andreas: did you decide that the AD586 is better? What's your final opinion on the LT1027?Randy

author: Andreas
author_rank: Super Contributor
timestamp: July 26, 2017, 07:00:28 pm
content: Hello,finally I still want to do some ageing measurements on ADC25+ADC26 with AD586LQYou can shurely not generalize from few samples.But what I measured is that the measured LT1027DCLS8-5 have more hysteresis and more ageing drift as that what I am used to from my AD586 ADCs.But quality may change from batch to batch.The latest batch of AD586 has a high number of references with popcorn noise which I do not want to have in my precision cirquits.If the manufacturer uses a different die attach (e.g. because of RoHS) this may have a influence on the hysteresis.And it is always a question of requirements:- I have large temperature and humidity changes in my lab (18-32 deg C and 40-70%)- my "benchmark" ADC has a standard deviation of around 0.25 ppm measured over 42 days (1000 kHrs) and I still want to do it better.- I always use battery supply. So I do temperature compensation with a NTC but no heating of the 5V references in my ADCs.So under normal room temperature a selected AD586 against a selected LT1027 may be a factor 2 better. (But the price is also higher).Essential is that you use a sort of "dead bug" mounting. Which is easier with a CERDIP8 package.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 23, 2017, 05:07:09 pm
content: Hello,now after (finally) having adjusted ADC25+26 with AD586 I can do some comparative ageing measurements.I selected ADC21 as "reference" because it was the most stable compared to LTZ#4 the last 1000 hours in the ageing measurement.But there are several surprises:ADC21 had stabilized to around reading 3597.810 mV for LTZ#4 with a 2:1 divider after 3kHrs. see;https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1112234/#msg1112234And now after a longer off time reading has changed to 3597.842 mV (9 ppm drift).When comparing a 24 hours measurement between the 3 ADCs then it gets obvious that also the temperature compensation of ADC21 has a problem. Nearly 3 ppm drift over a 4 deg C temperature excursion.After adjustment of ADC21 I had around 1 ppm over a 30 deg C range. See:https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1061241/#msg1061241I never observed such a large T.C. drift on any other of my ADCs.Now ADC25+ADC26 with AD586LQ (with few operating hours for adjustment) are running against ADC21 (with more than 3kHrs operating time) for a new ageing test.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 26, 2017, 04:54:36 pm
content: Hello,I now have found the reason for the T.C.-Drift on ADC21:The T.C. correction was switched off on ADC21.(probably I have done a ratiometric measurement and forgotten to re-activate the correction).With active correction ADC21 looks much more stable (light blue) similar to the other ADCs.But there is still that drift during off-timenow with active T.C. correction it is from 3597.810 mV mV to around 3597.836 mV (or 7.2 ppm)with best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: August 27, 2017, 07:19:20 pm
content: Hi Andreas,is it drift during turn off time or is it drift because of turning the reference off and on again? I wonder which mechanism makes the reference drift without any power supplied. A simple test could be to vary the power at both limits, from normal supply to minimum, maximum and back to normal supply and measure the output voltage.-branadic-

author: Andreas
author_rank: Super Contributor
timestamp: August 27, 2017, 09:26:32 pm
content: Hello branadic,the ADC21 has been switched off since end of January.But of course there were some temperature changes up to around 32 deg C in my "lab" which might also introduce some ageing effects.(the temperature itself is compensated by a 3rd order correction).Before it was contiously active for around 140 days. (ADC21 readings of LTZ#4 drifted down so the LT1027 drifted up during these days).And now it seems that it has drifted back to around the value which was in the beginning of these 140 days.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: September 23, 2017, 08:14:12 pm
content: Hello,next 24 hr stability measurement (after some stabilisation time) on my LTZ#4 reference.ADC25 red (AD586LQ) has execellent stability and lowest noiseADC26 green (AD586LQ) shows some popcorn noiseADC21 blue (LT1027DCLS8-5) also shows some instabilities.and so no surprise also the Allan Deviation reflects the results.Note that all measurements are done after 2:1 divider. So to compare with other instruments all results have to be multiplied by 2.ADC25 with averaging over some minutes shows a stability of 0.15 uV (2:1 or 5V range) or 0.3uV (1:1 or 10V range).With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: October 01, 2017, 09:28:12 pm
content: Hello,now ADC25+ADC26 with AD586LQ are running for their first 1 kHr.So attached the ageing drift compared to ADC21 with LT1027DCLS8-5.(absolute (mV readings) and relative VREF drift).And also attached the equivalent sheet of ADC21-ADC24 (LT1027DCLS8-5).Drift of the AD586LQ is at least a factor 3 lower than LT1027DCLS8-5.with best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: October 02, 2017, 08:53:32 pm
content: So the picture has slightly changed? ADC26 performs better then ADC25, even though it has popcorn noise? Or has the popcorn noise vanished? In your last post ADC25 was more stable (see Allan Deviation).Have you performed Allan Deviation on ADCs with slotted reference as well? How do they perform?-branadic-

author: Andreas
author_rank: Super Contributor
timestamp: October 03, 2017, 10:17:44 am
content: Hello branadic,thats exactly the reason why I have started another stability measurement (will be around 30 hrs).So today in the evening I will know wether ADC25 or ADC26 is more stable now.On the other side the standard deviation of my ADCs (when well pre-aged)is around 0.25 ppm for daily measurements of 1 minute integration time.So 0.5 ppm jumps (+/- 0.25 ppm) is nothing unusual.If you mean ADC23 + ADC24 with LT1027DCLS8-5 and slotted PCB:I have only some older stability measurements which I have done immediately after T.C. adjustment.(so before the 3 kHr ageing phase with still high initial drift).Here ADC16 (also a AD586) is given as reference for ADC24 -> minimum a factor 2 better in stability.So a more recent comparison with ADC24 would be interesting.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: October 05, 2017, 09:18:02 pm
content: Hello,The result of the 33 hours stability measurement.the popcorn noise is still on ADC26 (perhaps not so frequent fluctuations)All in all: ADC25 has moved slightly worse (most probably due to the larger temperature gradients)ADC26 slightly betterADC21 appears more long term stable as in the previous 24 hours measurement even if the shape of the noise over time has not changed.But I think all is just statistical variation. (so would need more observation time).I have started ADC24 to be included in the measurement. (after several months being switched off).But at the moment the daily drift is more in the 1-2 ppm range.I think it will need a week to stabilize until a stability measurement can be done.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: October 25, 2017, 07:46:32 pm
content: Hello,next 31 hours stability measurement.After ADC24 now had the chance to stabilize somewhat.ADC21 (LT1027DCLS based) dead bug mounted is now rather stable. (nearly as ADC25)ADC24 (LT1027DCLS based) with slotted PCB still drifting medium+long termADC25 (AD586LQ based) still the "best"ADC26 (AD586LQ based) suffers from popcorn noise.with best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: October 25, 2017, 08:01:01 pm
content: Looks like ADC24 has some temperature dependence superimposed on its drift.-branadic-

author: Andreas
author_rank: Super Contributor
timestamp: October 25, 2017, 08:25:36 pm
content: Hello,you are right.it looks around 1 ppm over 2.5 deg C or 0.4ppm/deg C.So either at the T.C. calibration something went wrongor the T.C. characteristic has changed over time.(which would be the first of all my ADCs where this gets visible).with best regardsAndreasEdit:adjustment was ok but ADC24 is suffering from a relative large hysteresis over temperature as you can see here at calibration check. (and of course some days before the actual calibration).https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1037076/#msg1037076

author: branadic
author_rank: Super Contributor
timestamp: October 25, 2017, 09:07:51 pm
content: I can't identify the hysteresis you mentioned. Have you tried compensating the data by temperature afterwards? What is left then?Still have my problem interpreting the allan plots, as I don't see any quantization noise nor random walk in the diagrams. The values are directly starting with bias stability, what is somewhat unusual as the ADC should introduce some quantization noise and I would expect some random walk by the reference itself. Am I'm wrong?

author: Andreas
author_rank: Super Contributor
timestamp: October 25, 2017, 09:31:45 pm
content: Hello,for me the hysteresis is clear visible:at 30 deg C I have 15 uV or 4 ppm hysteresis at the ADC input (after 2:1 divider).https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/?action=dlattach;attach=258961;imageFor the Allan diagrams:Be aware that with my ADCs I use the 1 minute average values. (1 minute integration time).So around 330 measurement values averaged which is also near the maximum stability already.So the quantisation noise and the angle random walk are already removed from the diagram.When doing the Alan diagram with raw values I can see the angle random walk with sqrt(n) decrease.But then I am starting at 2uVeff noise. (not at 0.2uVeff as in the 1 minute integration time).A different picture is shown when I use either a short (zero volts) or the own reference voltage.Then I have a looooooong falling sqrt(n) measurement.(see example with ADC14 measuring own reference with a 1:1 buffer amplifier and raw data)with best regardsAndreasEdit: quantisation noise is probably not visible with my ADCs since the LTC2400 has 24bits + 4 sub-Bits further I gain around 4 bits due to the averaging over 1 minute. So quantisation noise would be at nV.

author: branadic
author_rank: Super Contributor
timestamp: October 25, 2017, 09:48:18 pm
content: Okay, taking this additional picture into account I understand your conclusion. Have you tried multiple temperature slopes one after the other? Does hystersis decrease or increase? With constant hysteresis the residuals should show some ellipse but it looks like your residuals show some helix, so there seems to be some previous history left.Concerning Allan diagrams: Didn't notice that and was thinking if this different pictures are resulting from using voltage values as frequency data instead of phase data in Plotter. But with increased averaging this makes sense. Thanks for explanation.-branadic-

author: Andreas
author_rank: Super Contributor
timestamp: November 14, 2017, 10:09:30 pm
content: Hello,now we have around 2000 hrs on ADC25 + ADC26Both AD586 based ADCs have drifted around 1-1.3 ppm/kHr.It will still take some time until they reach 1-2 ppm/year.ADC21 in comparison (LT1027LS8 based) has drifted 6 ppm or 3 ppm/kHr.And since ADC21 + ADC24 have been already aged for 3 kHrs this is for 4th and 5th kHr of ADC21.-> nearly no chance to get into the 1-2 ppm/year region.ADC24 (on slotted PCB) has been included into the measurement for its 4th kHr. Average drift is similar to ADC21 but with more noise.Interesting find:I had accidently put ADC24 up side down on the table for the first week which can be seen as up to 2ppm/day variations.After 1 week it was put in normal orientation and the variations calmed down.But I am not shure if this was really the reason since the "noise" of the measurements increased later again.with best regardsAndreas

author: Noopy
author_rank: Super Contributor
timestamp: March 13, 2020, 10:10:30 pm
content: I took some more pictures of branadic´s LT1027:https://www.richis-lab.de/REF07.htmNot really better but different.Besides the voltage adjusting the die has fusible links in the upper right corner. The links connect resistors to the opamp. Probably some adjustment of the opamp characteristics.The Zener. They have two of them. The upper one seems to be responsible for some constant currents.Does anybody know what that big thing does?

author: magic
author_rank: Super Contributor
timestamp: March 14, 2020, 08:35:02 am
content: wtf1, wtf2 are the resistor networks in the corner near the factory test pads.

author: Noopy
author_rank: Super Contributor
timestamp: March 14, 2020, 08:39:01 am
content: Quote from: magic on March 14, 2020, 08:35:02 am...have to think about that…

author: razvan784
author_rank: Regular Contributor
timestamp: March 14, 2020, 03:20:13 pm
content: Quote from: Noopy on March 13, 2020, 10:10:30 pmLooks like a huge diode in series with some elements used for post-packaging trim, maybe Zeners than can be zapped (short-circuited with a high enough voltage & current). I don't know why they're not using metal fuses like in the left section, maybe those require much higher current? Or maybe they leave contamination behind?

author: babysitter
author_rank: Frequent Contributor
timestamp: March 14, 2020, 03:22:24 pm
content: Any idea who or what JS is?

author: iMo
author_rank: Super Contributor
timestamp: March 14, 2020, 03:56:39 pm
content: .. and, again 3 pins with an unknown functionality (but no fuses this time)..

author: Noopy
author_rank: Super Contributor
timestamp: March 14, 2020, 04:40:46 pm
content: Quote from: imo on March 14, 2020, 03:56:39 pmIn my view there are at least two fuses next to two of the bondpads connecting the "not used" contacts. They look like these smaller "poly-fuses".

author: iMo
author_rank: Super Contributor
timestamp: March 14, 2020, 06:33:38 pm
content: Hmm, I do not see any around the 3 bonded unused pins..

author: magic
author_rank: Super Contributor
timestamp: March 14, 2020, 07:51:12 pm
content: Branadic's own photograph shows this device in more detail and it doesn't look like any of the fuses.There is more of them, here's one in the middle of the die.By comparison with transistors, I would say that the outer "frame" with dark and thick borders is N+ doped over bare isolation island (it looks like NPN collector connections) and the central rectangle is P. The P appears to be contacted by both metal traces entering the device, so it functions as a resistor, though a suspiciously thick and short one. The island is biased to the potential of the metal which contacts N+. That's normal for resistors, though nobody goes to such effort to distribute that potential over the whole perimeter of the resistor with N+ diffusion because no current normally flows through the isolation island.Maybe I'm wrong and the doping is different. Or maybe the isolation island forms a diode with the substrate to clamp the input to GND. Total speculationAt any rate, somebody tell me how that circuit at the bottom of the input stage works

author: Noopy
author_rank: Super Contributor
timestamp: March 15, 2020, 09:04:41 am
content: Hm... Seems that I was wrong...

author: magic
author_rank: Super Contributor
timestamp: August 14, 2021, 09:57:19 am
content: I found the picture of this IC on my computer and decided to take another look.I think you are right, these small structures like the one I posted above are zeners, possibly acting as fuses for trimming. If you look closely you may notice that there are thick lines visible on metal wherever it crosses the border of N+ diffusion. However, there is no line between the N+ frame and the metal finger which reaches to the P diffusion inside the frame. Therefore, N+ must also be present in that finger, under the metal. The contact window covers only the N+, not the P. The P is contacted by the lower metal trace. Together, they make a diode with low reverse breakdown.I attach a picture of diode-strapped NPN for comparison. Here the collector contact N+ diffusion (right) also partly overlaps with the base P diffusion (left). The edges of both diffusions can clearly be seen on the overlap area. The whole area is covered by metal and there is a contact window extending over both parts.So what we have in the upper right corner of the die is a bunch of resistors and some (presumably) fuses and DNC pads. Parts of the circuit are obscured by bonding wires. There seems to be some sort of connection between the pads and ground, but I think it's not very low resistance because in such case the pads wouldn't really be necessary. The resistors go to the wtf1,wtf2 points in that weird transistor network which biases the emitters of the differential pair.The big structure with multiple fingers is different. The central part appears to be a P diffusion because its color is the same as the P parts of the buried zener. There seems to be an N+ diffusion aligned exactly under the metal rectangle, because there is a line where the rectangle joins the trace coming from the DNC pad. So it looks like a diode-strapped NPN so far. But the emitter fingers leave the base area and join an N+ frame which surrounds the base and makes contact with the collector.A similar structure (plus another zener zap?) is found near the TRIM pad in the bottom right corner.Regarding the reference, it seems to roughly follow the datasheet schematic. It consists of the buried zener and NPN located on the central axis on the left side. The NPN is a Vbe-multiplier whose output is averaged with that of the zener. The topmost resistor in the chain is adjustable by all those fuses on the left side. The divider chain has a few more taps and some other transistors hang off of them. The PTAT generator in the bottom left corner appears to actually be a proper bandgap reference (Brokaw cell topology), because I simulated the circuit with transistor and resistor ratios as seen on the die (4:1, 8:1) and it produces close to 1.2V, depending on which transistor model I choose.