REF70 new from TI - Page 1
Forum section: electronics / general

author: fcb
author_rank: Super Contributor
timestamp: April 22, 2021, 11:26:44 am
content: Just came across this:https://www.ti.com/product/REF70

author: Grandchuck
author_rank: Super Contributor
timestamp: April 22, 2021, 11:38:44 am
content: It is in a ceramic package. The typical drift is 28 ppm from 0 to 1000 hours. The REF70 will experience the highest drift in theinitial 1000 hr. Subsequent deviation is typically lower than 13 ppm for the next 1000 hrs.

author: kleiner Rainer
author_rank: Regular Contributor
timestamp: April 22, 2021, 01:15:55 pm
content: There is also a new application note on long term drift:https://www.ti.com/lit/pdf/sbaa436Greetings,Rainer

author: Andreas
author_rank: Super Contributor
timestamp: April 22, 2021, 06:13:16 pm
content: Quote from: Grandchuck on April 22, 2021, 11:38:44 amHello,it is pin-compatible to the LTC6655. And the data is also copied from the LT datasheet except for the supply voltage range which is better for the REF7025.The large question is: how sensitive is the chip in the SMD package to FR4 board stress due to humidity.With other LS8-packages (LT1236, LTC6655) I have always had not so good results as with CERDIP 8 packages. (AD586LQ)with best regardsAndreas

author: ch_scr
author_rank: Frequent Contributor
timestamp: April 22, 2021, 07:50:28 pm
content: Quote from: Andreas on April 22, 2021, 06:13:16 pmHmm, shooting from the hip but: flex pcb interposer between LS8 and rest of the FR4 PCB?

author: jbb
author_rank: Super Contributor
timestamp: April 22, 2021, 10:38:59 pm
content: QuoteLT recommend routing slots around the references, but I guess there’s still some stress to be had in the FR4 right underneath the chip.I wonder if a ceramic SOIC would be a good compromise? That would be surface mount for reasonable assembly, with legs to take up some of the strain and reduce stress in the package body.

author: Andreas
author_rank: Super Contributor
timestamp: April 23, 2021, 04:39:13 am
content: Quote from: jbb on April 22, 2021, 10:38:59 pmI fear: not really.costwise you would need a lead frame from Kovar instead of copper (for the plastic case) and a glass seal. (so no advantage against CERDIP).For reducing the board stress something like gull wings would be proably a better choice for a SMD package.with best regardsAndreas

author: jbb
author_rank: Super Contributor
timestamp: April 23, 2021, 07:05:50 am
content: Sorry, I wasn’t clear. I meant a to compare a hypothetical CERSOIC to the LCC. Clearly the CERDIP has already been done.As you say, carefully forming the CERDIP legs to gull-wing is probably a good way to go.Edit: the thought of a flex PCB interposer is interesting. I guess the question would be how the polyimide compares to FR4 in terms ofmatchingthe LCC package. Maybe it could be slit in creative ways.Anyhow, good to see someone else offering the LCC packaging.

author: ch_scr
author_rank: Frequent Contributor
timestamp: April 23, 2021, 08:25:04 am
content: Quote from: jbb on April 23, 2021, 07:05:50 amA quick search shows that polyimide does take up water as well. But I would hazard a guess that the polyimide flex pcb is well a lot moreflexible.The polyimide has to have a degree of "stretchiness" to conform to the forces bending it - my guess would be that as a consequence of that forces on the LCC package would be a lot lower.Another point where geometry comes into play is: how to arrange the soldered contact between rigid and flex so that the stresses are not only decoupled by stretch but reduced by geometry.Imagine a flex pcb with LCC + say a decoupling cap. It is now in one case soldered "all around" with contacts on 4 sides. Stresses would only be reduced by stretching of flex.But when having all contacts to one sides of the flex, stresses from rigid to flex are completely decoupled in one direction and concentrated in the area of the solder joints in the other direction.Further shenanigans could be played by having a "bend" in the flex pcb, but mechanical assembly would be more cumbersome.

author: 2N3055
author_rank: Super Contributor
timestamp: April 23, 2021, 10:33:01 am
content: Quote from: kleiner Rainer on April 22, 2021, 01:15:55 pmThat accelerated aging method of estimating long term drift was disputed long time ago.https://www.analog.com/media/en/reference-design-documentation/design-notes/dn229f.pdfAccelerated aging is a real thing and helps references to settle down quicker. But it cannot be used to project long term drift from those measurements.

author: kleiner Rainer
author_rank: Regular Contributor
timestamp: April 23, 2021, 10:54:43 am
content: I would put a reference in LCC package on a hybrid circuit - no problems with humidity, and the temperature coefficient should be close enough to alleviate stress in the solder joints. Thermal conductivity is also much bettter, you could even print a heating resistor onto the hybrid, together with a SMD NTC to close the loop. Hybrid circuit manufacturing, anyone?Greetings,Rainer

author: dietert1
author_rank: Super Contributor
timestamp: April 23, 2021, 05:08:57 pm
content: What i don't understand in the TI paper on LTD: They find that the square root prediction overestimates aging. Why didn't they try to fit an exponential decay? In my own experiments with LTFLU references i found that exponential decay with a time constant of about 7000 hours described the observations very well. The idea behind this would be that the process isn't a random process but a decay of some initial disturbance(s). Like in radioactive decay the disturbance with the longest half life dominates after some time. Another observation i made: "Random" deviations from the exponential decay get smaller during the aging process as well. For me another indication that LTD is less than the square root estimate. TIs logs also look a bit bumpy between 3000 and 4000 h and a bit better afterwards. To be honest, their data look bad considering the use of a 8.5 meter and of a 0.01 °C oil bath.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: April 23, 2021, 08:58:57 pm
content: The curve for the LTD look in deed rather noisy, with quite a bit off correlation. Quite some reference noise would be expected for the reference chips, but the correlated part would be more like a problem with the meter or the switches . So there may be a hidden fail of some kind (e.g. using a wrong settting at the meter or EMI problems or cable thermal effects). The test for 85 C looks good though.The square root law is only one possible case for long term drift. There are other commonly used law to describe aging effects. An exponential decay is one example. There is also the so called streched exponential often used to describe glasses. The epoxy is to a large part in glassy structure.It really depends on the system how aging proceeds and there can be several contributions superimposed. So predictions are diffcult.Besides just aging, there can also be an effect of humidity, that can be relatively slow at low temperature. This is especially important for plastic cases.

author: KT88
author_rank: Frequent Contributor
timestamp: April 23, 2021, 09:11:19 pm
content: Did you notice the differences distribution and trend of the LDT in comparison to the LTC6655?The drift has mostly settled after 1kHr with an eyballed median of about +15ppm after 2kHr. The REF70 is still moving after 2kHrs with a median of about -35ppm...Looks like TI's copy machine has some issuesCheersAndreas

author: tszaboo
author_rank: Super Contributor
timestamp: April 23, 2021, 10:06:41 pm
content: Cheap LTC6655, cool. Although I still dont understand the pricing of precision components. If I buy a crystal oscillator in a 2 pin hermetic package, its 10 cents. If I want VPG to place a resistor in the same package its 20 EUR.I know, supply, demand... But still, I think it shouldn't be that difficult to place them into hermetic packages.

author: exe
author_rank: Unknown
timestamp: April 23, 2021, 10:12:37 pm
content: Why would they copy it? What market is it for? Maxim released max6226 in a similar package, but with incompatible pinout. Not as good as ltc6655, but twice less the price (in single qty), and only a fraction of power consumption. But I haven't seen any reviews of it. I have two of them, but I'm still to put them into service.Anyway, I kinda like ceramic references and welcome alternatives to ltc6655 which was hard to get. Had to hunt it for months until I got them. Also expensive. Ref70 seems to be a bit cheaper. But I also noticed TI clones not just cheaper, but also have worse specs, often hidden in plots.PS This IC was discussed a bit here with one guy having hands-on experience with it:https://www.eevblog.com/forum/metrology/ref70-family-voltage-reference/.

author: KT88
author_rank: Frequent Contributor
timestamp: April 23, 2021, 10:36:07 pm
content: QuoteThe level of accuracy requires extensive testing -in case of the LTC6655 at least two, if not three temperatures. Test time is an expensive commodity...CheersAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: April 24, 2021, 08:23:30 am
content: There are plenty of high high resolution SD ADCs offered that need a suitable reference. So there is some need for good 2.5 V or similar references.For low frequency application one may even like low lower noise, if it would be available.The long term drift on the Ref70 does no look very good. Even with the ceramic LCC case it looks like the board may have an effect too. So it may not be just the fault of the chip itself.The hysteresis part also does not look that good. Still testing -40 to +125 is kind of extreme.Given the way they describe the noise measurement system, the noise data may be a bit optimistic: 2 filters with 10 Hz cross over in sereis give a cross over a little less than 10 Hz. Still the higher frequency noise looks very good and the lower frequency noise is thus likely not as good as with the LTC6655.

author: Gyro
author_rank: Super Contributor
timestamp: April 24, 2021, 09:39:42 am
content: I wonder how long, if ever, it will be before they put a decent burred Zener reference in one of those packages. If it's TI then they've at least got the REF102 available.I know it's supply and demand but surely somebody must need decent long term stable references, especially as they phase out Cerdip packages.

author: fcb
author_rank: Super Contributor
timestamp: April 24, 2021, 10:23:03 am
content: Do any of the reference vendors sell unpackaged die?Need an excuse to buy a 2nd hand Kulicke & Soffa.

author: KT88
author_rank: Frequent Contributor
timestamp: April 24, 2021, 11:48:52 am
content: They do:https://www.analog.com/en/products/adr225.html?doc=adr225-kgd-chips.pdf.Search for *voltage reference "known good die"*TI and others will show up as well.It won't be a bargain though....CheersAndreas

author: MiDi
author_rank: Frequent Contributor
timestamp: April 24, 2021, 12:46:37 pm
content: Quote from: Gyro on April 24, 2021, 09:39:42 amLT1236LS8?

author: Gyro
author_rank: Super Contributor
timestamp: April 24, 2021, 01:30:19 pm
content: Good catch. The datasheet seems to show a fair bit of thermal hysteresis, maybe that's the penalty of the small ceramic package - although I don't see hysteresis figures specifically stated in the cerdip AD587 datasheet.BTW, the AD587 is also available in die form. Edit: Make that 'was' (Datasheet Rev. D)

author: exe
author_rank: Unknown
timestamp: April 28, 2021, 07:29:06 pm
content: One thing I noticed about ref70. Looking at the diagram onhttps://www.ti.com/product/REF70, I see it has something called "digital core". I wonder what that is. Can they do some sort of software correction of temperature drift and ageing? Chopping? That could also explain weird 10Hz oscillations reported in another thread (unless that was a measurement error). I wish I could send one to Noopy for decapping and die shots.

author: fcb
author_rank: Super Contributor
timestamp: April 28, 2021, 07:35:35 pm
content: Quote from: exe on April 28, 2021, 07:29:06 pmYeah. I wondered what that was too. That's one of the reasons I got interested in this part.I think they'd be on a sticky-wicket if they tried to compensate out ageing, but you can compensate out thermal drift (we've just released a low-cost voltage reference that does this).

author: KT88
author_rank: Frequent Contributor
timestamp: April 28, 2021, 09:31:37 pm
content: A lot of analog parts like references but also opamps, LDOs and others use some sort of trimming. Often times the parts feature DACs/digipots together with OTP memory. During testing more or less parameters will be adjusted and then stored in OTP. Especially precision parts like the REF70 won't be viable without that capability...Lasertrimming is rarely used these days except for thin film resistors.CheersAndreas

author: 2N3055
author_rank: Super Contributor
timestamp: April 28, 2021, 10:41:21 pm
content: For years digitally trimmed crystal oscillators are available. They are mostly lookup table with interpolation to trim tempco.Same can be used with voltage reference, provided that tempco shape doesn't change as it ages.....These trimming techniques usually are very useful to get decent accuracy across large temperature range, more than to provide highest possible sub ppm tempco, lab style..

author: dietert1
author_rank: Super Contributor
timestamp: April 29, 2021, 07:56:07 am
content: Similar methods are applicable for metrology setups as for integrated precision devices. One example is here:https://www.eevblog.com/forum/metrology/esi-resitance-standard-sr1010-and-standard-resistor-sr104/msg2719638/#msg2719638.I determined a 1.84 ppm/K temperature coefficient using small ambient temperature changes. Later with the same setup i could determine drift/aging and even the quadratic term of TC.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: April 29, 2021, 08:30:58 am
content: I would expect the digital trimming to be static. That is some memory and DACs / digit pots to adjust the reference with constant values. So no change in the settings depending on temperature or similar. This way no real option to trim for long term drift, but an option to trim the voltage level and TC, possibly even higher order coefficients.

author: Andreas
author_rank: Super Contributor
timestamp: April 29, 2021, 07:06:38 pm
content: Quote from: Kleinstein on April 29, 2021, 08:30:58 amI would also expect some similar to the "SVR-T" reference by having 2 pots (DACs) one for the offset and the other for some temperature dependant influence from the PTAT voltage.Otherwise you would need high ADC resolutions.If having only 1 deg C resolution (~2mV of the PTAT output of the band gap reference) and a uncompensated TC of about 2 ppm/K each compensation step would give a 5 mV step on the output which is much larger than the noise of the 2.5 V reference.So effectively for a pure digital system you need minimum a 0.1 deg C resolution over a 165 deg C temperature range. And even better if the uncompensated TC is more than 2 ppm/K.with best regardsAndreas

author: exe
author_rank: Unknown
timestamp: May 12, 2021, 06:31:01 am
content: It seems ref70is now called ref7025 and itis in production. The datasheet was updated. Some noticeable changes:1. About long-term stability: "Corrected typical shift from 0.021% to 0.009%". That's good.2. "Changed Figures 7-2 and 7-20, Long-Term Stability (First 1000 Hours), to longer duration (2000 hours)."3. "Added clarification on how to connect OUTF and OUTS in specific load current condition". OUTS current is... wait for it... 4mA! (this is not part of main specification, they mention this in "10.2.1.2.2 Force and Sense Connection"). Can't make sense of it, this is comparable to maximum output current of 10mA. This also matches quiescent current of the reference. Does it mean they use OUTS as a supply pin? Or this 4mA are in addition to 4mA supply current?The price from TI website is $12.696 for single qty + VAT. Quite expensive, I was hoping for better pricing. This matches LTC6655 pricePS LTC6655 also has quite big OUT_S current, which is 2mA.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 12, 2021, 07:10:04 am
content: The sense current should be included in the supply current. Still the high sense current can cause a nasty supprise in some cases. An important point may be the question how stable this current is. Just a stable current would only give a little addition to the actual ref. votlage. A change in the current would compete with the change in the sense resistance.They seem to quite proud of there figure 7-2, it comes up 3 times in the datasheet.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: May 12, 2021, 08:04:24 am
content: this is max6126 for comparison (and about 1/2 the price)from this articlehttps://www.maximintegrated.com/en/design/technical-documents/app-notes/7/7009.htmlthe drift for this is interesting, some are +ve, some are -ve. with the right amt of luck, maybe 2 units will cancel the drift out.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 12, 2021, 08:59:15 am
content: The drift curves look surprisingly good with the max6126, especially for a SO8 part. Looks like they somehow got lucky with the design to get low stress sensitivity.However the noise is quite a bit higher than for the Ref70 / LTC6655. So it would need some 5 pieces max6126 combined to get a similar noise level.It is still a good reference, but for different uses.

author: exe
author_rank: Unknown
timestamp: May 12, 2021, 09:02:38 am
content: Quote from: Kleinstein on May 12, 2021, 07:10:04 amOn LTC6655, as I understand, the high current is due to low-value resistor divider (they also say that in the text). So, if we assume that REF70 works the same way, then the load resistance should be stable (but I cannot confirm this without a die shot). And this also means that the rest of the circuit doesn't consume much. This gives me a hope that MAX6226 with 0.38mA supply current is not much worse (but I don't have equipment to to verify this).Quote from: 3roomlab on May 12, 2021, 08:04:24 amThere's also MAX6226 in ceramic packaging and still 1/2 the price. And also MAX6079 with proper kelvin connection, not that high-side OUT_S rubbish.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 12, 2021, 09:28:39 am
content: A badgap reference needs quite some current to get really low noise. So it is really odd the Ref70 uses so much current on the sense pin, unless they actually use it to power the actual reference curuit part.A resistive divider would need some current, but this should not be so much, more like maybe 0.1 - 0.5 mA or so, but no real need for 2 or even 4 mA.

author: Karel
author_rank: Super Contributor
timestamp: May 12, 2021, 10:19:39 am
content: I recently used an ADR4540 in my DVM project. Looks like the REF70 isn't any better.

author: exe
author_rank: Unknown
timestamp: May 12, 2021, 10:34:43 am
content: Quote from: Karel on May 12, 2021, 10:19:39 amREF70 is a bit cheaper than ADR45* in LCC hermetic package:https://nl.mouser.com/Search/Refine?Keyword=ADR4525DEZ. But yeah, seems all parts cost approx the same, which is sad, not price competition

author: chuckb
author_rank: Frequent Contributor
timestamp: May 12, 2021, 05:09:28 pm
content: I could not get a REF70 chip from the normal distributors yet, so I got a REF70 Demo PCB from the factory.It supplies a solid 2.4998V (K2002) from 6V down to 2.7V input voltage. At 2.6V input (no load) to output voltage dropped 50uV. The supply current was 4.34ma.I removed R2 on the demo pcb which shorts the output to the sense terminal and installed a 1 ohm resistor. There was 2.8mV drop across the 1.005 ohm resistor (0.5%, 25ppm) for 2.8ma through the sense pin. The installed resistance was verified with a Keithley 580 Micro-ohm meter.Later checks will include noise and temperature stability.At the end of the testing the lid will come off and we will gets some die pictures.

author: chuckb
author_rank: Frequent Contributor
timestamp: May 12, 2021, 05:14:27 pm
content: The REF70 datasheet talks about 4ma for the sense pin. That may be for the 5V version of the chip. Just guessing.

author: exe
author_rank: Unknown
timestamp: May 13, 2021, 08:43:42 am
content: Thanks, chuckb! The datasheet says the dropout voltage is 250mV (but under load). So, if this 50uV sag happens due to insufficient voltage headroom, or it's caused by line regulation. If you have time, can you please check how much output voltage shifts when changing supply voltage from, say, from 2.9V to 12V? (Probably 12V is on the extreme side).The datasheet says that at 25C REF70 has 4ppm/V line regulation. For 6=>2.6V change in supply voltage this translates to, if I'm not mistaken, 34uV. It's not far off to what you measured, but it never hurts to verify datasheet figures :).

author: chuckb
author_rank: Frequent Contributor
timestamp: May 13, 2021, 05:17:45 pm
content: Looks like the line regulation starts working well at 3.5V for the 2.5V reference.I started at 2.6V input voltage and worked up to 15V. Then I returned to 3V input voltage and the output voltage had shifted less than 1 ppm. That's pretty good.The Keithley 2002 has been turned on for the last few weeks for other testing.

author: chuckb
author_rank: Frequent Contributor
timestamp: May 14, 2021, 01:39:54 am
content: I did the first test of the REF7025 (2.5V) TempCo today with a 5V supply. From 25C to 61C the output voltage changed exactly -2ppm / deg C. It was right on the spec limit and very linear.When the temperature was reduced back to 25C the hysteresis was +56ppm.I will do several more temperature cycles over the next several days to see if the hysteresis reduces.

author: Andreas
author_rank: Super Contributor
timestamp: May 14, 2021, 04:49:36 am
content: Quote from: chuckb on May 14, 2021, 01:39:54 amPlastic package or LS8-Package?with the plastic package it is likely that humidity makes the difference (drying of the package with heat).If you repeat the test the package is already "dry" so less influence.If you wait several weeks before repeating the test the package "soaks" humidity from the environment.The game starts from new.with best regardsAndreas

author: chuckb
author_rank: Frequent Contributor
timestamp: May 14, 2021, 12:49:17 pm
content: It is the LS8 type ceramic package

author: Andreas
author_rank: Super Contributor
timestamp: May 14, 2021, 02:59:11 pm
content: Hello,for a LS8-package the change of your sample is rather high.Eventually there is some influence from the (EPOXY) PCB where it is mounted.(which humidity do you have in your area?)compared to the LTC6655 its even worse than my first measurement of a LTC6655 in MSOP package (ADC14)The pure LTC6655BLS8 (without influence from the PCB) I measured with +/-4 ppm hysteresis.see:https://www.eevblog.com/forum/metrology/ltc6655b-long-term-drift/msg2355873/#msg2355873with best regardsAndreas

author: exe
author_rank: Unknown
timestamp: May 14, 2021, 03:44:53 pm
content: The datasheet suggests that consecutive thermal cycling should have smaller impact on the voltage. Still, 56ppm shift is more than I'd expect.I'm fighting my greediness and I'm willing to experiment on my max6226 for the sake offunscience. Stay tuned.

author: chuckb
author_rank: Frequent Contributor
timestamp: May 16, 2021, 03:49:36 am
content: My lab has a nice variable speed HVAC system with some humidity control. It is typically 23C +-0.2C. I have a number of circulation fans running so the temp control has been verified in the different corners of the lab. Most of the time it has a 40% RH +-2%. The last few days have been cool and raining so the humidity was 43%.I have the REF70 demo PCB in a small heated chamber. For this test I use a SciGene Hybex Incubator ($50 'bay) with some 1/4" steel plates in the bottom to add thermal mass. There is no humidity seal on the chamber. I have not noticed any electrical interference between the heater controller and the DUT on previous sensitive electronics tests.Attached is the plot from the last several days of testing. The chip temperature was monitored with a small, stick-on, OMEGA DIN 100 Pt Class B RTD. Datasheet attached. The resistance was converted to temperature by a 4-wire connected K2001 set for D100 RTD. A Fluke Model 52 with a thermocouple crosschecked the temperature. The two sensors agreed within 1 deg C.Setup -5V applied to the REF7025 demo PCB. The output voltage was monitored with a Keithley 2002.Procedure -I would change to a new temperature, wait about 2-3 hours, verify that the system had stable temp and voltage, and take a reading. The DUT was suspended in the air inside the chamber.The testing started in the lower left corner of the graph at 2.4996V. As I raised the chamber temperature the output voltage decreased at 2ppm / deg C. I stopped testing at 61 deg C. I let it cool over night and the voltage had increased to 2.4997V.As I increased the temperature in steps on the second day the voltage dropped in a similar manor of about -2ppm /C.This time I let the PCB soak at 61C for 23 hours. The voltage increased quickly at first then it ended up with a +150uV total shift. The first 100uV of movement happened in 4 hours.Over the next 8 hours I tested the chip as it cooled. With 1kg of steel in the chamber it does not cool off very quickly. It ended up 200uV above where it started 36 hours earlier.Later, I will do a few more temperature cycles to 61C then a few going up to just 30C.The first large hysteresis step is within the REF70 data sheet limits for a 0-70C temperature change. However the factory expects the voltage shift to reduce with cycles. I have not seen that yet. Linear Tech recommends stress relieving cuts in the PCB for their SMD Voltage references. That may help.