Need help with my first LTZ1000A-DIY-project - Page 1
Forum section: electronics / general

author: Storm
author_rank: Contributor
timestamp: June 20, 2019, 01:38:24 pm
content: Dear metrology experts,today was supposed to be the day where my first LTC1000A project based on Dr. Frank's PCB should be running for the first time... Unfortunately I made a big mistake and probably destroyed one of my two new LTC1000A devices...I misread the datasheet and did not just omit this 400KOhm resistor (R10 in Dr. Frank's PCB) between pin1 and pin4 of LTC1000A but SHORTED it. Although I used a 25mA current-limit on my power supply and only kept it on for very short time until I noticed the Constant-I kicking in, it was too late I guess.I removed the short for R10 leaving it unstuffed and tried it again but now the 12V power supply needs to deliver around 50mA of current to come back to Constant-V-mode and my expectation for a normal running unit would be 25mA or less. I measured the outside case temperature of the LTZ1000A with a TC-sensor around 60°C at 25°C room-temperature and the zener voltage reads around 7.0158V. I do not think that this is correct behavior and before I use my second brand-new LTZ1000A device I would like to get your opinion first and make sure that the second try will work as expected.For the first try I did not stuff the 10V-Amp and I got advice from Edwin G. Pettis that I should use a 13K/1K temperature setpoint resistor pair instead of a 12K/1K for the LTZ1000A...I attached a picture as an attachment, since I did not figure out yet how to add it into the post directly...I am not that familar with posting to this board yet, but i think this will improve over time.Best Regards,Walter

author: Andreas
author_rank: Super Contributor
timestamp: June 20, 2019, 02:25:09 pm
content: Hello,50 mA is a bit on the high side.It could be that the base emitter junction of the current regulator has suffered.I would measure all voltages around the LTZ to see wether all regulator loops are working correctly.you can compare to the values from here: (except for the temperature setpoint ratio)https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg874560/#msg874560with best regardsAndreas

author: tszaboo
author_rank: Super Contributor
timestamp: June 20, 2019, 03:40:44 pm
content: It could be still OK, depending on the definition of "short amount of time". If it is a few seconds, it is still going to be fine. Your 12K/1K sets the temperature high, so 60 on the outside could still be OK.BTW, are you sure you have the "A" version?

author: borghese
author_rank: Regular Contributor
timestamp: June 20, 2019, 03:52:15 pm
content: I believe that instead the setpoint is lower; between 40 and 50 ° C.

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 20, 2019, 04:45:43 pm
content: That's really a pity.. that short drove an excessive current of many mA through the BE diode of the reference transistor. Usually, a pnp should handle that (Ib max ~ 1/3 Ic max), but probably not that special chip. I fear that it suffered a degradation of beta.50mA is too high, I think, but Andreas has somewhere published the current consumption for the A version with 13k/1k in the LTZ thread.This 13k/1k gives ~65°C, which is the correct selection for the A version, btw.The case gets too hot, so I think the LTZ runs much hotter inside.Please check the oven regulator also, as well as all the resistor values.Maybe you should check first all potentials at all of the LTZ pins, related to common ground , before you desolder it. You may post your measurements here, in the circuit diagram, so we could have a look on it.The build otherwise looks ok.Frank

author: iMo
author_rank: Super Contributor
timestamp: June 20, 2019, 06:41:25 pm
content: First of all I would doublecheck your PSU's current limit shows the proper currents. It could be it shows twice the real currents, hopefully..

author: Storm
author_rank: Contributor
timestamp: June 20, 2019, 09:51:06 pm
content: First of all: A big THANK YOU to all members answering on my request in such short amount of time!Second: I made two stupid mistakes on my build that tought me this lesson: Read carefully and double-check everything before switch-on...The shorted R10 was No.1 and the second one was using a 2N3904 for Q1 instead of a BC639, which has a different pinout... Oh what a shame...After desoldering Q1 and correcting the pinout I get 7.08677V between +REF and -REF on my Fluke 8502 and the current consumption is now 27mA@12V, which looks acceptable now.The temperature setpoint and regulation now look OK (0.506V) at pin5 and pin6 of LT1013 and 0.428V at pin3 and pin2 look OK for the current regulation as well.I still think that the excessive heating at the first power-on will result in performance degradation in the long term but let's see. I will run the device overnight and capture the min/max values, well knowing that my 8502A is not really suitable for this task but it is the best instrument I currently own.I will continue to install a paper-tissue filled plastic cover over the LTZ1000A and start constructing a suitable metal case for the device.I plan to run the unit for some weeks now from the lab power supply.Because I want to have an adjustable 10V-Standard, when does it make sense to calculate and order the booster resistors R12/R13/R14?In order to be consistent, I plan to use Edwin G. Pettis precision resistors for the booster stage again. I hope this makes sense?

author: TiN
author_rank: Super Contributor
timestamp: June 20, 2019, 11:04:22 pm
content: Welcome to the club.QuoteGood, we all did that, been there. Meaning no disrespect, just matter of fact - more expensive mistakes = better remembered lessons.I usually have 100mA limit for LTZ refs, current goes up for few seconds at the start to 50-60mA than drops fast to ~28mA when heater kicks in to good temperatures.LTZ1000Achip should be barely warm (~40c) because great thermal insulation between die and case. LTZ1000 non-A could be 60-ish. If A chip is 60c case - then thing is dead.You could try measure resistance between case and heater/zener pin (with circuit powered OFF). There should be megaohms on good A-chip.Good idea to post the schematics, so expert eyes on forum can help you to check things. One head is great, many are betterStill sleepy, didn't see it's Dr.Frank's design.

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 21, 2019, 05:50:12 am
content: So the chip is not completely defect, but at least will have acquired a big hysteresis by the elevated temperatureSuch an over heating, Andreas and I encountered on several different references, so welcome in the club!This will probably manifest as a big permanent timely drift, so that the reference voltage will rapidly increase in the next months.I suggest that you first store the unpowered PCB in a refrigerator at around 0°C for several hours, wrapped in a plastic bag to avoid water condensation after retrieval.Then you will probably see a remarkably higher reference voltage afterwards, a proof for the before acquired hysteresis. This shift will be on the order of about +5..10ppm.The future drift rate , noise and dip performance will show, whether it has survived, so you need to build your 2nd ref for comparing both against each other and vs. your reference voltmeter. And as soon as you have a proper setup, you may let it be measured by other volt nuts out of this group.Appropriate cases have been named, so no need to invent something else.The booster resistors can be ordered as soon as you have determined that the monthly change is less than, let's say 1ppm.To avoid future such catastrophes, please assemble that buffer amplifier with preliminary resistors, or x1 configuration, and measure the reference voltage only at the intended node.Frank

author: Storm
author_rank: Contributor
timestamp: June 21, 2019, 12:33:17 pm
content: Today the temperature in my home-lab rises according to the outside temperature in germany... my lab-thermometer reads 25.8°C.Running the device powered on over night for 14hrs revealed a min/max reading from 8502A of 7.08672V/7.08675V. The measured top case temperature of LTZ1000A by means of a Fluke 179 TC-probe is 39.0°C at 25.8°C room temperature and the device consumes 27.0mA@12.00V from my lab-power supply.I will take the advice from Dr. Frank and put the unpowered unit in a sealed plastic bag into our refrigerator today. Tomorrow I will add the buffer amp LTC1052 with preliminary resistors and continue to measure at +REF_buf against -REF, or (even better) use a preliminary x1 buffer configuration using the metal casing, thermal isolation and soldered Pomona 3770 jacks with low-thermal EMF-cables.In order to be able to measure a 1ppm monthly change (which is ~7.1uV) I need to use one of my (uncalibrated) Fluke 8505A meters, which can do 7.5digits in AVG-mode in the 10V-range. I recently repaired these meters but since I do not yet have a reliable CAL-Standard, I have no knowledge about this meters stability and drift...but that is the best option I can currently think of. In case I get reasonable long-term readings from this setup I at least know that the standard AND the meter show good stability...

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 21, 2019, 02:01:17 pm
content: To decide this 1ppm /time criterion, you need the 2nd LTZ as well.Typical drift of virgin LTZ at 50°C is -1..-3ppm / year, whereas such temperature excursions lead to initial higher drift rates, but probably recovering to these typical rates later.The 8505A may have higher drift, due to internal resistor dividersFrank

author: Andreas
author_rank: Super Contributor
timestamp: June 21, 2019, 02:02:46 pm
content: Hello,Quote from: Storm on June 21, 2019, 12:33:17 pmI would even repeat the procedure for 3-5 times.Quote from: Storm on June 20, 2019, 09:51:06 pmIf you plan to do a T.C. adjustment of the reference (with R10) you should do this first.(because the output voltage changes due to the T.C. adjustment)with best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: June 21, 2019, 02:28:30 pm
content: QuoteMh, that depends on your strategy... you can compensate zener voltage only, but most likely end up with some residual tc error by the divider for 10V boost or you can compensate overall tc, but are then left with an output voltage different from 10,00000V. So you have to choose your strategy.-branadic-

author: ArthurDent
author_rank: Super Contributor
timestamp: June 21, 2019, 02:42:24 pm
content: I made a couple of changes to my LTZ1000A board to try to increase the output current capability slightly and also eliminate any possible drops on the wires that run between the board and the 10 volts output terminals. I did this by running the negative lead from my 15 volt bench supply directly to the -10 volt output terminal of the reference and running the negative lead for the LTZ1000A board power from the -10 volt output terminal to the board. This means that any minute IR drop on the length of wire between the board and the negative output terminal that would be caused by any varying external load is eliminated.I also added a transistor, wired as an emitter follower, within the output opamp feedback loop so the output current from the +15 volt bench supply is handled by the emitter follower and not the opamp. This was an easy change to the existing board and shown in the photo below. I separated the run on the board where the top of the 10/7 volt divider string was connected and connected the top of the string directly to the +10 volt terminal. This way any minute IR drop on the +10 lead going from the board to the positive terminal is eliminated because the sensing is now right at the positive terminal.I’m happy with the way these simple changes have worked and testing show the differences are measureable. Unfortunately once when I was making one of these various changes I forgot to power down when I was soldering a component or two and the ‘click’ and red overcurrent light on my bench supply told me I was in trouble. The reference voltage from my aging LTZ1000A jumped from 7.xxxx volts up to 12 volts (deep sigh). Version 2 with a new LTZ100A with its foam cap is working just fine and has been aging for several months now.

author: Hermann W
author_rank: Contributor
timestamp: June 21, 2019, 02:58:07 pm
content: Quote from: Andreas on June 21, 2019, 02:02:46 pmHow do I do this TC adjustment? Place the whole reference in an oven (for example 30°C) and measure the internal temperature? Or is it better to measure moderately over the course of the day?But in which direction do I change R10? I can not find anything in the manual.

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 21, 2019, 03:30:39 pm
content: I forgot the T.C. compensation, Andreas mentioned it correctly.For that, you also need a 2nd LTZ reference.You should go through the long LTZ thread, about October 2017, maybe around page 76 or so, there I have described in great detail how to iterate the compensation, and how to bootstrap the problem, as you need something being virtually more stable temperature wiseFrankhttps://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg1336573/#msg1336573PS: For these experiments you need a real lab with constant temperature, otherwise you are completely lost.Do not in your basement

author: Andreas
author_rank: Super Contributor
timestamp: June 21, 2019, 03:33:39 pm
content: Quote from: Hermann W on June 21, 2019, 02:58:07 pmI have built a temperature chamber out of a (peltier) car cooling box + a heater foil for around 10 - 40 deg C.(photos see in the resistor T.C. measurement thread).Ramp speed is around 0.12 K/minute.You can increase T.C. with R10 by around 40 ppb/K for each 1 Meg resistor in parallel.(Yes you need many temperature cycles to get below 20-30 ppb/K).with best regardsAndreas

author: Hermann W
author_rank: Contributor
timestamp: June 21, 2019, 05:04:50 pm
content: Thanks to Frank and Andreas.Too bad that I am completely lost.Quote from: Dr. Frank on June 21, 2019, 03:30:39 pmI had hoped that I could measure the TC with a 2nd LTZ as a comparison. But I only have a bad 6.5 DMM. When my 2nd LTZ is finished, I will dare to try to find a TC. If not, it is not important to my basic equipment.I'm surprised that you, Frank, trimmed the TC with R9 and not with R10.

author: Macbeth
author_rank: Super Contributor
timestamp: June 22, 2019, 09:59:23 am
content: Quote from: TiN on June 20, 2019, 11:04:22 pmMy LTZ1000 (non A) starts at 49mA and takes a few minutes to settle at 34mA. I am using branadic's board / Dr. Frank's circuit.

author: branadic
author_rank: Super Contributor
timestamp: June 22, 2019, 10:38:18 am
content: QuoteYou are not lost, all you need is a temperature chamber and some constant lab temperature for your meter and your second reference during temperature cycling.You can build yourself a temperature chamber or you can simply buy one (wine cooler, reptile incubator, mini fridge...).They can be bought used and very cheap on ebay and the like and most of them can heat an cool. I bought myself a Lucky Reptile Herp Nursery II (temperature setting between 2°C and 60°C), drilled a 40mm diameter into the wall to get all the cables in/out and mounted 4x SMT172 temperature sensors to one wall (top, middle, bottom, DUT). Together with a SMTAS04 USB and the Smartec LogSmart temperature logger software you have a complete temperature chamber.See also EEVblog #101 video-branadic-

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 22, 2019, 10:42:52 am
content: Quote from: Hermann W on June 21, 2019, 05:04:50 pmThe reference designator changed, obviously.. of course i'm also always trimming the nominal 400k resistor, that is R10 in my schematic.The problem is, that you try to measure extremely small changes, all sub-ppm. Final goal for the DUT is achieving < 0.05ppm/K, whereas the initial T.C. of an LTZ circuit, or for your DMM is about ten times as hig, on the order of 0.5ppm/k. Please have a look into the specification of your Fluke 8502A, its T.C. is not necessarily much worse than the 3458A.So if your comparison reference, that is either another LTZ circuit, or also you DMM, are also exposed to smaller temperature changes, you maybe can't distinguish between the drift of the DUT, or that of the reference. Strategy is to keep the environmental temperature constant to a few tenths of °C, and changing the DUT by +/- 10°C in contrast.Frank

author: Hermann W
author_rank: Contributor
timestamp: June 23, 2019, 11:06:00 am
content: Another thanks to brandic and Frank.Quote from: branadic on June 22, 2019, 10:38:18 amThat's what I intended. The temperature chamber I have (long before I learn in this forum) is a modified pizza oven with a cheap temperatur controller which keeps the temperature at about 1 degree. I do the cooling with cooling pads on the oven. This works because the insulation is very thin. Of course I measure the temperature at the DUT.This reptile inkubator is very interesting, it makes it a lot easier.Quote from: Dr. Frank on June 22, 2019, 10:42:52 amThank god, now the world is alright again.The sub-ppm range, I can perhaps just measure in the 200mV range of the DVM. (my solartron 7150 is a 5.5 meter with 6.5 digits over averaging) More difficult are the constant ambient conditions for DVM and 2. LTZ.Important for me was the expected initial T.C. of an LTZ circuit with about 0.5 ppm/k. This is already better than anything that is available to me so far.But I'm just starting with the exact measurements and have already learned a lot in this eevblog.