10V Reference (I Did It My Way) No1 - Page 1
Forum section: electronics / projects

author: blackdog
author_rank: Frequent Contributor
timestamp: February 09, 2015, 09:20:28 pm
content: Hi Volt-Nuts, :-)I got several emails from people on this forum why I dont start my own topic over "Voltage References"Your Wish Is My CommandBut, please bear with me, I'm a dyslexic monkey, it takes a lot of time, to make reasonable sentences in Englels.And yes, Google is my Friend.OK enough blah blah...The first design that I want to show here, I already posted in another topic.It is designed approximately 3 to 4 years ago by me.I've been playing for over 30 years with voltage references and the last 5 years more intensive.But only in the last view years, I was able to buy the instruments, to test voltage references.I still have no Fluke 732a reference, therefore I myself am trying to build a voltage reference with good specifications.i have not the resources like Fluke to test my reference design.But i have a fresh calibrated HP3458A and two new KEYSIGHT 34461A DMM's.This is an older design of mine, but there are good decisions,such as multiple safeguards against the misuse.Let us begin the upper left, where the power supply is made for the 4x LT1021.The power supply is built around a LM10 IC, which has a 200mV reference, I scale to approximately 8V. R8 and C3 filter the noise of this voltage, this is a low pass filter of about 5Hz.The alert viewer falls on, that R 1 and R 8 has the same value, in order to avoid bias errors.At the output of the opamp, which is pin 6, there is a 47 Ohm resistor (R2) which, together with R1, C1 and C2 also create a low pass filter.The effect of this circuit is that there are only about 5uV RMS noise is present on the 8V power supply for the LT1021. The bandwidth of the measurement was 20Hz to 22Khz.The 8V power supply is clean, there is no visible popcorn or schutgun noise.The circuit might be a bit overdesigned.It is necessary to have a good quality capacitor for C3, as it is not DC offset corrected, like C1.Next part, mixing the references...To make the noise and drift lower of the already good LT1021, I use four of them in parallel mode.R4, R5, R6 and R7, together with the capacitors C4 and C5 create a low pass filter, having a frequentie of about 3.9 Hz.This filter is a "bootstrapt", R10 of 10K, ensures that the upper 330uF only sees a very low DC voltage, the leakage current is than very low.Also, use good quality capacitors.Gain of 2 amplifier.I used a selected LT12ACN opamp, i want to test with a LTC2057 or a Max44241.The problem is, there is not much info, about about the low frequentie noise, especially the current noise is not properly specified...I'm always fighting with the values of R12 and R17 (10K, down 0.01% Vishay S102) and the current noise.The lowest noise and the greatest stability, you need low value for these resistors.Low value for these resistors also means, that the dissipation in these resistors will go up.D1 and D2 is for protection, normaly there is no voltage over the inputs, so no leakage.To trim the Reference i use small 0.1% film resistors in parallel with a 10-turn potentiometer.Perhaps it is better to inject current through a resistor a high value in the -input of the opamp?Less connection @ the -input.BufferThe output of the opamp goes to a 1x buffer.As a result, the opamp has its maximum open loop gain.At the output of the buffer are 4 resistors of 100 ohms, C11 and 330UF again make of a low pass filter.C9 and R17 are also involved in this filter.ProtectionThe reason for the 4x100 Ohm is this, it is an inexpensive way to ensure that the reference remains intact, when power is put on the output. Bad Boy!!!L1 keeps HF away from de output of de buffer.F1, fast blow 150mA, u can choose a lower value, if you want.D5 PGKE18CA protects IC3 a buffer IC like EL2001, BUF634, LT1010 etc.And this is a high resolution picturehttp://www.bramcam.nl/Diversen/Quad-LT1021-10Vref-01.pngPlease shoot @ it, make it better ect. ect.Make it a community project if you wantMy version is housed in a 42C heathed BimBox, but about ovens, i wil show you later my designs.Kind regarts,Blackdog (Bram)

author: nctnico
author_rank: Super Contributor
timestamp: February 09, 2015, 10:47:52 pm
content: I got a grazy idea: how about putting a (thin) reference circuit board on a piece of aluminum (with thermal conductive rubber in between) and keep it at a constant temperature say 45 deg. C. That would eliminate temperature differences between components (including the reference chip).

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 09, 2015, 11:02:14 pm
content: Hey Blackdog,Nice to see you on this forum. I am looking forward on your posts and pictures of your references!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 10, 2015, 02:44:06 am
content: Curious use of TVS and FBs...Why use a bidirectional TVS in parallel with a diode? The unidirectional (-A) version is probably much larger (lower series resistance) than a 1N4001, let alone a 1N4007. Likewise, D5 missed the opportunity to be placed at the output, where it will absorb the surge and the series resistors will prevent the buffer from exploding; as shown, it might still blow the buffer (I'd have to check the datasheet). It should also be unidirectional, for similar reasons. Surge protection on the output, with all that capacitance there, is probably not a big deal, in the end, either way.As for the FBs, they're in series with ground (doesn't really matter?), a 100u inductor (may matter at HF, but generally not a big amount), or a 10k resistor (no effect at basically any frequency). So, they're probably not doing much..Why not also filter / regulate the amp/buffer supply? PSRR is good, but not fantastic. I suppose you could even go overboard and bootstrap the amp/buf with the ref voltage itself... who needs an outboard regulator when you have an even better inboard one?Bringing together multiple references is often a good idea (if not often used). Same goes for amplifiers in parallel (not op-amps -- due to differing Vos, but JFETs in parallel for a low noise buffer / preamp is a classic). Noise/error goes down as sqrt(N) for N sources; but, only for independent error. So temperature drifts will remain. As always, keeping convection currents away from the refs will help keep low frequency noise down.Speaking of large capacitors, with all those beefy 330uF caps around, some diodes to allow them to discharge into the supply (if it collapses) might help. The good old "back-diode around the 7805" trick.Tim

author: quarks
author_rank: Frequent Contributor
timestamp: February 10, 2015, 04:08:59 am
content: Looks very good to me.Thx for sharing

author: 3roomlab
author_rank: Frequent Contributor
timestamp: February 10, 2015, 04:26:03 am
content: sorry i have noob questionwhat does C4 do if thru R3/6/7, C4 will seem to see evened out voltages on both ends? EMI / noise / spike reduction ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 10, 2015, 05:11:10 am
content: Quote from: 3roomlab on February 10, 2015, 04:26:03 amLeakage reduction strategy: C5 takes "most" of the voltage (minus what gets dropped through R3, which is larger than the other resistors, to have less effect on the refs*), so that C4 gets 0V and almost exactly 0 leakage.*Except, if the voltage drop is small, the value of R3 will have little impact, because... that's the point, 100% of C5's leakage is drawn from the refs. So it's exactly as good as having C4 straight to ground, except with its leakage supplied by only one ref (more or less).What should be done is, instead of using C5 and R3, use a unity gain (not gain-of-2) amplifier, and use its output as the "guard ring" voltage: for the bottom end of C4, for traces around the board, stuff like that.Good catch.Tim

author: EEVblog
author_rank: Unknown
timestamp: February 10, 2015, 06:30:16 am
content: Didn't the EL2001 buffer go obsolete a decade or two back?

author: janaf
author_rank: Frequent Contributor
timestamp: February 10, 2015, 08:37:48 am
content: If crazy, I'm one! Got copper plate and large silicone pad in the drawer :-)You can get single-layer PCBs made on aluminium base material. Price $60 setup plus 0.02usd/cm2http://www.ebay.com/itm/290575172294Quote from: nctnico on February 09, 2015, 10:47:52 pm

author: blackdog
author_rank: Frequent Contributor
timestamp: February 10, 2015, 09:10:03 am
content: Hi,I will try to explain my decisions for the construction of this 10V reference.T3sl4co1lFirst, good remarks!I dit not make good labnotes, yes i know, a beginners mistake...Why use a bidirectional TVS in parallel with a diode?My bad, corrected :-)The ferrite bead and the 100uH keeps the HF out of de reference, this is one purpose of these components.The second one is, to keep the peak current down (100uH helps).By misuse, the fuse, 100uH coil, R19 / R22 and D5 to keep the energy from the sensitive circuit.If you use for the resistors R12 and R17 a LT5400 (Nice LT part) the voltage over these resistors are Max 80V.I fault conditions you can reach this voltage because the inductions of your cables who are connected to the voltage reference.T3sl4co1l, I already told you, good comments! 2 points ;-)To ensure that the peak voltage is not too high on the output, in fault conditions, i have placed an extra TVS across the output.I have not told you how the reference is used, normaly is powerd by the mains supply.The battery is only used to keep the sensitiv parts under power (reference + heather)About the 7805 diode trick, I thought of this, but am afraid of leakage currents...The schematic is fed from a good regulated power supply, built around a good reference IC.DaveEL2001 a beautiful part, I'm also more than two decade's old, and still working fineAbove the schematic i explained that you can use a BUF634 LT1010, etc. choose a buffer, it is not really critical part.High Res Picture..www.bramcam.nl/Diversen/Quad-LT1021-10Vref-02.gifKind regarts,Blackdog

author: EEVblog
author_rank: Unknown
timestamp: February 10, 2015, 12:52:08 pm
content: Quote from: blackdog on February 09, 2015, 09:20:28 pmIf you were really keen you could parallel opamp to lower the noise a bit.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 10, 2015, 01:04:43 pm
content: Can one assume that R12 and R17 have the same temp. coef. ?If they have the same temp. coef, are they thermally coupled?What I mean is, as they drift both indifferentdirections, lets say with 10 ppm/K, the drift due to this resistors will be much greater than the drift of the references?

author: codeboy2k
author_rank: Super Contributor
timestamp: February 10, 2015, 01:19:15 pm
content: Quote from: blackdog on February 09, 2015, 09:20:28 pmThe 8V regulator is quite good, very low noise. My calculations show it does indeed have about 5uV RMS noise, but that's at the reference amplifier output. The buffer amp adds another 15 uV RMS noise (which is still good!)Reference:Reference Voltage = 0.2 Vfilter cutoff in the feedback loop = 1/(2*pi*RC) = 1/(2*pi*2k21*15uF) = 4.8 Hzreference amp gain = 1+(39k2/1k) = 40.2reference output voltage = Vref * gain = 0.2 * 40.2 = 8.04 Vnoise gain = reference amp gain = 40.2spectral noise density @ 4.8 Hz = 45 nV/sqrt(Hz)RMS voltage noise @ BW 4.8 Hz = 45 nV/sqrt(Hz) * sqrt(4.8 Hz * 1.57) = 124 nV RMSRMS voltage noise gained up by noise gain = 124 nV * 40.2 =4.98 uV RMSOp-Amp:Source impedance = 2k21filter cutoff in the feedback loop = 1/(2*pi*RC) = 1/(2*pi*2k21*1nF) = 72 KHzopamp gain = 1noise gain = 1from figure 8: input referred noise @ 72 KHz is approx. 45 nV/sqrt(HZ)RMS noise @ BW 72 KHz = 45 nV/sqrt(Hz) * sqrt(72e3 * 1.57) =15 uV RMSTotal RMS noise = sqrt ( 4.98uV ^2 + 15uV ^2 ) =15.8 uV RMSThen, the LT1021-5 has a PSRR of about 94 dB according to the charts (and 12 ppm/V max according to the tables)So this 15 uV RMS noise on the power supply rail will equate to about 12 ppm/V * 15 uV = 0.00018 ppm of voltage change at the output of the LT1021-5. It's hardly noticeable, and you could even use a much noisier regulator and still be fine (which is why you admit it might be over-designed)If I made a calculation mistake, please point it out and I'll update my math.Thanks!

author: blackdog
author_rank: Frequent Contributor
timestamp: February 10, 2015, 03:27:34 pm
content: Hi, :-)I made some changes to the schematic, but that wil come later...Dave,More opamps is also more current noise...It's really hard to determine what is the best solution.I have at present no measurement setup to properly measure from 0.1 to 10Hz noiseAnd it takes a lot of time!codeboy2kYou are not far of the real world, about the noise calculations :-)Yesterday i made a small setup, to measure the noise of the 8V regulator.The test setup of the LM10, yes i know, breadboards en references, bad choise, but its not about DC stability in this test...RMS Noise measurend by the Audio Presisoin, Portable One Plus.About 5uV RMS, 22Hz-22Khz bandwith.This is a picture of de analyser output, don't mind de scoop scale, the analyser is in autorange mode.Look at the trace, no popcorn or shutgun noise, 1.2 Seconds data...JohnnyBergThe whole reference is in a 42C oven, temp drift is not so important,uneven aging of these resistors is important, since the oven does not help to reduce it.Kind regarts,Blackdog

author: codeboy2k
author_rank: Super Contributor
timestamp: February 10, 2015, 04:26:21 pm
content: Measured real-world noise is always going to be better than max noise.My numbers came from the datasheets, and I think the datasheets try to be conservative when they list noise numbers. They don't ever want someone to say they measured 15 uV noise when the datasheet said it would only be 5 uV noiseBut still nice to see that it comes in better than the datasheet specs.PS: Can you measure the noise at pin 1 (the ref-amp output) over the same spectrum ?

author: blackdog
author_rank: Frequent Contributor
timestamp: February 10, 2015, 06:42:47 pm
content: Hi codeboy2k,I dit the measurement you ask for.The noise on pin-1 is higher than your calculations, about 190uV RMS :-)I had to change the powersupply to batteries, because of the high commonmode signal at my workbench at the moment.I hope this measuremt helps you.Kind regarts,Blackdog

author: codeboy2k
author_rank: Super Contributor
timestamp: February 10, 2015, 07:10:02 pm
content: Wow.. thanks! That's the gained up reference noise, before the 5Hz filter.Pin 3 would be the filtered noise, input to the final op-amp.

author: Andreas
author_rank: Super Contributor
timestamp: February 10, 2015, 08:22:16 pm
content: Hello,instead LT1021 with metal can housingone could use the (today easier to get) LT1236AILS8-5 with hermetically ceramic housing.But you would need some dead bug style mounting to a standard pcb to reduce humidity influence.From behaviour the LT1021 and LT1236 are very similar.Except that the LT1236 is available with low TC and at the same time with high accuracy.(The LT1021 comes either with low TC or with high accuracy but not both).With best regardsAndreas

author: blackdog
author_rank: Frequent Contributor
timestamp: February 10, 2015, 09:34:20 pm
content: Hi andreas,I cant say anything negativ about you remarksI already started making changes to the schematic, i wil post it later this week.I propose diffferent 5V reference, like the LT1236, LTC6655, LT1027, APEX VRE310A, ect.Kind regarts,Blackdog

author: Andreas
author_rank: Super Contributor
timestamp: February 10, 2015, 09:39:32 pm
content: Hello,best of all would be my "famous" dead bug in hole mounting method which can be seen here:https://www.eevblog.com/forum/projects/oshw-24bit-adc-measurement-system-for-voltage-references/msg474504/#msg474504With best regardsAndreas

author: blackdog
author_rank: Frequent Contributor
timestamp: February 10, 2015, 10:09:48 pm
content: Hi Andreas,I do not want to polute this topic, but how about this...Because i like to play with referencesA ceramic resistor BPR10, 50 Ohm.The oven controler on the BPR10 resistor, nice blue WLK 30 glue!The reference schematic.Dead bug style, mounting, the reference is the LT1236-LS8Kind regarts,Blackdog

author: Andreas
author_rank: Super Contributor
timestamp: February 10, 2015, 10:53:34 pm
content: Quote from: blackdog on February 10, 2015, 09:34:20 pmVRE310A is a 10V reference.With VRE3050A I have no good experiences (too noisy, much hysteresis)LT1027 is only available in plastic. (its a pity: the chip seems to be first class of unheated references).LTC6655 also has much hysteresis. The only advantage is the low minimum supply voltage. (if one needs it).So LT1236AILS-8 or AD586LQ would be my choice.What I ever wanted to do but had no time up to now:use the (chip - temperature dependant) voltage at the trim pin of the LT1236 for temperature compensation.When using glue you might introduce already some mechanical tension to the package.You have to try this.With best regardsAndreas

author: codeboy2k
author_rank: Super Contributor
timestamp: February 11, 2015, 06:04:58 am
content: Quote from: Andreas on February 10, 2015, 10:53:34 pmThe adhesive might shrink a little, but if it's a ceramic package, shouldn't it be able to handle any mechanical stresses better than the plastic packages ?and 2nd) even if there was a mechanical stress, would it be ok to trim the output 10.0000000 AFTER the glue dried? or is there a risk still?

author: Andreas
author_rank: Super Contributor
timestamp: February 11, 2015, 10:14:46 pm
content: Quote from: codeboy2k on February 11, 2015, 06:04:58 amBetter: yes. Perfect: no.E.g. in the first notes to LT1236AILS8-5 LT stated 12ppm humidity sensitivity for a 30% rH change. This is only due to the swelling of the epoxy pcb used where the LS8-ceramic package is soldered. On the current datasheet the rH spec is deleted (because it is only related to the pcb material used).Even with my "in hole" mounting method. I can get output voltage shifts of 5-7uV when I apply mechanical tension to the pcb. And there is only one wire connected directly to the pcb.With best regardsAndreas

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 11, 2015, 10:33:44 pm
content: Quote from: EEVblog on February 10, 2015, 06:30:16 amWow, élantec. Yeah, 20 years.

author: blackdog
author_rank: Frequent Contributor
timestamp: February 11, 2015, 10:50:46 pm
content: Hi,I'am sorry...That i used a good quality product, from the 1990's, i'am a bad boyThe next schematic that I present here, will only have components that are not older than 6 months.Kind regarts,Blackdog

author: babysitter
author_rank: Frequent Contributor
timestamp: February 12, 2015, 05:09:45 am
content: Preferrably Infineon, which have at least had enough short engineering-sample-to-obsoletification-cycles for me to realize, so if people are not fast they will immediately be punished

author: branadic
author_rank: Super Contributor
timestamp: February 12, 2015, 06:55:01 am
content: Quote from: Andreas on February 10, 2015, 09:39:32 pmIn addition, you can also use thin base material such as 0.1mm FR4 or Polyimid the reference is soldered to and solder this flexible pcb to your main pcb or use a semiflex pcb, with the flexible part for the reference. This ensures a much smaller stress relief because of less swelling of the pcb by humiditiy, less thermal mass, but causes maybe other problems. However, this solution is more process reliable.branadic

author: babysitter
author_rank: Frequent Contributor
timestamp: February 12, 2015, 09:10:06 am
content: Instead of going for a quite expensive flex-rigid processed PCB, consider using cheap flat foil cable.(Says someone who finally decided for a flex-rigid pcb at work after starting his design with ffc)

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 24, 2015, 08:31:39 pm
content: Quote from: 3roomlab on February 24, 2015, 08:25:28 pmHmm .. you certainly have me confused .. what is this supposed to do?Is it some kind of noise generator?Generally speaking when doing voltnut stuff: "less is more"

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 24, 2015, 09:25:35 pm
content: Quote from: 3roomlab on February 24, 2015, 08:25:28 pmHmm .. a opamp as a voltage follower behind a reference will add offset, offset drift and noise to the reference voltage.When amplifying the voltage, you get the TC of resistors added to the TC of the reference. You need very expensive resistors, or put the whole thing in a oven.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2015, 03:48:36 am
content: Any time you see "three terminal model", it means the output is unhindered by conventional notions of things like "power supplies", or "breakdown voltage of the quantum vacuum".Tim

author: babysitter
author_rank: Frequent Contributor
timestamp: February 25, 2015, 06:18:52 am
content: Quote from: JohnnyBerg on February 24, 2015, 09:25:35 pmDid right that with a second LT1013 as a x1 buffer for my LTZ1000A (see in the ultra-thread), thus preventing heater going berserk if I ever short it out. Influence is small. You even can find it in the datasheet how small you can expect it to be. Comparisons against a bunch of unbuffered LTZ's showed, if all, contributions to a very small overall impact.

author: blackdog
author_rank: Frequent Contributor
timestamp: February 25, 2015, 08:17:35 pm
content: Hi Group....I've done a number of tests on the LT1236-5V references.This version => LT1236A-5V LS8I placed them on a smal oven to heath them to42,5Cand back to room temp.I have some pictures so you guys can see how the LT1236 behaves when cycled between these temperatures.The first image is LT1236 number-1, and shows the drift when cooling down.At the beginning of the chart is power of the oven is turned off.At the end of the chart where i stopt measuring, the temperature is cooled down to 29 Celsius.The temperature sensitivity from this LT1236, is a little more than 2PPM / CLT1236A LS8 Number-4Sorry different style of the picture.Look at the middle of the chart, there i started the oven.In the middle to the end is about 90 seconds (fast oven!!!)Afther 90 seconds the temperature is stable and you can see there is a hump in the graph.This graph is totally different, the other graph was linear and this looks like a camelAt the beginning of the chart, the power of the oven is switchd off.At the end of the chart where i stopt measuring, the temperature is about 42.56 Celsius.The temperature sensitivity from this LT1236, far below 1PPM / CNext time also some pictures of the "oven" i used.Kind regarts,Blackdog

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 25, 2015, 08:21:20 pm
content: Are those LT1236's from the same batch?

author: blackdog
author_rank: Frequent Contributor
timestamp: February 25, 2015, 08:36:53 pm
content: Hi,Not all LT1236 references are measured, it cost a lot of time...I try to find a good way to do these measurements.If I know how I can do good measurements, which will be done on 10x LT1236-5V IC's, with half of them from one SMD plastic strip.You guys will have to wait until I'm done.Kind regarts,Blackdog

author: Andreas
author_rank: Super Contributor
timestamp: February 25, 2015, 11:42:35 pm
content: Quote from: blackdog on February 25, 2015, 08:17:35 pmHello blackdog,the typical T.C. for the LT1236 is around 2-4 ppm/K (relatively linear but with a slight hysteresis).From my measured 20 LT1236 I have only 2 which are deviating to this rule.So when having extremely low T.C. I would consider a mounting anomaly of the chip within the housing.(perhaps too much die attach or a edge of the chip which has contact to a sidewall).The one of these low T.C. references (#2) I had large initial ageing drift. Now I am using this chip in one of my ADCs (ADC17).(I really do not know if I can trust this candidate or if he will start larger ageing drift some day).The other (#8) has still large 0.1-10 Hz noise. About a factor 2-3 more than the usual 2-3uVpp.(see oscilloscope photo with 2 uV/DIV and 10 sec sweep time).With best regardsAndreas

author: blackdog
author_rank: Frequent Contributor
timestamp: February 26, 2015, 02:25:07 pm
content: Hi Andreas :-)At the moment i have only 10x total LT1236, in a and b version.The problem is that you are never sure of how a component behaves over time ...Check, check, check over time, References are a BitchWat kind of preamp are you using to measure the noise, one of Jim Williams designs?Gain 60, 80 100dB?OK, Picture time!Let me start first with the oven, it is built on 2x a BPR10 resistor (50 Ohm).On the picture below you see 5 pieces and i use in this oven 2 pieces.I remove the connection pins and glue on each site a thin double sided printed circuit board (0.25 mm thick board)This gives some extra thermal mass and it is good solderable.The copper of both board (top and bottom) helps distribute heat very well.This is one of my designs for a oven controler, this one, is using a TI, INA125 Chip, little expensive,but you have good control over characteristic's (compensation)The LT1236 are powerd bij a 9V battery.The breadbord holds the decoupling capacitors and a easy way to connect my testclips.Close up...There are three PT1000 sensors, the sensor in the center, goes to the TEK DMMM4050 multimeter to measure the temperature of the oven.The two other sensors are connected in series and are for controlling the oven temperature.The "big" red cap is for power supply decoupling.This is the oven controler "Manhattan Style", on the right side of the sensitive components are buried under a layer of thermal adhesive (WLK30 Fischer)At the bottom left, one can see a small power FET, which regulated the current through the resistors.LT1236 Pictures, Power Up and Power DownI measured six LT1236, as the oven heats up very quickly, this measurement takes only 1 minute.In the first picture you can see that there is a hump in the graph.This little anomaly is due to the oven current wich drops when reaching the selected oven temperature.This has nothing to do with the LT1236, it is a measuring abberation.In the first picture i placed the temperatures at two points, in the middle that is about 36C and at the little hump, thats about 41C.Power Up LT1236-01Power Down LT1236-01This is the oven current, you can see, its nicely damped.And the current drops at the same time the is a nanomy in the reference output.Power Up LT1236-02Power Down LT1236-02Power Up LT1236-03Power Down LT1236-03Power Up LT1236-04Power Down LT1236-04Power Up LT1236-05Power Down LT1236-05Power Up LT1236-06Power Down LT1236-06It is clear, LT1236-6 is a BAD BOY!Yes its whitin the specs, but not as nice as the other ones.With the 34461A multimeter, it is not possible to measure hyteresis at the 20 degrees temperature variation,these variations are too small at the six measured LT1236 references.Shoot @ it!Kind regarts,Blackdog

author: Andreas
author_rank: Super Contributor
timestamp: February 26, 2015, 09:32:50 pm
content: Quote from: blackdog on February 26, 2015, 02:25:07 pmHello,I was inspired by AN124 but did a much simpler design.Just 2 simple OP-Amps with a 4rth order Bandpass. And selected standard 85 deg C capacitors.80 dB together with 2mV/div lowest resolution on my analog oscilloscope give 0.2mV/div maximum sensitivity.Noise floor is around 0.12uV rti.So the noise of a LTZ1000 can be easily measured.But you will have to add a series resistor (3k6) and a switch (closed after charging) to charge the capacitor in order not to kill the LTZ1000 by the inrush current. (heater setpoint will go to "infinity" when the output is loaded by more than about 2mA so the device ages very fast).With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: February 26, 2015, 10:19:22 pm
content: Quote from: blackdog on February 26, 2015, 02:25:07 pmHello,try harder !Perhaps you have to use a additional temperature stabilized reference and measure the difference with a higher resolution. (in the 100mV range).On the other side you would have to stay at minimum for half an hour or so on the hot temperature so that the die attach on the reference can begin to creep. Half an hour is about the time constant that I found at least on my #02 sample.With best regardsAndreas

author: blackdog
author_rank: Frequent Contributor
timestamp: February 26, 2015, 10:47:20 pm
content: Hi Andreas, :-)Thanks for the schematics of the measuring amp!I wil built one this weekend!Tomorrow I'll show my adapted schematic of your preamp.And I understand your comments on the load of the reference measured.But, I have a question, I do not understand the filter at the output of the LT1012.Strange corner frequenties...Can you explain this for me?Thanks!Blackdog

author: Andreas
author_rank: Super Contributor
timestamp: February 27, 2015, 08:27:52 pm
content: Quote from: blackdog on February 26, 2015, 10:47:20 pmC6 + C10 are only for EMI supression (the oscilloscope has a mains line) no filtering function.All other parts including the reference are battery supplied.And do not forget the (metal) cookie box. (or a paint tin can if measuring TI devices).I use a additional TEKO aluminium box for the amplifier.with best regardsAndreas

author: blackdog
author_rank: Frequent Contributor
timestamp: February 27, 2015, 09:06:38 pm
content: Hi Andreas, :-)First of all, i will try harder for the hysteresis effect...OK noise amplifier question!I'am talking about R12, C10 (1,6Khz) and after R12 there is a resistor 2k2 or 1k? in series with C8?I know that C8 and R10||1Meg scoop resistor, makes a ~60mHz filter.But why the two resistors in series?At the moment i'am testing some capacitors, but its difficult to find some with low leakage currents.What brand and type you've selected??I tested already 10 pieces, the lowest 1000uF, was approximately 60nA...Afther some time formatted at 15V.Kind regarts,Blackdog

author: Andreas
author_rank: Super Contributor
timestamp: February 27, 2015, 09:45:53 pm
content: Quote from: blackdog on February 27, 2015, 09:06:38 pmHello,Try it out. Short the resistor comming out from OP-Amp to the 100nF capacitor.You will need at least 2 days forming/formatting? at >= 10 V.And do not heat the capacitor during soldering.It´s also a good idea to reform the capacitor 2 days before measuring.(otherwise there might be some popcorn noise).I used just that what I had in the drawer. (2200uF Elna NOS and some 1000uF YAGEO?)Higher voltage rating than needed could be better.From 10 capacitors there should be 2-3 which meet the spec.By the way: for decoupling the amplifiers I use 1000uF + 100nF for each supply.And between pre-amplifier and end amplifier 100 Ohms for further filtering.With best regardsAndreas

author: blackdog
author_rank: Frequent Contributor
timestamp: February 27, 2015, 10:01:24 pm
content: Hi Andreas,I'am aware about the load a opamp can't handle, R12 and C10 are clear to me.But why is there a resistor between C10 and C8?Is this resistor 2K2 or 1K?RFI filtering was not needed?, chokes ect.Kind regarts,Blackdog

author: Andreas
author_rank: Super Contributor
timestamp: February 27, 2015, 10:17:27 pm
content: Hello,Resistor (name = 2K2) with value 1K (as can be seen on the photo) prevents together with C10 noise comming from the mains line to be demodulated on the OP-Amp output.I rather use resistors than chokes if possible. They have lower parasitic capacitances.The whole cirquit is double shielded (cookie box and TEKO A3 enclosure).With best regardsAndreas

author: blackdog
author_rank: Frequent Contributor
timestamp: March 08, 2015, 02:44:40 pm
content: Hi AllI've been a little busy...Now an update on the changes of the Quad LT1021 reference.First update of the schematic.Link to the big schematicwww.bramcam.nl/Diversen/Quad-LT1021-10Vref-03.pngIf anyone is interested in more than 50 hours of data, below are two files for the voltage and temperature in the OLD situation.10V data, there is one big spike, sorry ;-)www.bramcam.nl/Diversen/Quad-LT1021-51Hour-10V-2015-03-07.csvTemperature data, the sensor is between two of the four LT1021 references, see picture below.www.bramcam.nl/Diversen/Quad-LT1021-51Hour-Temp-2015-03-07.csvThis is the old situation of the reference circuit board.This is the new setup, the 2x opamp is now a LTC2057, you can just see him, in the blue circle.The temperature sensor is mounted between the middle two LT21021 references in the red circle.I've tried various circuits for the oven controler circuit, but I went back to the LT1006 circuit.The problem is not the LT1006 controler, but the mechanical layout of the Power FET and the two heather resistors and the sensor place.If the oven is cold, there is about 1.2Amp of current through the resistors and there is almost no voltage over the Drain/Source of the Power FET.The sensor is glued between the two resistors and that is OK but...When the oven is reaches its equilibrium, most of the voltage is across the FET and de place of de sensor is now not optimal.The distance is about 3cm and the thickness of the aluminum is not enough for good thermal coupling.When the oven is powerd up, the resistors dissipate about 0.1 Watt and the FET about 1.2 Watt!Between the two Rifa 6,8uF and the blue 150uF capacitor is the Power Fet.On the right side you can see the two Vishay 5 ohm power resistors, and in between, the sensor LM335.The yellow ntc on the right side, was only for testing, its not there anymore.And what can I do now to improve the oven...Make the bottom of the oven thicken, so give a lower thermal impedance.I orderd some sheet copper, 0.1, 0.8 and 2mm.And have a piece of copper cut out the size of the bottom of the oven.Here you can she the piece of copper, and the polystyrene box, fore size reference i placed a 9V battery on the oven.Yes, 2mm of "Low Impedance"This is a second use of isolation transformersIs it now a good oven, it is OK but i will not use this setup anymore.Later more about ovens and the problems.Some more pictures, this one is about 150 seconds of measuring time.1uV/Div.Almost three hours, After 30 minutes, the sun began to shine in my LAB, the voltage went up.This can be the reference drifting, but i think, a big part is the 34461A DMM :-)5uV/DivShoot at it!Kind regarts,BlackdogPSTemp and Voltage a little more then 3 hours.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 08, 2015, 03:03:15 pm
content: Nice pictures, again!I have done some experiments with ovens lately. Of course, they oscillate like hellFrom your descriptions I read that the location of the sensor is critical. But what if you have no freedom on the place of the sensor.It is on the die, for example?A lot of bandgap references give a PTAT signal for free, something we Dutch like a lotI noticed that your temperature regulation is a simple P regulator. I guess, by using a PI or even a PID regulator, sensor location will be less critical and the regulation more stable? I remember the professor saying that for a P regulator to work, there must always be a error signal, eg a difference between set and feedback.Furthermore, you test a lot to see if you got a long term stable situation. My experience is that when cycling the temperature, thermal hysteresis is a very important factor. 10ppm difference after a cycle is not unusual. Unless your reference is 24/7 on, how do you solve this?

author: Andreas
author_rank: Super Contributor
timestamp: March 08, 2015, 03:20:02 pm
content: Hello Blackdog,for ovens I usually use 2 temperature sensors.The first immediately at the heater.(fast response for the P-controller).The 2nd is at the point of interest.(slow I-Part for the I-controller).Usually I use the I-part to tweak the setpoint ofthe P-controller so that the final temperature is reachedat the point of interest.with best regardsAndreas

author: blackdog
author_rank: Frequent Contributor
timestamp: March 08, 2015, 05:50:12 pm
content: Hi,Thanks for the remarks.Most of the ovens i make, are small like the other topic on this forum.https://www.eevblog.com/forum/projects/small-oven-for-2x-lt1236-%28python-reference%29-no1/The small ovens almost never need capacitief compensation in the loop, this is because the good coupling between the power source and the sensor.Also becouse there is only a little thermal mass.JohnnyBergWhen the TI REF5050 came out, i played with the temp output, but you have to use a smal thermal mass and good coupling between the REF50550/REF5010 and the heather.Someting like a TO220 transistor.Cut of the thick wires of the TO220 housing, and use thin wires.Do not mount the TO220 connections on a print, you wil lose so mutch energie and it wil be inposible to get is stable :-)This is one of the controlers i designed, and i like the most, simple and good controle over the setting.R1 in the schematic controles the temperature and R3 sets the gain, the do not interact like most of the schematics you see.Use max. gain as posible, without oscilation.R4 and R5 keeps the output relatively in the middle of the commonmode range of the INA125.If you want to use a transistor for the FET, no problemo, just change R4 and R5.And if you dont like the BPR10 resistor, kick it out, and use only the transistor as the heath source.I use about 10x thepoweras Max to setup the current limit at normal LAB temeprature.I am not designing for military use, in my LAB its between 18C in the winter and 30C in the summer.Some pictures of oven design...This is a piece of ALU tube, the wall is 5MM and you can see a small hole wherethe NTC fitted in, not tested yet.This is a bad way to do it, a small sheetmetal box...This is a project i stil have to finnish, a 1Khz AC reference, yes, in the middle a LT1021 as a DC reference.On the right the LM10 oven controller and on the left you can se a small ceramic TCXO for the 1Khz reference generator.On the left en right site of the LT1021 you can see a PT1000 sensor for controling the temperature of the oven.It is al mounted on a big sister of the BPR10 resistor.On the other site is the control circuit for making 1Khz 1V 0.01% and the distortion in below 0.001%, the 1 Khz wil be within 1PPM.Maybe this week i finnish this setup of a oven, i uses 4 powerfets to distribute the heat for the oven.I hope that is wil not be a 0.1Hz oscilatorThe black and the white twisted wires are both 5K NTC resistors for controling the oven.This alu tube is 4mm thick and on top is a thin layer of FR4 mounted.This are the last ones i show today, smal thick wall alu box.The design is such that it there is no setting for the temperature.It wil be whitin 41 tot 44C by design, sweetspot around 42C.Im made a tester, to test the temp sersor before mounting it in the box.You can see the sensor in the middle bonded to the aluminum.This is stil a design in progres, and maybe i wil sell is as a product.HysteresisI know, for my own use, i select the parts.This week i received more LT1236A references and i am testing these.It is difficult to see the hysteresis on a 34461a and i want to ask Andreas how he is using the 3458A DMM.I have a measuring computer W7 USB-GPIB of HP and a PCI HP GPIB card a modern one.In the Agilent IO, i can talk to the 3458a, simple commands...I also installed IZGPIB on this system, but i have yet to learn the command structure.So Andreas can you kick me in the wright direction so I can make pretty graphs with the 3458a?If the output is a .cvs, then i am happy Excel wil do the rest.Kind regarts and thanks,Bramsorry for the many grammatical mistakes, strong headache today...

author: Andreas
author_rank: Super Contributor
timestamp: March 08, 2015, 08:11:17 pm
content: Hello Blackdog,sorry I do not have a 3458A like Frank. (so probably you should ask him).My ADC´s which I use have a host program on PC which generatesa CSV out of the binary data which is transferred by the ADCs.The rest is spread sheet calculation.With best regardsAndreas

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 08, 2015, 09:21:46 pm
content: I've done this before;No pictures, unfortunately.Construction is a copper sheet of modest gauge, bent into a trough, with a piece of copper clad PCB soldered in the middle for deadbug construction of the circuit. Also soldered to the sheet are the transistor and regulator, at opposite ends. Since both the regulator and heater carry essentially the same current, heat will therefore be balanced between them when the supply is a nominal 10V or so. (Under ambient conditions, it regulates from 7V to 18V and "naked" to "wrapped with as much fluffy stuff as I can find".)Compensation is slow and stable, with a time constant around 30 seconds. I think it had mild overshoot, but nothing crazy.Tim

author: blackdog
author_rank: Frequent Contributor
timestamp: March 18, 2015, 10:50:14 am
content: Hi Heren <=DutchSome pictures of the data afther the rebuild of my Quad LT1021CH-5 Reference.Measuring thime is almost 240 Hour.The top graph in this picture shows the drift from "0" function (9,6e-07V) <Stupid BenchVue software, code written by hobbyists!!!1Div. = 1PPM (10uV), the mountains in the graph, the sun is shining in my LAB.The bottom graph in the temperature drift @ the LT1021 references.1Div. = 0,1C, you can see very well when the sun is shining.Hi Resolution foto link.www.bramcam.nl/NA/NA-Quad-LT1021-10V-Ref/NA-Quad-LT1021-10V-Ref-Drift-Temp-238H.pngHistogram of almost 240 Hour.And this is nice...I isolate the Polystyrene box better with a small towel and the temperatuur in the oven went up, about 0.8C!And nothing to see at that point at the voltage output.So the DC graph, indicates for the most part, the drift of the KEYSIGHT 34461a DMM...I left out the "trim" possibility in this reference, it makes it more stable and its only about 3.8 PPM of from 10.000000Time to build de Quad LT1021 reference in a nice box.Kind regarts,Blackdog

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 18, 2015, 11:07:00 am
content: Quote from: blackdog on March 18, 2015, 10:50:14 amNice pictures!May I conclude that keeping the temperature of the oven is not that exiting?After all, a 0,8°C temperature jump did not change the output voltage.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 18, 2015, 11:20:05 am
content: Hi JohnnyBerg,I used 4 selected aged LT1021CH-5 and the LT1021 is already a good part, thats why i good let you see this nice dataKind regarts,blackdog