Building a 7 decade voltage calibrator - Page 1
Forum section: electronics / projects

author: gerrysweeney
author_rank: Regular Contributor
timestamp: September 22, 2013, 08:06:25 pm
content: Does anyone know of any projects to build a high quality seven decade voltage calibrator? I am thinking about having a go at building one but was wondering if it had been done DIY with any degree of success?Gerry

author: Andreas
author_rank: Super Contributor
timestamp: September 22, 2013, 08:20:18 pm
content: Linear Technology AN86 may be a starting point.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: September 22, 2013, 08:34:55 pm
content: Another posssibility may be this:http://www.edn.com/design/other/4326640/DC-accurate-32-bit-DAC-achieves-32-bit-resolutionBut this design leaves much room for optimization.The given values for accuracy and linearity are very optimistic.With best regardsAndreas

author: saturation
author_rank: Super Contributor
timestamp: September 24, 2013, 11:50:36 am
content: You could modify the basic form of any one of the series of defunct pure analog EDC calibrators. They are 6 decades, but have great specs and haven't been made since the 1980s. Dave reviewed and tore down the MV106 version some time back.Here's the full manual, includes schematic and theory of operation, for one variant that goes from 300VDC to 300mVDC ~ 30ppm 1 year accuracy and 5-10ppm ?? 1 hr transfer accuracy. Note those specs are ~ identical to all models EDC produced as voltage references.http://ko4bb.com/Manuals/09)_Misc_Test_Equipment/Krohn-Hite_VS330_DC_Voltage_Standard_Operator_Manual-1979&Before.pdf

author: JoeyP
author_rank: Frequent Contributor
timestamp: September 24, 2013, 05:12:44 pm
content: Also take a look at the Data Precision 8200. They've been out of production for a while, but full schematic can be found on line. They use mostly cheap simple CMOS switches to get surprisingly good results.

author: quarks
author_rank: Frequent Contributor
timestamp: September 24, 2013, 07:40:36 pm
content: Hello Gerry,that will be a very interesting DIY project. What are your thoughts so far about the voltage ranges, accuracy and stability you like to achive?If you want the best possible result, you probably want to use a LTZ1000 reference.Then have a look herehttps://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg180731/#msg180731and for some ideas about Kelvin Varley Deviderlook herehttps://www.eevblog.com/forum/projects/kelvin-varley-divider-(and-precision-voltage-source)/msg70859/#msg70859And look into the articles from Conrad Hoffman (also member of this forum) about metrology, KVDs, Hamon Divider and bridges.http://conradhoffman.com/electronics01.htmhttp://conradhoffman.com/mini_metro_lab.htmlhttp://conradhoffman.com/HamonResistor.htmlbyequarks

author: Andreas
author_rank: Super Contributor
timestamp: September 24, 2013, 07:57:23 pm
content: Quote from: JoeyP on September 24, 2013, 05:12:44 pmIn this design the quality of the switches does nearly play no role by design.But you will need many (>15) highest quality resistors (hand trimmed for accuracy + linearity) to achive the good results for the 10V output range.In the mean time 20 bit DACs are available (laser trimmed) which will have same or better data.With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: September 24, 2013, 08:54:17 pm
content: QuoteWow, this is pretty interesting:DAC1282 - 24bit -http://www.ti.com/product/dac1282DAC1280 - 24bit -http://www.ti.com/product/dac1280DAC1220 - 20bit -http://www.ti.com/product/dac1220AD5791 - 20bit -http://www.analog.com/en/digital-to-analog-converters/da-converters/ad5791/products/product.htmlI wonder why I couldn't find parts from LinearTechnology.

author: sync
author_rank: Frequent Contributor
timestamp: September 24, 2013, 09:40:41 pm
content: Building a n-decade voltage calibrator is on my to-do list since a long time. I think 7 decades are out of my skills and needs. I want usable 5-6 decades. Which should be hard enough. I want to experiment with amspires sigma-delta PWM1and Linear Technology AN86 ideas. One goal is not using resistors for anything affecting the output voltage (at the basic range). Ultra precision resistors (hybrids) are hard to get and expensive.1)https://www.eevblog.com/forum/projects/general-purpose-power-supply-design-7488/msg99807/#msg99807

author: sync
author_rank: Frequent Contributor
timestamp: September 24, 2013, 10:15:07 pm
content: Quote from: branadic on September 24, 2013, 08:54:17 pmIt has a differential voltage output. So an ultra precision differential amp is needed, right?QuoteDifferential current output. Worse than above.Quote0-5V single ended output - fine.QuoteA "high" voltage DAC. It can use a 7V Reference directly. Nice!Edit: Wow! These are really expensive (60$ or 100$).

author: Andreas
author_rank: Super Contributor
timestamp: September 25, 2013, 06:12:16 am
content: Quote from: sync on September 24, 2013, 10:15:07 pmThe AD5791 is the one that I had in mind.Compared to the around 11 high precision resistors (10-60$ each depending on long term stability) that it replaces + the trimming time + the equipment for trimming the 100$ are a good invest.All resistors on the single chip have same temperature and this results in < 0.05 ppm/°C temperature drift which cannot be done with discrete resistors without a temperature stabilized oven.And the 1ppm linearity spec is better than many 6,5 digit precision instruments have.With best regardsAndreas

author: sync
author_rank: Frequent Contributor
timestamp: September 25, 2013, 02:20:22 pm
content: It's surely a very nice IC. But I'm not comparing it with a discrete resistor solution. That's just impractically. I hope that an AN86 type solution will give similar results.I would using the AD5791 directly with a 7V reference. The resolution would be 6.7uV. For the calibrator I would use 10uV resolution. Actually i find the 6.7uV too coarse for this. The AD5791 has 1ppm typ. and 3ppm max. INL. That is an error of 0.7 typ. and 2.1 max. "counts".Unfortunately the LTC2400 can not used with a 7V reference. I would use a capacitive divider to get 3.5V. It has 4ppm typ. and 15ppm max. INL. With the same 10uV calibrator resolution it's an error of 1.4 typ. and 5.25 max. "counts".The LTC2400 has also the advantage of a much finer resolution. And the AD5791 has a DNL up to 2.5 LSB. If i understand this correctly it's not guaranteed to be monotonic.Please correct me if I'm wrong or wrote complete nonsense.

author: saturation
author_rank: Super Contributor
timestamp: September 25, 2013, 03:15:37 pm
content: Just a side comment, if you consider building around the EDC design, you have a proven performer tested in the field by industrial users for several decades because said models contributed to the economic viability of the EDC Corporation for many decades plus allowed them to create and sell variations of these voltage calibrator models based on adjusting the basic modules. Thus, the design is fairly flexible and proven. The components are fairly common, but they will need replacement with modern versions.

author: branadic
author_rank: Super Contributor
timestamp: September 25, 2013, 05:25:57 pm
content: As far as I can see on the picture of the evalboard:they have spent a voltage reference circuit based on the LTZ1000. What is missing is the schematic for that specific part of the circuit. I'm sure that they didn't use theQuotefor characterization.Up to now I couldn't find a diy project with LM399/LTZ1000 and the AD5791. Could be worth developing such a circuit.

author: mrflibble
author_rank: Super Contributor
timestamp: September 25, 2013, 06:41:23 pm
content: Regarding the AD5791, thisrelated appnotemight be of interest as well.

author: quantumvolt
author_rank: Frequent Contributor
timestamp: September 25, 2013, 07:04:08 pm
content: Quote from: saturation on September 25, 2013, 03:15:37 pmThis is correct. However, all (?) these EDC/Krohn-Hite boxes are nothing but a super low tempco zener diode and a chopper amp. And add a fortune in precision resistors. Building the reference is cheap (1N829A and LT1050), building the multi decade rotary switch resistor divider is hundreds of dollars. The schematics of two different versions (positive / negative supply for the current source for the ref diode) is in a tear down video and on kobb - both linked on this forum.

author: Andreas
author_rank: Super Contributor
timestamp: September 25, 2013, 07:12:42 pm
content: Quote from: sync on September 25, 2013, 02:20:22 pmI would use a capacitive voltage multiplier * 1.5 having a 10.5V maximum range.For the LTC2400 I would use a voltage divider * 0.666 having a 4.6V reference and up to 5.1V measurement range.See:ppmgeekThe LTC2400 linearity is given as "best straight fit" so if you do a "end point calibration" of the range the actual value is practically doubled against the datasheet value. So 4 ppm are a 40uV missing to the 2.5 V mid range value.(You can find this in carefully analyzing the assembler code of AN86).The LTC2400 has a noise voltage of 10uVpp (= 0.3ppm, eff in best case). So the settling time to the final value below 10uV will need averaging many measurement values. So the much finer resolution is only valuable when you can use a integration time of 1 minute or longer.So the settling time of the AD5791 is much better. And if the switching noise between neigboured output values is low enough one could have the idea to do some PWM with 10uV neighboured values.With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: September 25, 2013, 07:34:34 pm
content: Quote from: quantumvolt on September 25, 2013, 07:04:08 pmThats not all. You need very specialized know how and a lot of work to build a good device with theese cheap parts.First you have to age and select some out of a bunch of the zeners for ageing and noise. This will take around 1 year and from hundreds of zeners there will be few dozens left. Then you have to determine "zero tempco current" and build your current source accordingly for each device. And perhaps there are more treats to do that I still do not know.With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: September 25, 2013, 07:53:52 pm
content: QuoteThis means the use of 2x LTC1043, one half for the divide by 2 and another complete part for the multiply by 3 section.And what about the negative voltage reference rail? Just a simple inverter with gain=1?Edit: The second half of the first LTC1043 could be used as an ultra precision voltage inverter.

author: quantumvolt
author_rank: Frequent Contributor
timestamp: September 25, 2013, 07:56:58 pm
content: Quote from: Andreas on September 25, 2013, 07:34:34 pmAlso correct from apuristpoint of view. However - for those of us that can only afford to buy one diode (mine was USD10 +shipping 4 on fleabay) the knowledge that the current will be around 7.5 mA for all diodes and that the tempco is monotonic increasing in current leaves running in the device to a question of investment of free hobby time. Just set the current approximately, log the tempco behaviour and adjust the current up or down accordingly. Iterate for a year or more. Typical diagram page 2 inhttp://www.aeroflex.com/ams/Metelics/pdfiles/1N821-1N829-1_TC_Zener.pdf

author: grego
author_rank: Frequent Contributor
timestamp: September 25, 2013, 08:15:25 pm
content: EDN had a video on building a 5-decade KV reference - neat as a starting point:http://video.linear.com/28

author: quantumvolt
author_rank: Frequent Contributor
timestamp: September 25, 2013, 08:25:30 pm
content: Quote from: Andreas on September 25, 2013, 07:34:34 pmDetermining the current is easy due to the monotonicity in tempco vs current. Changing the current source is only adjusting a trimpot. I have seen two different versions of the schematics. No more magic. The sum of Rt and the trim determines the current, and the 'strange' bias/feedback network is the same for both schematics. Maybe there is matching of transistors, but that is not mentioned in the manual.

author: Andreas
author_rank: Super Contributor
timestamp: September 25, 2013, 09:43:42 pm
content: Quote from: branadic on September 25, 2013, 07:53:52 pmWhy using 2 LTCs for a factor 1,5 multiplication?with 1 LTC1043 you can: Divide by 2 + add the divided voltage to the input voltage.The inverted voltage for 10V will not work due to limited power supply range. (maximum 18V).For what do you need it?With best regardsAndreas

author: EEVblog
author_rank: Unknown
timestamp: September 25, 2013, 11:04:55 pm
content: Quote from: saturation on September 24, 2013, 11:50:36 amThe problem with this is that the switches are really schmick. Not sure where you'd get suitable ones for a 7 decade box. I greatly doubt youd get good 6 decade performance with run of the mill switches, let alone 7.

author: EEVblog
author_rank: Unknown
timestamp: September 25, 2013, 11:07:27 pm
content: Quote from: Andreas on September 25, 2013, 07:34:34 pmThe switches. EDC didn't just go down to Tricky Dicks and pick those up. They could be the most expensive thing in the whole system, by far.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 25, 2013, 11:22:02 pm
content: Hi,One of the best solution DIY solutions is to use a high resolution DAC.You can view this as set of matched resistors and precision switches. A micro can be used to supply the SPI signal to set the output voltage.A good example of a DAC is the LTC2756http://www.linear.com/product/LTC2756#This part has softspan. and can generate + and - outputs.This opens up the possibility of making an accurate, low frequency AC reference.This will be much cheaper than the precision resistors and switches needed to build a Kelvin Varley divider.Jay_Diddy_B

author: branadic
author_rank: Super Contributor
timestamp: September 26, 2013, 06:28:19 am
content: QuoteThe question is, what is more noiseless, a summing amp or an additional LTC1043 multiply by 3 circuit.QuoteIs this your suggestion or have you simulated this? The -10V are needed for the negative reference voltage supply on the AD5791.

author: quarks
author_rank: Frequent Contributor
timestamp: September 26, 2013, 09:47:42 am
content: Quote from: grego on September 25, 2013, 08:15:25 pmVery good hint, I also thought of this one. If up to 10Volt is what you are after, it is a very nice design. And you can easily change the design to the reference of your choice (LTZ1000, LM399, ...). I have played around with it (on breadboard), with a Geller SVR and my KVDs. If I remenber right, there were a few minor errors in the schematic, so watch out if you build it. Also I had trouble to get the LT1881, so I finally ordered directly from LT in US.Besides "only" 10V and the nice/easy free choice of reference you want to use, all depends on the KVD. If you are after great accuracy, a DIY KVD is a challenge.

author: sync
author_rank: Frequent Contributor
timestamp: September 26, 2013, 12:45:11 pm
content: Quote from: Andreas on September 25, 2013, 07:12:42 pmThanks for your response!10.5V is a good value.What calibrator resolution do you want to use?I have some questions about the capacitive multipliers/ dividers. Are there alternatives to LTC1043?In the LTC1043 data sheet they show typical applications and their precision, e.g. multiply by 2 has +-5ppm. Is this error stable?I had planed to use a capacitive multiplier at the output of the AN86 to get a >=10V range. Do they work at low voltages (near 0V)? What's about the precision then?QuoteIs this “best straight fit” somewhere noted in the data sheet? I'm looking for terms or descriptions so i can detect these data sheet optimizations.QuoteIs there a rule of thumb between the noise and the settling time?The AN86 says 1.4s settling time to 1ppm.btw: Do you have an opinion about the DAC1220? It's not as good as the AD5791. But much cheaper. Or do you know other suitable DACs?

author: sync
author_rank: Frequent Contributor
timestamp: September 26, 2013, 12:53:12 pm
content: Quote from: branadic on September 26, 2013, 06:28:19 amWhat is more stable?QuoteNo. The data sheet specify it with 0V for the negative Vref too.

author: Andreas
author_rank: Super Contributor
timestamp: September 26, 2013, 06:28:54 pm
content: Quote from: branadic on September 26, 2013, 06:28:19 amWhy a summing amp? On a charge pump you can simply put the negative output pins on the positive input pins.You only need a high impedant voltage follower (in any case).The maximum voltage supply for the LTC1043 in the datasheet is given with 18V.The negative reference supply (-10V) for the AD5791 is only needed if you want to output negative voltages (but in this case the resolution is 20uV between the steps). I would use +/-5V as reference in this case and use the internal multiply by 2 of the AD5791.By the way you could also get 7V * 3/4 = 5,2V out of one full LTC1043. (3.5 + 1.7V)With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: September 26, 2013, 07:04:58 pm
content: Quote from: sync on September 26, 2013, 12:45:11 pmMany questions.I still have not fully planned my calibrator. Perhaps 2 interleaved 16 Bit DACs which I have already here together with a LTC2400 for self calibration or a interleaved PWM version (2*16 Bit PWM). In the first case I will use a resistive voltage multiplier * 1.5 after the summing point of the DACs and adjust the full scale (and temperature drift) with a 2:1 LTC1043 divider and the LTC2400.The error of LTC1043 mainly depends on leakage currents (PCB + Capacitors) and the injection currents.At least in the divider version the value is very stable over temperature and time (below my noise limit of around 1-2uV)The charge injection will give some (offset) error. But the OP-Amps which you use also have some error. So you will have to do some offset (+ full scale) calibration in any case.Stability: Generally you can say the stability is better if a lower count of LTC1043 (and flying capacitors) is used.A division of voltages is better since the output impedance is lower than the input impedance in this case.A multiplication has more errors. (higher output impedance).For the best straight fit look DS of LTC2400 on page 4 footnote 6. But the real meaning of the value can only be calculated by analyzing the software given in AN86.The settling time in this case is mainly determined by the conversion rate of the LTC2400.For the DACs I generally look for low TC and good linearity.E.g. the AD5545BRUZ which I have already here before the AD5791 came up.With best regardsAndreas

author: sync
author_rank: Frequent Contributor
timestamp: September 27, 2013, 03:13:34 pm
content: Quote from: Andreas on September 26, 2013, 07:04:58 pmYes, sorry about that. Many thanks for your answers!

author: branadic
author_rank: Super Contributor
timestamp: September 28, 2013, 02:41:03 pm
content: QuoteQuoteQuoteQuoteSorry, I don't get it, maybe because I'm not familiar with switched-capacitors.

author: BravoV
author_rank: Super Contributor
timestamp: September 28, 2013, 03:25:49 pm
content: Quote from: branadic on September 28, 2013, 02:41:03 pmThough I'm just an enthusiast, I think he means like this below using LTC1043 to get quite high precision voltage multiplier or divider, and quite reliable in term drift/stability against temperature.I used this once while ago, very accurate, though I only have 5.5 digits bench DMM which too overkill for my lame dmm. cmiiwExample applications quoted from LTC1043 datasheet and there are others as well :

author: branadic
author_rank: Super Contributor
timestamp: September 28, 2013, 03:38:46 pm
content: I'm able to read datasheet and found the several circuits for /2, /3, /4 and *2, *3, *4, thats not the problem, but what I didn't get is how Andreas wants to realizemultiplication *3/2multiplication *3/4with only one LTC1043. So I was asking for that.

author: Marco
author_rank: Super Contributor
timestamp: September 28, 2013, 04:53:50 pm
content: For 3/2 can't you just use the second part of the chip as a buffer to add 1/2 to Vin?

author: Andreas
author_rank: Super Contributor
timestamp: September 28, 2013, 06:24:51 pm
content: Quote from: branadic on September 28, 2013, 03:38:46 pmIts easy ;-)I show the example for *3/2 the *3/4 is then the homework.With best regardsAndreas

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 28, 2013, 10:23:50 pm
content: Hi,I did a little simulation on the circuit proposed by Andreas.I want to draw attention one of the challenges with this approach.I have modified the LTspice model to step one of the capacitors through a range of values. You see that the output changes. This means that capacitors have to be matched.The results:There was also about 100uV of ripple at the LTC1043 oscillator frequency.Jay_Didddy_B

author: sync
author_rank: Frequent Contributor
timestamp: September 29, 2013, 01:34:24 am
content: Quote from: Jay_Diddy_B on September 28, 2013, 10:23:50 pmYou missed the connection to the S4A pin of U2.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 29, 2013, 02:01:40 am
content: Hi,It works a lot better !!My mistake !!Jay_Diddy_B

author: Andreas
author_rank: Super Contributor
timestamp: September 29, 2013, 04:42:27 am
content: Quote from: Jay_Diddy_B on September 29, 2013, 02:01:40 amHello,for a "perfect" LTC1043 switched capacitor design the tolerances of the components have only influence on the settling time of the output value, but not on the final output value itself.To reach this it is necessary that all the capacitors which are switched in series in the one state are switched to parallel in the other state. Only with this approach charge differences can be equalized.With best regardsAndreas

author: Spikee
author_rank: Frequent Contributor
timestamp: September 30, 2013, 05:41:05 pm
content: This looks promosing: (use google translate)http://bbs.38hot.net/thread-39900-1-1.html

author: Bryan
author_rank: Frequent Contributor
timestamp: September 30, 2013, 07:14:29 pm
content: Just presents a log in screen?

author: Spikee
author_rank: Frequent Contributor
timestamp: September 30, 2013, 07:27:33 pm
content: It worked on my PC (win7) but it also shows login on Android ....Ill fix it when i am home.

author: mrflibble
author_rank: Super Contributor
timestamp: September 30, 2013, 07:45:59 pm
content: Worked earlier on, now I get the login. On the same PC + browser. From the translated messages Ithinkthey may be doing some site maintenance.

author: senso
author_rank: Frequent Contributor
timestamp: September 30, 2013, 08:37:05 pm
content: I have seen it earlier also, nice schematics, can't make a thing of the text..Now its very slow and doesn't show much more than the top advertising.

author: Spikee
author_rank: Frequent Contributor
timestamp: September 30, 2013, 09:02:20 pm
content: Quote from: senso on September 30, 2013, 08:37:05 pmYou could try if my ripped version works :[old rip - see edit]EDIT:Now as pdf:https://dl.dropboxusercontent.com/u/100819959/ching%20chang/Reference%20devices%20Voltgen%20%28self-calibration%20of%20the%20six%20half-voltage%20source%29%20TR_V1.pdfEdit 2:His setup:Main schematic:https://dl.dropboxusercontent.com/u/100819959/ching%20chang/scheme1.pngCommunication schematic:https://dl.dropboxusercontent.com/u/100819959/ching%20chang/scheme%202.pngSelf cal schematic:https://dl.dropboxusercontent.com/u/100819959/ching%20chang/self%20cal.pngLTC2400 adc schematic:https://dl.dropboxusercontent.com/u/100819959/ching%20chang/ltc2400.png

author: branadic
author_rank: Super Contributor
timestamp: October 07, 2013, 11:26:52 am
content: QuoteOkay, got it. Here's the answer:

author: Jebnor
author_rank: Regular Contributor
timestamp: October 07, 2013, 02:16:46 pm
content: Hello Gerry,While you have likely already seen this video, it might be useful as a basis for a buffer circuit.I think a KV divider is an obvious choice, then using it in the feedback loop of an Opamp circuit.It's a good challenge. I like it.Good luck.

author: sync
author_rank: Frequent Contributor
timestamp: October 07, 2013, 03:24:24 pm
content: A KV divider will be difficult or very expensive. The resistors must be very good. I expecting the KV pot in the video costs several 1000 bucks. A DAC, PWM or AN86 should be easier and much cheaper. I experimenting with PWM at the moment with promising results. Attached a measurement from it.

author: branadic
author_rank: Super Contributor
timestamp: October 07, 2013, 04:05:41 pm
content: QuoteI'm working on a circuit too, thanks to Andreas who showed me his modified EDN version with 30bit resolution (2bit overlap) when he was at my lab for reference measurements. I also plan some modifications like the use of LTC2057 instead of LTC1151, AD586 or MAX6250 or optional LM399 with LTC1043 and LTC2057 for ~5,2V.Like suggested by Andreas I will use the ATMEGA1284 with a 6,5536MHz crystal. The advantage is that there is no internal pll circuit for the clock.I'm still drawing, but it will be a mixture of SMT and THT.

author: Spikee
author_rank: Frequent Contributor
timestamp: October 07, 2013, 04:06:11 pm
content: A long time ago i had the idea to build a kv divider. But it is a lot of work + expensive + time consuming. And if you eventually finish it , who says it will perform good ?I agree that AN86 dac method is better.

author: sync
author_rank: Frequent Contributor
timestamp: October 07, 2013, 04:44:20 pm
content: Quote from: branadic on October 07, 2013, 04:05:41 pmInteresting. Do you have any stability issues with the resistor ratio?I using a very crude approach. A 10 bit PWM clocked at 16MHz from an Arduino and dithering the lower 14 bits. A 74HC14 as driver which is directly powered from a LT1021C 5V reference. Then a passive second-order LPF and a LTC1050 as buffer. I was surprised how well it performs.

author: branadic
author_rank: Super Contributor
timestamp: October 07, 2013, 04:47:52 pm
content: QuoteAnd I agree that the use of an AD5791 is more simple than AN86.

author: Spikee
author_rank: Frequent Contributor
timestamp: October 07, 2013, 04:53:01 pm
content: Quote from: branadic on October 07, 2013, 04:47:52 pmAnd I can confirm that AD is out of samples (AD5791).

author: branadic
author_rank: Super Contributor
timestamp: October 07, 2013, 05:32:13 pm
content: QuoteDo you really wonder that AD don't sample a 80,-€ plus VAT part?

author: Spikee
author_rank: Frequent Contributor
timestamp: October 07, 2013, 05:44:36 pm
content: Quote from: branadic on October 07, 2013, 05:32:13 pmI have sampled more expensive parts before and it was no problem.BackorderEst Avail:2013-11-08

author: quantumvolt
author_rank: Frequent Contributor
timestamp: October 07, 2013, 07:45:06 pm
content: QuoteStrangely, the OP has not a single post here. He must be busy with his 1% resistor boards...This thread has enough momentum to get a life of it's own. I am very interested in the different ideas presented. Nice to see that there is activity here again

author: Andreas
author_rank: Super Contributor
timestamp: October 07, 2013, 08:27:33 pm
content: Quote from: branadic on October 07, 2013, 11:26:52 amHave you simulated it?I guess it has a not so good settling time.And perhaps dependancy on tolerances,because the main law of the capacitors is not met:in the one state the capacitors are in series and in the other they are in parallel.With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: October 07, 2013, 08:29:26 pm
content: QuoteI don't understand your problem to pay for something. It's not the task of AD or any other company to supply you with free samples, it's a friendly option. The people that work there also want to be paid.Those all-grabbing people are the reason that many companies don't offer free samples anymore. The samples are for engineering purpose with hopefully big quantaties in series production and not for your homebrew hobby stuff. Think about that

author: Andreas
author_rank: Super Contributor
timestamp: October 07, 2013, 08:32:34 pm
content: Quote from: sync on October 07, 2013, 03:24:24 pmAround 9 ppm nonlinearity is not too bad on the first try.My first cirquit was around 13 ppm. With carefully adjustment I could get around 3 ppm.Which analog switches do you use for the PWM output?Which Instrument do you use for 1ppm linearity measurement?With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: October 07, 2013, 08:33:55 pm
content: QuoteYes I did.

author: sync
author_rank: Frequent Contributor
timestamp: October 07, 2013, 08:56:39 pm
content: Quote from: Andreas on October 07, 2013, 08:32:34 pmI use a 74HC14 as driver which is powered from a LT1021C reference. The DMM is an old HP 34401A with unknown calibration status. It's A/D linearity is specified as 2ppm of reading and 1ppm of range. So it's not good enough but the best i have. I took the average of 10 reading at 100 PLC.

author: Andreas
author_rank: Super Contributor
timestamp: October 07, 2013, 09:22:40 pm
content: Quote from: branadic on October 07, 2013, 08:33:55 pmI think its time to show my proposal:The first half of LTC1043 (the lower) is a normal 2:1 divider from the datasheet creating 3.5V (at PIN S2A) out of 7V.The second half is also a 2:1 divider but switched between input and the 3.5V output of the first device.So the halved 7V-3.5V = 1.7V are added to the output of the first LTC giving 5.2V in total.To optimize settling time the output capacitor of the first LTC is set to 1.5uF.BTW. there should be a 2nd solution which subtracts the 1.7V from the 7V instead of adding it to the 3.5V.But I fear that there could be some overshoot to 7V at the output when the cirquit is switched on.With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: October 08, 2013, 07:01:56 am
content: I agree that your circuit solution settles three times faster.

author: mrflibble
author_rank: Super Contributor
timestamp: October 08, 2013, 11:57:42 am
content: Do you guys have an idea how they compare in terms of precision? As in how many ppm for the 3/4 circuit from Andreas compared to the one from branadic?

author: Andreas
author_rank: Super Contributor
timestamp: October 08, 2013, 09:22:12 pm
content: Quote from: mrflibble on October 08, 2013, 11:57:42 amDo you mean precision (repeatability over time + temperature) or accuracy (actual deviation from theoretical value)?I am not sure whether the values of the datasheet for a 2:1 divider of 1ppm are really accuracy or only precision.My 2:1 dividers built according to the datasheet have a typical gain of around -9ppm against the ideal value.But I do not know which (teflone ?) capacitors are used for the 1ppm value.I use small WIMA MKS-02 mylar capacitors which could be surely improved by low leakage types or even polypropylene.Further factors on accuracy are the PCB + LTC leakage + charge injection currents and the input bias current of the buffer amplifier at the output.Interestingly the gain is higher if the pins of the LTC1043 are flipped from the right to the left side. In this case I measure around -2ppm gain which corresponds much more to the calculated error budget of 3.5ppm for the leakage currents with my components.So I have the suspicion that the charge injection compensation circuit of the LTC1043 works in the wrong direction with the pinning of the LTC datasheet.With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: October 08, 2013, 10:19:16 pm
content: QuoteOne of the major problems. The LTC1043 comes in a SO18 package (okay, a DIP18 version is also available) on the other hand polypropylene capacitors are huge, for example the WIMA MKP-10 comes with 27,5mm pitch. Teflon capacitors with 1µF or more are no option, as they are not available for realistic prices. NPO ceramic capacitors with 1µF and more are not available.So what is the right choice?I found PPS capacitors in surface mount technology:http://de.farnell.com/panasonic/ecpu1c105ma5/kondensator-1uf-16v/dp/9694315Any suggestions to that? PPS is know to have small leakage, small dissipation factor and therefor good hf characteristics."You need low leakage types, primarily capacitors with outer film that can be tied to ground."Are there such capacitors available?

author: Andreas
author_rank: Super Contributor
timestamp: October 09, 2013, 06:25:23 am
content: Hello branadic,when you look at the datasheet of your tiny 1210 1uF PPS capacitor you will find a time constant of 300s as isolation resistance. That is in the range of a X5R ceramic capacitor.I have already used WIMA 2220 PPS capacitors together with WIMA MKS-02 in a 3:1 divider cirquit.http://www.farnell.com/datasheets/1674204.pdfThe PPS have >1250s time constantThe MKS-02 have also >1250 sStandard mylar capacitors have >3000s at least at higher voltages.Good polypropylene have >30000 s (and are available from 15 mm pitch)http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfiles/F3294_PHE426.pdf/$file/F3294_PHE426.pdfSo think BIG with precision components.Since modern capacitors are multilayer low inductance it will be hard to find some (rectangular) with a outer film.But you can think of copper taping the capacitor or use axilar types.With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: October 09, 2013, 07:29:06 am
content: QuoteI can't, I've studied microsystemtechnology, not heavy weight equipment construction. If it can be done small do itWhat I can fairly accept is something like caps from Vishay BC Components with pitch 5,00mm:100nF:http://www.digikey.de/product-detail/de/BFC241641004/BC2054-ND/502834and 15,00mm pitch:1µF:http://www.digikey.de/product-detail/de/BFC241671005/BC2076-ND/502856

author: quantumvolt
author_rank: Frequent Contributor
timestamp: October 28, 2013, 08:14:37 pm
content: IMO it is somewhat unrealistic to build a 7 decade Volt-box, but I will try to add two effective bits to the LTC2756 18 Bit Softspan DAC (+-1 LSB Error). This can be done (as in the AN86) by paralleling / attenuated cascading a second DAC or maybe by using a LTC1043 capacitive switch divider (two divisions by 2 adds 2 bits (if I am not wrong)).http://www.linear.com/product/LTC2756

author: sync
author_rank: Frequent Contributor
timestamp: October 28, 2013, 09:35:13 pm
content: Quote from: quantumvolt on October 28, 2013, 08:14:37 pmWith ADC feedback like AN86?QuoteWhat do you divide?

author: quantumvolt
author_rank: Frequent Contributor
timestamp: October 28, 2013, 10:08:35 pm
content: Quote from: sync on October 28, 2013, 09:35:13 pmYes - ADC feedback. As I now have the LTC2400 ADC running - and in addition have built the Linduino clone (expanded Arduino with a C++ library called LTSketchbook) to control / read all LT devices, all I need is another DAC (16 bit should be OK - I have one). I do not have to copy the Code Comparator PIC running ASM.I will try several references. The LTC2756 is able to output 0-10 volt based on the 5 Volt LTC6655-5 reference.[What will I divide?] Alternatively I can make a 2 Bit DAC from a LM399 board ouputting 10 Volt. Another possibility for reference will be 2 or 4 stacked LT6655 5V or 2.5V on separate batteries. This device will with quality relays be a programmable 2 Bit DAC with resolution 2.500.... Volt. I can then sum the output of this 2 Bit DAC to the output of the LTC2756 configured as a 0-2.500.... DAC. Since all devices are digitally controlled I can also have ADC feedback for a configuration like this.Me thinks ... Flaw detection and ideas are welcome.

author: mrflibble
author_rank: Super Contributor
timestamp: October 29, 2013, 01:02:18 am
content: The AN86 approach looks interesting. An similar but slightly different approach would be to PWM between 2 dacs. So same dual LTC1599 setup. You set one DAC to word N, and the other to word N+1. Then you use an analog switch to switch between the two DAC outputs, followed by an integrator/LPF. The duty cycle then determines where in the N...N+1 range you end up. And same as in AN86 you use an ADC for feedback.Since the two input voltages for the analog switch are very close this will (hopefully) minimize the amount of switching noise.

author: sync
author_rank: Frequent Contributor
timestamp: October 29, 2013, 01:47:28 pm
content: Quote from: quantumvolt on October 28, 2013, 10:08:35 pmThe AN86 code comparator is part of the control loop. With it and the ADC the precision is defined by the ADC not the DACs. That's the trick. You can use relatively imprecise DACs, resistors, amps.QuoteA clever idea. Maybe there are linearity and stability problems on the switching points (2.5V, 5V, ...). I would use the same reference for both DACs. Then the reference drift affects both in the same way.

author: quantumvolt
author_rank: Frequent Contributor
timestamp: October 29, 2013, 03:09:01 pm
content: "I do not have to copy the Code Comparator PIC running ASM."What I am saying is not that I can drop the Feedback Loop, I say that I can use C++ on Arduino in stead of the looooooooooong (for me cryptical) PIC assembler code in AN86."Maybe there are linearity and stability problems on the switching points (2.5V, 5V, ...)."Imo the idea is so simple it cannot fail. Imagine 4 batteries in series - each nominal 2.5 Volt. By tapping at the "end points and joints" - either directly - or through 3 capacitive dividers of the (4x2.5=)10 V, you will get something like 0, 2.5, 5.0, 7.5, 10 V.Now change one battery with a 0-2.5 V 18 Bit DAC (LSB for 2.5 V and 18 Bit is just under 10uV which the LTC2756 gives stable), and add relays so that you can tap the full span 0-10V (i.e. 0, (0-2.499...), 2.5+(0-2.499...), 5.0+(0-2.499...), 7.5+(0-2.499... ), 10) with 18 Bit resolution. You now have the topology for a 1PPM 10V Source (10uV resolution for 10V).The implementation is only technical details. You can use a rotary switch as in a KV divider - or you can use jumpers on a matrix/breadboard. But then it is only manually controllable ...If you use digital control by relays and ADC feedback, you can tap the physical joints of the 2.5 v references, and then correct the output via the 18 Bit DAC. You will not need LTC1043.If no feedback - open loop - is chosen, you must tap 2.5, 5.0, 7.5 V from resistor or capacitor precision dividers.I think one has to use different references for the 2 Bit and 18 Bit DACs because of galvanic isolation (different ground is needed for series summing). This is not a problem. The more LTC6655 you put in series, the more stable the end result (or fractions of it).But maybe it is a better idea to buy a 20 Bit DAC. Then you can add 4 Bit and get a (nominal) 0.1 PPM 7 Decade Voltage SourceThis idea came up because I already have an 18 Bit DAC.Maybe I am wrong in details - and personally I think it is almost impossible to make a stable 7 Decade 0.1 PPM Source - but all I am saying is that you can get "finer resolution" voltages between 0 and 6 Volt by series 4 pcs. AA batteries and put a multi turn potentiometer over ONE battery (divide 1.5 V) instead of putting the pot over ALL of them (the full 6 V).

author: edavid
author_rank: Super Contributor
timestamp: October 29, 2013, 04:09:44 pm
content: Quote from: mrflibble on October 29, 2013, 01:02:18 amWhy not switch the same DAC between the 2 codes? Then you don't need an analog switch.

author: sync
author_rank: Frequent Contributor
timestamp: October 29, 2013, 04:31:35 pm
content: Quote from: quantumvolt on October 29, 2013, 03:09:01 pmOk, i misunderstood you. Yes, that assembler in AN86 is not easy to read. They better used C.QuoteThe problem is when the DAC has a span of 0.001-2.498V and you add 2.501V. Then you can't get 2.499-2.502V (simplified to mV differences).QuoteA LTC1043 can provide galvanic isolation (within the supply voltages). The problem with different references is, that they drift differently. Today the 18 bit DAC reference has 2.501V and the 2 bit one has 2.498V Tomorrow they have 2.499V and 2.502V. How do you compensate this?QuoteGood 20 bit DACs are expensive. But you can use a 18 bit DAC and PWM/ dither between two values. That gives you more resolution. But i think the INL won't get better.

author: Andreas
author_rank: Super Contributor
timestamp: October 29, 2013, 08:53:38 pm
content: Hmm,just some thoughts:- when I use a DAC with 16 or more bits, I will do this with the only aim to get a low noise output signal.So I would never do some kind of switching noise or dithering at the output of the DAC.every DAC switching creates unwanted noise or glitches.- If I want to dither then I would use some kind of the EDN cirquit with 2 cascaded 16 Bit PWMs.- 7 decade resolution = 10V output with 1uV (RMS) noise can be done with some experiencewhen all is battery supplied and interfaces good galvanically isolated.- stability or even accuracy is more difficult.With the best (pre-aged) references, a good design somethingin the range of 5-10ppm/year as stability can be made with standard parts of catalog distributors.But of course it will not have the fast settling times of a professional calibrator.Accuracy will be even more limited if you dont have a josephson array at home.On the other side: even the HP 3458A with 8,5 digits is specced for 4-8ppm accuracy/year in the best range.And finally: with a LTC6655 you will not get near to 10ppm/year stability because of humidity and hysteresis effects.With best regardsAndreas

author: quantumvolt
author_rank: Frequent Contributor
timestamp: October 29, 2013, 08:57:57 pm
content: Quote from: sync on October 29, 2013, 04:31:35 pmI understand this. But if one accepts that this is a DC gadget and allow settling time in the order of 100 milliseconds or a second, then voltages and relays probably will have stabilized and the LTC 2400 will tell the software about the glitch and you can maybe use the offset to output the missing values."A LTC1043 can provide galvanic isolation (within the supply voltages)." Good point. I didn't think about that. I guess you are right about using common reference.But there is another possibility with the LTC2756 (at around USD 35 it is not too expensive). Let's say you have a reference 10 V, and you add LTC1043 chips to make a "ladder" 0, 2.5, 5, 7.5 and 10V. The LTC2756 is softspan so you can configure it as +- 2.5 V (it is powered +- 15 V clean external DC which is stepped down two more times in linear regulators). You now connect the +- 2.5 V DAC span successively to points 2.5 V, 5 V, 7.5 V on the ladder.Depending on your configuration you now have the glitch at 0V and 5V, at 2.5 V and 7.5 V, or at 5 V and 10 V. In all cases you get a span of 5 V w/o glitches (but maybe the LSB will be 19.1 uV - 2 ppm). Also the span is firmware programmable / changeable while the ADC is powered up (it is programmed by the first byte of the 4 bytes SPI data transfer). Furthermore it has 2 pins that allows trim of up to 2048 LSB for offset and also adjust gain by applying 5 V. By experimenting with it, learning it's behaviour and maybe also do a quick calibration against an external reference and meter before each working session, I think most of these problems can be solved in software / firmware.Anyway - I do not build usable lab equipment - I just experiment. So it is a very nice and flexible device for meHere is how I found it (the LTC2757 is parallel output - but the same guaranteed 4 ppm DAC with +- 1 LSB INL).

author: quantumvolt
author_rank: Frequent Contributor
timestamp: October 29, 2013, 09:14:51 pm
content: Quote from: Andreas on October 29, 2013, 08:53:38 pmI have given up all dreams of a fix-point voltage reference with high accuracy and stability in time.Whatever I do doesn't matter - I am doomed by practical and financial externals to a 10 yo uncalibrated Agilent 34401A and a (very good bang-for-buck) Geller ceramic reference 3458A transfer-calibrated board. I have enough frustrations as it is when I cannot reproduce a measurement.On the volt-nut email forum (which has many smart and knowledgeable people - but also a lot of morons with too much dollars and no brain), a clever guy recited the saying about a man with one clock who "knows" the time versus the man with two clocks showing different time (however small the difference between the clocks - you will never again know what time it is ...)

author: mrflibble
author_rank: Super Contributor
timestamp: October 29, 2013, 11:17:06 pm
content: Quote from: edavid on October 29, 2013, 04:09:44 pmBecause with just one DAC you'll have to deal with your DAC settling all the time. Not the best for a low noise output. So you use 2 DACs, let them settle, and then PWM between those 2 DAC outputs.

author: Andreas
author_rank: Super Contributor
timestamp: October 30, 2013, 05:37:15 am
content: Quote from: mrflibble on October 29, 2013, 11:17:06 pmAnd then you will learn something about digital feedthrough and additionally charge injection of a switch. So this will also be no low noise solution.With best regardsAndreas

author: mrflibble
author_rank: Super Contributor
timestamp: October 30, 2013, 06:49:52 am
content: Quote from: Andreas on October 30, 2013, 05:37:15 amAnd there I was thinking that by keeping the two analog inputs at almost the same potential + carefully chosing the switching patterns you could keep the errors due to charge injection effects to a minimum.Feedthrough will be an issue, agreed. But then again, the regular AN86 approach isn't static either...

author: RikV
author_rank: Regular Contributor
timestamp: May 31, 2021, 04:09:42 pm
content: Is this topic still alive or was it moved somewhere else?I'd like to build one for myself and the existing software would of course be of great help?

author: Andreas
author_rank: Super Contributor
timestamp: June 02, 2021, 05:00:32 am
content: Hello,I am aware of this thread:https://www.eevblog.com/forum/metrology/diy-acvdcv-calibrator-with-ad5791/msg3087976/#msg3087976But do you really want to build one by yourself? (and how do you adjust and calibrate it?)https://www.ianjohnston.com/index.php/onlineshop/handheld-precision-digital-voltage-source-2-mini-detailwith best regardsAndreas