VoltCard design with LM399: does this look sane? - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: May 07, 2015, 03:47:03 pm
content: This circuit is my design of a self-contained voltage reference card. It plugs into target circuit with a 7-pin connector. It takes a single 12V input rail and outputs a reference voltage. The output voltage, uncertainty, and a digital certificate of calibration is stored in the onboard EEPROM, which is living on a separate digital portion of the card. When this VoltCard is inserted into the target assembly the onboard microcontroller will read the data and control relays accordingly.My board used OP07C which is all I have. Any pin compatible op amp can be used in this circuit so if you volt nuts want one of this VoltCard with enhanced precision I can get OP177 onto that footprint.The two ten turn trim pots adjusts the constant current to the Zener, and the output voltage respectively. All other resistors in the analog section on this board are 0603 SMT resistors with 25ppm tempco (initial errors are trimmed out with the pots)EDIT: Changed board design to a simpler LM317 constant current source.The digital section in the bottom right corner is independent to the analog section.

author: Andreas
author_rank: Super Contributor
timestamp: May 07, 2015, 07:21:11 pm
content: Hello,just some annotations:the trimmers (200ppm/K) will spoil your 1ppm/K TC of the reference.(you could use a cheaper band gap reference for this).The LM317 needs a minimum load current of 5-10mA so a 1mA current source is a illusion.You should kelvin sense the output voltage of the LM399 otherwise the heater current will change output voltage.(this will need a additional ground pin).with best regardsAndreas

author: technix
author_rank: Super Contributor
timestamp: May 07, 2015, 10:25:34 pm
content: Quote from: Andreas on May 07, 2015, 07:21:11 pmIf the trimmer will ruin my day, what is the correct way of doing this then? Tap the Vz off directly with the OP07C as a voltage follower, write the measured voltage into the EEPROM, and let the digital side of things to compensate?If I bring the Zener current up to 15mA, will LM317 work?

author: Vgkid
author_rank: Super Contributor
timestamp: May 07, 2015, 10:37:58 pm
content: The lm399 zener only needs an iZ of 1.0mA. I was contemplating using a 10v voltage to power it, with a stable 10k resistor in series to provide the 1ma of current.

author: technix
author_rank: Super Contributor
timestamp: May 07, 2015, 10:49:55 pm
content: Quote from: Vgkid on May 07, 2015, 10:37:58 pmI want some sort of better constant current source so I used that LM317, and since LM399 can safely takes a 15mA current I am just pushing that current into it. The 12V was a choice of convenience as the final board will be powered using an ATX power supply where 12V is really a choice of convenience. This VoltCard is really a replaceable plug in board.

author: kjs
author_rank: Regular Contributor
timestamp: May 07, 2015, 11:16:24 pm
content: Quote from: technix on May 07, 2015, 10:49:55 pmbut the LM317 is a VERY noisy regulator and factors worse than the spec of the LM399. If you look at the LM399 datasheet the better solution is to use a real low noise and temperature stable voltage regulator and a low temp-co resistor.

author: radioFlash
author_rank: Regular Contributor
timestamp: May 07, 2015, 11:25:30 pm
content: I'm interested in this topic and found another thread on the matter of 10V LM399 references (see the link below). I'll be interested in what you end up with.https://www.eevblog.com/forum/projects/lm399-based-10-v-reference/

author: Vgkid
author_rank: Super Contributor
timestamp: May 07, 2015, 11:41:56 pm
content: This is the circuit that will be powering my gpsdo, instead of the lm317 it will use a lm338.any critiques, realize I left out the transformer/rectifier/10000uF of filter caps.

author: GK
author_rank: Super Contributor
timestamp: May 08, 2015, 02:27:40 am
content: You can use a variation of this circuit from the LM399 datasheet to trim the reference voltage without being bothered by the temperature coefficient of the trimpot.

author: Andreas
author_rank: Super Contributor
timestamp: May 08, 2015, 04:43:10 am
content: Quote from: technix on May 07, 2015, 10:49:55 pmHello,datasheet spec is 1mA +/- 0.1% for long term stability.Higher currents will not be useful.Best way to do this is a 3K resistor from 10V output to supply the zener.(you might need a startup cirquit).2nd best way is to use a 7K5 resistor to a stabilized 15V supply.For trimming I recommend the scheme of AD587 datasheet.(where the internal resistors of AD587 have a good T.C. tracking ).I would not use a switchmode supply together with a precision device in the LM399 class.With best regardsAndreas

author: babysitter
author_rank: Frequent Contributor
timestamp: May 08, 2015, 05:10:58 am
content: How is the digital side going to fix anything?You throw a lot of transistors on the problem of specifying the voltage references behaviour - a EEPROM.The digital side has something - relay-based KVM or switchable feedback resistor or silicon RDAC.So, Voltcard voltage times 1,428571429 might give 10V - are you sure its not doing Voltcard * 1,428571428?But aren't you just assuming that the digital controlled post-conditioning does what it is meant to do, or are you actually looking at the ratio of the reference to the output??

author: cellularmitosis
author_rank: Unknown
timestamp: May 08, 2015, 05:57:49 am
content: Quote from: technix on May 07, 2015, 03:47:03 pmWelcome to the path of becoming a volt nut!The last time I fooled around with the LM399 and read through the LM399 mega thread on this forum, if I recall correctly the stability of the LM399 voltage ended up being dominated by your ability to drive a stable current through it. I can't remember the exact relationship off the top of my head, but if your current source drifts by X microamps, the 399 drifts by Y microvolts. (I believe this means the LM317 is going to be the biggest source of error for your design).The design presented in the LM399 mega thread (https://www.eevblog.com/forum/projects/lm399-based-10-v-reference/) is a really interesting one, because it uses the LM399 itself to control how much current goes through it. Its kind of hard to wrap your head around, so I drew up a simplified version of the schematic in a way which makes sense to me:That circuit produces roughly 10 volts at the output, but you can simply change the two 3k resistors to whatever you want to get a different output voltage (e.g. 4k would give you 11 volts, 5k would give you 12 volts...).The key thing to understand is that, as soon as there is any current at all going through the LM399, it is near 7V. That causes the op amp to try to make its other input close to 7V, and the only way it can do that is to drive 1mA through the 7k resistor. And once that happens, you are also driving (a very stable) 1mA through the LM399 as well. Pretty neat!Here's an idea I've been sitting on for a while (which is based on the above circuit) to create a mini transfer standard to calibrate the various ranges on my HP 3478A bench meter:

author: Andreas
author_rank: Super Contributor
timestamp: May 08, 2015, 06:30:59 am
content: Quote from: cellularmitosis on May 08, 2015, 05:57:49 amHello,you bring it to the point. Thank you for the schematics.The relationship is the about 0.5 to 1 Ohms dynamic resistance of the LM399.So each uA will give around 1uV drift.With best regardsAndreas

author: GK
author_rank: Super Contributor
timestamp: May 08, 2015, 06:46:06 am
content: Quote from: Andreas on May 08, 2015, 06:30:59 amThe temperature coefficient of the LM317 is 0.02% per deg_C, which is 200ppm. For a 1mA current source this works out to:200*(1mA/1e6) = 200nA drift per deg_C.With an effective DC resistance of 0.5 ohms, that will contribute an additional voltage drift of 100nV per deg_C.The LM399's temperature coefficient is 0.5ppm, which works out to:0.5*(6.95V/1e6) = 3.475uV drift per deg_C.At almost 35 times less than the intrinsic instability of the LM399, the voltage drift term contributed by the LM317 current source is therefore relatively negligible.I haven't done the sums but the noise contribution from the LM317 should be negligible as well.

author: technix
author_rank: Super Contributor
timestamp: May 08, 2015, 08:27:50 am
content: Quote from: GK on May 08, 2015, 06:46:06 amSo I can roll with LM317 and normal 25ppm temp co 0603 SMT resistors and a Bourns ten turn pot tweaking the current without worrying about losing too many accuracy? That seem nice. And is it possible to match the resistors and LM317 to make their tempco cancel out?

author: GK
author_rank: Super Contributor
timestamp: May 08, 2015, 08:37:54 am
content: Quote from: GK on May 08, 2015, 06:46:06 amOK, I've just done some sums. TI's datasheet for the LM317 specifies noise in a 10Hz to 10kHz bandwidth to be 0.003% of Vout, or 30ppm. At Vout = 1.25V (the internal reference voltage) the total rms noise voltage in the 10Hz to 10kHz bandwidth is:30*(1.25V/1e6) = 37.5uV.To establish a 1mA constant current source with the LM317, a 1250 ohm resistor is required. The rms noise current will therefore be 37.5uV/1250 ohms = 30nA rms.Form a graph in the LM399 datasheet, the reference diode has a typical dynamic resistance at 10Hz of 0.5 ohms, rising to 1 ohm at 1kHz and 3 ohms at 10kHz. I'll be really (overly) generous and say that the average dynamic resistance over the 10Hz-to-10kHz range is 2 ohms.2 ohms resistance and 30nA of noise current gives a voltage noise contribution of 60nV rms in the specified 10Hz-10kHz bandwidth. The datasheet noise voltage spec for the LM399AH in a 10Hz-10kHz bandwidth is 7uV typical and 50uV max.So, unless I've stuffed my basic sums up, the noise contribution of the LM317 current source is even less significant than its drift with temperature.

author: GK
author_rank: Super Contributor
timestamp: May 08, 2015, 08:41:22 am
content: Quote from: technix on May 08, 2015, 08:27:50 amI wouldn't bother with a trimpot to tweak the current. The absolute value isn't important, just how much it drifts.

author: technix
author_rank: Super Contributor
timestamp: May 08, 2015, 08:49:35 am
content: Quote from: cellularmitosis on May 08, 2015, 05:57:49 amI am investigating the LM317 constant current source issue and I remembered I had a few LP2985 regulators. Will those allow me to build a constant current source at 1mA? Or do I have to go down the path of Zener/current mirror?

author: technix
author_rank: Super Contributor
timestamp: May 08, 2015, 09:24:08 am
content: Quote from: GK on May 08, 2015, 08:41:22 amSo LM317 have trouble following up the 1mA current, how about this circuit, using TL431 and a current mirror?

author: technix
author_rank: Super Contributor
timestamp: May 08, 2015, 09:43:48 am
content: Quote from: GK on May 08, 2015, 08:37:54 amAlso, don't bother too much on the sum now as the voltage follower op amp I used here is OP07C, not your fancy OP177. (Probably I will get a few of those later)And about the "getting 10V out of this" situation if I am using the standard cell replacement circuit how do I calculate the resistors? And what kind of tempco I need to aim for? Will those 25ppm 0603 parts good enough?

author: GK
author_rank: Super Contributor
timestamp: May 08, 2015, 09:45:06 am
content: Quote from: technix on May 08, 2015, 09:24:08 amDude, an LM317(L) will work fine here. No problem supplying 1mA. However if you want a better current source that's fine, just be aware of the very modest overall improvement in performance. However that said, I don't think that your TL431/PNP CC circuit shown there would work any better.

author: technix
author_rank: Super Contributor
timestamp: May 08, 2015, 10:13:14 am
content: Quote from: GK on May 08, 2015, 09:45:06 amLM317 seem have trouble reaching proper regulation at 1mA and that is why I am thinking about this TL431/PNP CC which will work with proper regulation at 1mA. Also this version consumes way less board space and I need to reclaim some board space to fit the trimable amplifier according to the LT app note (noticed the big blank area to the left of the op amp? That's where the precision amplification will happen)

author: GK
author_rank: Super Contributor
timestamp: May 08, 2015, 11:14:17 am
content: Oh, you're right, minimum load current is specified at 3.5mA typ, so that rules out an LM317, unless of course you just run a heavier quiescent current through the LM399 or shunt the excess with a parallel resistor.

author: technix
author_rank: Super Contributor
timestamp: May 08, 2015, 11:45:59 am
content: Quote from: GK on May 08, 2015, 11:14:17 amQuiescent is 1mA or the noise can get out of control. So TL431/PNP CC it is. The space in the middle is going to be that amplifier but how should I choose the resistors?

author: GK
author_rank: Super Contributor
timestamp: May 08, 2015, 12:06:51 pm
content: If Iq just has to be 1mA you can set the LM317 current to 5mA using the recommended 240 ohm resistor and put a 1k8 resistor (to shunt away 4mA) in parallel with the LM399.As for choosing the resistors, etc, you'll just have to draw out your circuit and sit down with pen, paper and calculator and compute/determine the significance of the tolerances and drifts. For the most part it just boils down to ohms law and calculations of percentages along the lines of my prior posts. I have to call it quits for this evening.

author: technix
author_rank: Super Contributor
timestamp: May 08, 2015, 12:18:48 pm
content: Quote from: GK on May 08, 2015, 12:06:51 pmI understand that but then the board size comes into play. Notice that the board is VERY small yet?

author: c4757p
author_rank: Super Contributor
timestamp: May 08, 2015, 12:24:01 pm
content: Don't forget that despite the datasheet recommending 240 ohms, the minimum current is actually specified as 10mA, which requires a 120 ohm resistor

author: technix
author_rank: Super Contributor
timestamp: May 08, 2015, 01:18:58 pm
content: Quote from: c4757p on May 08, 2015, 12:24:01 pmNope I changed the const current source to a TL431 and a PNP current mirror. LM317 discussion no longer valid here.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 08, 2015, 07:08:34 pm
content: The current mirror from discrete transistors is not very accurate and stable. Also the circuit uses something like 2 mA of extra supply current. A minimal circuit would be a JFET with a suitable source resistor.The simple OP-amp to generate something like a buffered 10-12 V is likely the better solution, if there is sufficient supply voltage available. As a first guess even a LM358 might do. Usually an OP07 does not hurt.The circuit shown above may need some extra circuit for startup: A diode at die OPs output, and a resistor to 12-15 V would do it.The circuit should still use a separate GND line for the reference and supply / heater.

author: technix
author_rank: Super Contributor
timestamp: May 08, 2015, 07:22:37 pm
content: Quote from: Kleinstein on May 08, 2015, 07:08:34 pmThat current mirror is not two discrete parts, but a matched PNP pair internally connected into a current mirror (the 4-pin SOT23 on the PCB) so I think that is not really a problem any more.I am looking at generating 10V from the reference, but I need to figure a way to do so with a trim pot but not sacrificing stability.

author: technix
author_rank: Super Contributor
timestamp: May 08, 2015, 07:42:47 pm
content: Here is another revision that probably will work better.The trim pot works, but I think the temp co of the pot does not really matter here because the entire pot drifts mostly uniformly and it is all about the ratio on this pot - the temp co cancels itself out. Maybe the wiper resistance will bite but the offset current of the op amp is low enough to prevent it from doing anything too significant.I don't really have split ground in this project including the heater, so it is all about the layout when talking abou the heater. The PCB have GND on is front, and +12V on its back , so I think the heater will not contribute too much to the noise. Also note the extra decoupling at the op amp?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 08, 2015, 07:55:36 pm
content: The separate ground for the heater is nor because of noise, but because of the DC feed through of the heater current, because of wire and contact resistances. At low temperature the heater may take up more than 50 mA, so that 10 mOhms of resistance at the ground plane would cause an avoidable error of something like 500 µV. In this case it would be likely better to turn of the heater all together.So using a separate return pin is not an option. There is essentially no way around having an extra "ground" pin for the ref. output.

author: technix
author_rank: Super Contributor
timestamp: May 08, 2015, 08:05:58 pm
content: Quote from: Kleinstein on May 08, 2015, 07:55:36 pmCome to think it again, that just doesn't matter at all. If this inrush exists this device have not warmed up yet! The part I bought have a built in heat shield.I just need to whack a line like "let the VoltCard warm up for 30 minutes after turning on before start using it" or so on to the manual of the thing.

author: GK
author_rank: Super Contributor
timestamp: May 08, 2015, 11:44:15 pm
content: Quote from: c4757p on May 08, 2015, 12:24:01 pmI read the ST datasheet lest night and that specifies 3.5mA typical, 5mA worse case:http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00000455.pdfHowever the spec is all over the place:And Fairchild specify 3.5mA typical, 12mA worse casehttps://www.fairchildsemi.com/datasheets/LM/LM317.pdfTI specify 3.5mA typical, 10mA worse casehttp://www.ti.com/lit/ds/symlink/lm317.pdfbut only 5mA worse case for the LM317Lhttp://www.ti.com/lit/ds/symlink/lm317l-n.pdfThough Onsemi specify 10mA for the LM317Lhttp://www.onsemi.com/pub_link/Collateral/LM317L-D.PDF

author: GK
author_rank: Super Contributor
timestamp: May 08, 2015, 11:52:56 pm
content: Quote from: technix on May 08, 2015, 07:42:47 pmThat circuit isn't going to work well at all. Your trim-pot is going to adjust the op-amp gain from AVolto unity. How are you going to accurately trim the output voltage with that? Ideally, you want an adjustment range just sufficient to cover the worse case error due to component tolerances. Study the datasheet circuit I posted in reply#8 to see how it is done properly. Also, your supply V is 12V and you have a 1k resistor in series with the LM399 zener. How is that going to give you your desired 1mA Iq?

author: paulie
author_rank: Frequent Contributor
timestamp: May 08, 2015, 11:53:38 pm
content: Quote from: technix on May 08, 2015, 08:05:58 pmRegarding LM399 inrush 5 seconds might be more reasonable. The current on mine drops and stabilizes after about 3 seconds.

author: technix
author_rank: Super Contributor
timestamp: May 09, 2015, 01:51:12 am
content: Quote from: GK on May 08, 2015, 11:52:56 pmI cannot find SMT resistors with low enough tempco with a reasonable price and size. That is why I'd rather not use that circuit, due to form factor limitations. This circuit may be a bit tricky to adjust, but it is stable and cheaper to build, and it does not occupy too much board space.About that 1k resistor, noticed the current mirror circuit?

author: GK
author_rank: Super Contributor
timestamp: May 09, 2015, 02:07:01 am
content: Quote from: technix on May 09, 2015, 01:51:12 amI don't know why you keep mentioning form factor limitations given the amount of unutilized space on your PCB. Your circuit will be more than a bit tricky to adjust with any degree of accuracy. Yes, I noticed the current mirror circuit; early morning brain fart on the 1k.

author: technix
author_rank: Super Contributor
timestamp: May 09, 2015, 02:13:47 am
content: Quote from: GK on May 09, 2015, 02:07:01 amI think I have mentioned that SMT resistors with the required less than 2ppm temp co are difficult to find and expensive like hell. Using through hole parts is not exactly a solution as the remaining space on the board won't really fit any through hole resistors at all. The Vishay SMR1 resistors are even more expensive than the second hand LM399 reference chip. I want to minimize precision parts used on this board.

author: GK
author_rank: Super Contributor
timestamp: May 09, 2015, 02:32:06 am
content: If you rearranged the layout you could fit half a dozen through-hole resistors on that board without issue.To what degree of accuracy do you think you are going to be able to trim the 10V to? For a large proportion of its range that trim-pot will have the op-amp output slammed up against the positive rail. As you adjust through the targeted 10V you'll be able to see the step resolution of the wire-wound trim-pot.

author: GK
author_rank: Super Contributor
timestamp: May 09, 2015, 02:55:57 am
content: A more practical circuit for trimming the output V would look like this. The trim-pot is 10k and is in parallel with 100R, which largely mitigates the trim-pots large temperature co-efficient and the adjustment range can be reduced to the required +/- a few %.Also, using exclusively, say, just for example, either 25ppm or 50ppm resistors in this circuit will not respectively automatically return an output drift of 50ppm or 100ppm. The actual drift contribution from the feedback network will be somewhat lower due to the fact that the resistors will drift by the same proportion if in the same thermal environment.

author: technix
author_rank: Super Contributor
timestamp: May 09, 2015, 06:20:48 am
content: Quote from: GK on May 09, 2015, 02:55:57 amSo using this design if I got all 25ppm SMD precision resistors from the same series the tempco will cancel out right?

author: kjs
author_rank: Regular Contributor
timestamp: May 09, 2015, 06:31:11 am
content: Quote from: technix on May 09, 2015, 06:20:48 amAs usual: it depends... Resistors are not like semiconductors where there is some uniformity on the wafer. They will only be within the +/- limit but the curves can be quite different. We found that it would be a crap-shot to rely on that. One batch it will work and then you have several ones where it doesn't. If it would be that easy there wouldn't be a need for multiple resistors on one substrate to get close matching of tk.

author: technix
author_rank: Super Contributor
timestamp: May 09, 2015, 06:56:54 am
content: Quote from: kjs on May 09, 2015, 06:31:11 amSo how should I get those matched resistors? One of those precision low tempco resistor network? Those are expensive too.

author: Vgkid
author_rank: Super Contributor
timestamp: May 09, 2015, 07:14:17 am
content: you can easily hand match resistors, use a wheatstone bridge circuit.

author: kjs
author_rank: Regular Contributor
timestamp: May 09, 2015, 07:20:57 am
content: Quote from: Vgkid on May 09, 2015, 07:14:17 amFor one-offs that's what I do but technix needs a lot of matching ones and then running the resistors over temperature and picking matching ones is rather time consuming.

author: technix
author_rank: Super Contributor
timestamp: May 09, 2015, 07:42:50 am
content: Quote from: Vgkid on May 09, 2015, 07:14:17 amI need to match not only the value but also relative temperature coefficient.

author: GK
author_rank: Super Contributor
timestamp: May 09, 2015, 08:34:54 am
content: I've had a great deal of success (both for temp co. and value matching) with through-hole precision resistors adjacent each other on the same bandolier. Don't know how smd chip resistors from the same package fare in comparison.At the end of the day you simply aren't going to make a precision-trimmed 10V reference with true single-digit-ppm drift based on the LM399 without either some degree of component sorting or forking over the bucks for extremely stable resistors.The LM399 is old hat anyway and there are numerous other parts available these days. Just look at the ADR01 for example:http://www.farnell.com/datasheets/1536598.pdfIt's about 1/3rd the price of an LM399, has 3ppm typical drift for the SOIC package (1ppm for the more expensive B grade if you can source it) and the output voltage is 10V with 5mV initial accuracy - and the output is easily trimmed.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 09, 2015, 08:42:24 am
content: It is not only the inrush current that matters for the LM399. Even under normal operation the current depends on the external temperature, the wire resistance usually has a TK of some 3000 ppm/K and the contact resistance is not that stable and predictable. So to get a reliable reference there is not way around having a separate ground connection for the voltage signal and the power. You might get away with the 1 mA current for the Ref. over the voltage contact, but even this is better avoided.For stability of the resistors it's also important to look at humidity - some resistors also change there values with humidity. Also SMD resistors have the additional problem, that expansion (thermal and humidity) of the substrate causes stress in the SMD parts and thus can influence values.The ADR... Refs. in SMD case are not a real alternative they have significant more long term drift and hysteresis. Also humidity is likely a problem.

author: GK
author_rank: Super Contributor
timestamp: May 09, 2015, 08:50:27 am
content: Quote from: Kleinstein on May 09, 2015, 08:42:24 amThe negative heater terminal of the LM399 can be tied negative with respect to the anode of the zener by a substantial number of volts and the heater can accommodate a supply voltage as high as 40V. So if you can furnish split (+/-) supply rails, the heater can be powered directly between them, keeping the heater current out of the ground completely.Quote from: Kleinstein on May 09, 2015, 08:42:24 amThe ARD01 will deliver better short and long term performance than an LM399-based reference followed up by a simple and cheap voltage-scaling amplifier.

author: technix
author_rank: Super Contributor
timestamp: May 09, 2015, 11:43:22 am
content: Another revision of the VoltCard. It is no longer standing off the main board (to accommodate the heater power without running that track all across the board) and I trimmed a bit of size off it. Actually if I get rid of the digital portion this module may be made even smaller, thanks to the SMT parts (the entire constant current source is two 0603 and two SOT-23 packages)

author: Kleinstein
author_rank: Super Contributor
timestamp: May 09, 2015, 12:13:50 pm
content: If you have the amplifier to 10 V (or similar) you can just use a resistor from the 10 V as a current source. Depending on the OP you may need a startup circuit (e.g. a resistor from the OP output to supply).The Trimmer position is better, but still not good enough - the pot may have a TL of something like 200 ppm/K and even the voltage fraction at the pot may change by some 10 ppm. So you likely want the pot to only go from something like 9.9 -10.1 V or even less. This may mean you need to use individually selected resistors as well.Also the up to 250 K input impedance at the OP can give extra trouble. So better have the full 10 V at the trimmer, and use a resistor at the wiper to reduce the influence of the trimmer. This also only needs 2 precision resistors.

author: GK
author_rank: Super Contributor
timestamp: May 09, 2015, 12:46:31 pm
content: The 500k trimpot ads up to 125k in series with the (-) input, not 250k (with the wiper at center position there is effectively two 250k resistors in parallel (disregarding the much smaller 200R). In any case 500k is much larger than necessary. 20k (in parallel with the 200R) would be large enough, effectively reducing the coefficient by a factor of 100. An adjustment range of 9.9 to 10.1V isn't enough to cover the LM339's initial tolerance of +/-2%.

author: technix
author_rank: Super Contributor
timestamp: May 09, 2015, 12:58:54 pm
content: Quote from: Kleinstein on May 09, 2015, 12:13:50 pmI don't want all that bootstrapping trouble so current source is intentionally made independent to the voltage reference.I am okay with the accuracy of this version of the circuit already. OP07C is a good OPA with low input offset current so that 250K will not give me too much trouble. I spun a new board revision, double side loading components to shave off even more board space, and go back to the breadboard-friendly single-in-line form factor. This revision also makes the module almost the same length ang width of a 40-pin DIP chip

author: technix
author_rank: Super Contributor
timestamp: May 09, 2015, 01:25:39 pm
content: Final revision before sending off to the board house.oes this seem to work?Ended up in a 0.1in SIP form factor with pins:1. Heater V+2. Heater V-12. Reference Out13. Reference Ground14. Reference +12V17. EEPROM GND18. EEPROM SDA19. EEPROM SCL20. EEPROM VccThe digital section and its pins 17-20 can be omitted.

author: technix
author_rank: Super Contributor
timestamp: May 09, 2015, 01:41:37 pm
content: Quote from: GK on May 09, 2015, 08:50:27 amActually I found someone offering ADR01B at US$1.5/each... Volt nuts, if you need those, call me.I have already bought the LM399's and the percision resistors so this LM399 VoltCard ("Original") will be spunned and built. If I got interested in another voltage reference again I will spin an ADR01-based VoltCard with a compatible pinout.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 09, 2015, 02:10:03 pm
content: The better way to combine trimmer with fixed resistors is shown here:https://www.eevblog.com/forum/projects/bought-several-used-lm399-how-to-use-it/msg669177/#msg669177This only needs 2 of the stable resistors, not 3. The other advantage is that its easy to adjust the range covered by the trimmer. If needed a 3rd. stage with 2 more fixed resistors could be used to do a coarse trim.If you keep the high impedance feedback, the circuit should have a capacitor in feedback, to improve stability against oscillation.The divider with only 1.3 K and 7 Ohms uses 5 mA of current, that's quite a lot for the ref. ground pin. The strange current source also adds about 2 mA, that can be avoided.The Bootstraping current source is far from complicated. With the OP07 to already delivering 10 V, this is just 2 resistors: One to set the current and one to the supply to ensure startup.The layout has a asymmetric connection to the LM399: one contact is just on top, the other has two lines going from the pin. This can result in avoidable temperature differences.

author: technix
author_rank: Super Contributor
timestamp: May 09, 2015, 03:52:00 pm
content: Quote from: Kleinstein on May 09, 2015, 02:10:03 pm1) The current resistor set consists of only E24 and E96 values: 1.3k, 200 ohms (both E24) and 499 ohms (E96) so they are way cheaper. And I never bothered to sit down and calculate the resistors.2) Thanks for the info, and the current layout allow me to tuck that cap right across R6 on the other side of the board (I am double side loading anyway) but what is the optimal capacitance? And will a cheap MLCC affect the circuit's accuracy?3) I am not bothering that, and the TL431 current mirror is good enough for me.4) This asymmetric-looking connection have its roots at four-wire measurement. The trace on the back is a constant current source, and it merges with the trace on the front (sense pin to the op amp) right at the pad to the Zener.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 09, 2015, 05:00:30 pm
content: The capacitor in feedback is not critical. It just sets the upper bandwidth limit for the amplified part. Anything from 100 pF to 1 µF would be OK - possibly less in a high impedance setup. A normal cap. should be OK, as long as leakage is not extremely high. The position across R6 is not helping much, the critical part is the high impedance from the trimmer. The right place is from the inv. Input to the output.As for the layout at the reference, symmetric would be preferred, even if this would mean having an extra blind connection at the other pin. It's also better to have the 2 lines close together, not separated as in the current layout. Another option would be to make the bottom a complete ring around the 4 Pins to reduce thermal gradients.If there is place is might also help to have something to fix a cover to the board, as to prevent turbulent airflow around the OP.

author: technix
author_rank: Super Contributor
timestamp: May 09, 2015, 05:27:21 pm
content: Quote from: Kleinstein on May 09, 2015, 05:00:30 pmAdded another blind ground connection to the reference (and I happen to have a ground via to connect it to.) The drive/sense side is a bit sad as the board layout comes into play and I cannot just run the traces parallel across the board itself to each other for a small part of the circuit.This board will be small enough to apply a heat shrink tube across the mid-section (about the same physical size as a PDIP-40 chip body not counting the reference chip, headers and the pot,) covering up the OP and constant current source (before the reference chip or the pot is soldered of course) so the cover is no issue.

author: GK
author_rank: Super Contributor
timestamp: May 09, 2015, 11:05:00 pm
content: I already showed you that alternative pot/divider resistor arrangement in reply#8, but you didn't seem to like it. The advantage of the second arrangement, however, is that the adjustment range can be set symmetrically +/- about the target voltage if you choose the resistor values correctly. This is potentially an advantage if you desire to minimize the influence of the trim-pot. with the first arrangement this is only possible if the targeted output voltage is 2 times the reference voltage (or half the reference voltage when used as a passive divider).

author: technix
author_rank: Super Contributor
timestamp: May 10, 2015, 12:43:42 am
content: Quote from: GK on May 09, 2015, 11:05:00 pmThis design already have the pot's tempco effect within 0.5ppm and that is already good enough for me (or anybody, since 0.5ppm is within LM399's own tempco and noise threshold.) No need to guild the lily here.

author: Andreas
author_rank: Super Contributor
timestamp: May 10, 2015, 07:44:18 am
content: Hello,the 0.5-1ppm/K for the LM399 is the average spec over the whole temperature range.I measured around 7-8uV output voltage change over 10-40 deg C (>30 K range) for my LM399#2 reference.(see picture: X-axis temperature, the right y-scale is the voltage difference in mV against a second LM399 kept at room temperature).So around room temperature you might be much better than spec for the LM399 alone.With best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: May 10, 2015, 11:51:40 am
content: The current trimmer setup has two disadvantages:1) It needs a 3rd highly stable resistor2) the output impedance is relatively high. This gets better if a slightly lower value trimmer is used, but the range is limited. This gives extra noise (especially from current noise of the OP) and also drift due to the bias current.Depending on the quality of the trimmer, the extra instability may not be more than the LM399's intrinsic stability, but it still adds up and prediction how stable the trimmer setting is, is difficult. It is anyway difficult to do the adjustment over the full 2% uncertainty range of the LM399 with one analog trimmer alone. It's not only the TK, but also the mechanical stability of the trimmer. The classical solution is using a 2 step trimming: coarse trimming with a individually chosen resistor and only fine tuning with the trimmer.

author: GK
author_rank: Super Contributor
timestamp: May 10, 2015, 12:09:21 pm
content: I'm sorry, but the noise is still negligible in comparison to the LM399's noise. Figure 6, page 4 graphs ~90nV/sqrt-hz above the flicker (1/f) corner at ~ 100Hz:http://www.ti.com/lit/ds/snvs751c/snvs751c.pdf90nV is equivalent to the thermal noise generated at 25 deg.C by a 500k resistor.And don't forget that uncorrelated noise sources sum as the square root of the sum of the squares. E.g. two 90nV noise sources sum to 127nV not 180nV. So even if the OP's amplifier circuit was as noisy as a 500k resistor it would only worsen the overall noise performance by 3dB.In any case the LM399 is a terribly noisy reference and any truly serious application requires filtering. So far the OP hasn't mentioned anything about his noise performance requirements. A no holds barred approach would use auto-zeroing op-amps in differential/instrumentation amplifier configuration to allow low feedback impedances while rejecting ground currents and attendant I*R voltage drops and incorporate very low-freq. filtering and multiple trim-pots for the scale calibration. AD make some truly low noise auto-zeroing op-amps. How far do you want to go?

author: technix
author_rank: Super Contributor
timestamp: May 10, 2015, 12:53:09 pm
content: Quote from: GK on May 10, 2015, 12:09:21 pm1uF 0603 MLCC to ground at the input of the OPA will be added and that is it, none if board space is not adequate. The OPA also have a cap on its feedback path.

author: GK
author_rank: Super Contributor
timestamp: May 11, 2015, 02:00:53 am
content: 1uF directly in parallel with the LM399 won't do much due to the LM399's low dynamic impedance. I'm also not sure that the LM399 is stable with a large capacitive load. A series resistor will also be required to make a worthwhile filter. Make sure the cap is a low leakage type.Having substantial noise filtering prior to the scaling amplifier will make the scaling amplifiers noise dominant, so then you might want to reconsider its design.A capacitor in parallel with the feedback resistor will not do a lot either, as the high frequency noise attenuation in this case will plateau at only -3.2dB.If you're adverse to implementing filtering you may also want to test your individual LM399 for noise performance as the specified parameter spread is pretty large. The datasheet specifies 50uV worse case rms noise in a 10Hz-10kHz bandwidth. What?? That's 500nV/sqrt-Hz!

author: cellularmitosis
author_rank: Unknown
timestamp: May 11, 2015, 03:15:24 am
content: Quote from: GK on May 10, 2015, 12:09:21 pmGK, I wanted to say thank you for actually doing the math and keeping us honest. Some of the results have surprised me (e.g., the LM317 drift being more than adequate).

author: technix
author_rank: Super Contributor
timestamp: May 11, 2015, 03:05:06 pm
content: Quote from: GK on May 11, 2015, 02:00:53 amIf that is the case, I will leave the filtering to the main board. I am running out of space here.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 11, 2015, 04:44:55 pm
content: If you already have amplification, it would be easy to add the filter in front, it's just a resistor and a low leakage capacitor. The main board might need an extra amplifier.

author: technix
author_rank: Super Contributor
timestamp: May 11, 2015, 06:00:28 pm
content: Quote from: Kleinstein on May 11, 2015, 04:44:55 pmThe main board will have at least a voltage follower on it (for the VoltCard DMM, there will be a reducing amplifier that emits a percise 2.5V using two matched resistor divider stages, for the adjustable voltage standard there is the DAC chip) so noise filtering can be moved to the main board safely.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 11, 2015, 07:24:50 pm
content: Every extra amplifier adds drift. This is one reason why the ref. voltage without any modification, except possibly a buffer if needed (the LM399 does not absolutely need one - the LTZ1000 might like to have one).Also with a higher resolution DAC, there is often no need to have an exact value of the ref. voltage. Trimming can be often done in software, choosing the right DAC values. So no troublesome mechanical trimmer is needed. Its only if something like a decimal KV divider is uses directly to set values when you need something like a 10.000 V reference. Otherwise 10.x is often ok as long as you know x.