7.xV to 10.00000V AutoCAL circuit - Page 1
Forum section: electronics / general

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 10, 2016, 08:56:31 pm
content: Hi,I'm afraid there is quite a lot disscusion about 7V to 10.00000V in the LM399 and LTZ1000 Threads, but my "idea" is related to both references.It seems to be quite hard to get a relieable and stable (long-term and tempco) amplifier to generate 10.00000V out of the ~7V from a LM399 or LTZ1000. VHP100 are quite expensive, hard to get and not adjustable. So even if you get these things you can't adjust the voltage to 10.00000V without extra effort.I was thinking about something cheaper and much easier to get (in germany most things are available at Reichelt Elektronik).What if one uses a LTC2400 ADC as "reference divider"? The tempco is quite small (0.02ppm/K) and I could imagine it is also long-term stable. My approach is to create an amplifier which is able to generate about 10V out of the 7V and fine tune this amplifier with a DAC. A rather cheap (or two) DAC is sufficient, because it sits in a closed loop and will only tune the last digits.I would use a LTC1043 to divide the 10V output down to 5V. This 5V will be the digital/LTC2400 GND. The 10V output of the amplifier will be the reference voltage of the LTC2400 (which seems to be 5V for the LTC2400). Now, I'm able to measure the about 2V (7V-5V Ref) with the LTC2400 while the output voltage is tuned to 10.00000V by the DAC.After this calibration it is only neccessary to tune the DAC after some drift back to the ADC value measured at the calibration. Therefore, even the INL error of the LTC2400 doesn't matter.Furthermore, it is possible to start this AutoCAL procedure, tune the DAC and switch off the microcontroller, so there will be no swichting digital circuits until using the reference.What do you think about it? Is it a old story or doesn't it work?Philipp

author: uncle_bob
author_rank: Unknown
timestamp: March 11, 2016, 01:16:31 am
content: Quote from: e61_phil on March 10, 2016, 08:56:31 pmHiFloating the 2400 in "mid air" means that all your digital stuff will be sitting on your 5V "2400 ground" rail. You need a pretty good amp to do the buffering on the output of the 1043 and then you are right back "into a mess". Amps don't like to drive cap loads. Rails want to be highly bypassed to be quiet. You now have multiple loops and compensation around the amp.Since the 2400 is a cap sampler, it pulls bursts of charge off the source. That's worked out in the app notes from the 2400 side of things. It's still a very "spiky" current flow. To keep your ref from seeing these ... more op amps.The "big guys" decided long ago to simply take whatever the reference put out and use that as a standard. Checking a 8.39205228002894 V calibration reference is not any worse than a 10V ref with modern gear.Bob

author: Vgkid
author_rank: Super Contributor
timestamp: March 11, 2016, 01:37:29 am
content: If you want some old time circuits look at the Systron Donner m106, or the newer m107.

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 11, 2016, 05:57:28 am
content: Quote from: uncle_bob on March 11, 2016, 01:16:31 amI thought about a buffer with an auto-zero opamp around to stabilize the 5V rail (LT1010 togehther with LTC2057 for example). Perhaps, the LTC2400 isn't the right ADC for this.I think the nice thing which an approach like this is you can get rid of any switching noise during measurements. This ist not possible with PWM, so you have to put much more effort into this to get it very quiet. I can't believe that this will be less effort than maintaining a rough 5V for the ADC. But, perhaps I am on the wrong track.

author: montemcguire
author_rank: Regular Contributor
timestamp: March 11, 2016, 07:02:34 am
content: Quote from: DiligentMinds.com on March 11, 2016, 12:46:05 amOne way to extend PWM resolution is to use a high resolution PWM duration register and add high quality random dither to that 'real' duration value before truncating it to the actual low resolution duration value. Since we're concerned with essentially DC, the small but random variation of the dithered PWM edges is just a HF component that will disappear when lowpassed. But, the edge timing dither will allow a much higher effective resolution of the PWM.One requirement is to generate some high quality random numbers with an average of zero. I'm doing this in an NXP MCU by using their 12 bit ADC to sample the Johnson noise of some high value resistors, amplified by a few chopper stabilized op amps (OPA333). The combination of NiCr resistors and the noisy chopper amp produces very close to perfectly random noise with no 1/f component. The bottom 8 bits are extracted from the 12 bit ADC and stuffed into a RAM circular buffer, to be clocked out as needed when each new PWM edge timing is generated. While this circuit and ADC generates low bandwidth random numbers (~40KB/s), if the circular RAM buffer is large enough, there should be no problems with periodicity, even though the buffer will be cycled through a few times as the random data is slowly being updated.There are still some gotchas here - the DC error of the chopper amps (and the ADC) will pollute the random noise signal, slightly altering its average value, but some coupling caps can minimize that for all but the last amp in the noise generator chain. Further, this dither is scaled down somewhat before it is added to the high res register, reducing this DC error. Finally, when the bottom 8 bits are taken from the 12 bit ADC values, this randomizes the random signal's DC error - it is effectively modulated by the value of the 4 bits that are thrown away from each sample. As long as the analog noise is large enough to make those top 4 ADC bits move, but small enough to prevent the ADC from clipping with any regularity, any DC error (causing a non-zero µ in the random data) should disappear.Not sure if this scheme is what you came up with (and there's no need to confirm or deny!) but I'm using this method to randomize and extend the resolution of a PWM application and it seems to work reasonably well at low cost with a modern MCU.

author: Andreas
author_rank: Super Contributor
timestamp: March 11, 2016, 07:01:02 pm
content: Hello,the LTC2400 is not bad.The self calibration and thus the low drift are the advantages.Also a low power consumption helps to keep switching noise away from the analog part.On the other side this ADC is relative noisy (10 uVpp) so you will have to average manymeasurements (around 1 minute) to get a stable result.For the DAC you should think twice. A cheap 8 Bit DAC will not be able to compensate fora 20 ppm temperature + around 20-50 ppm ageing drift with a 1 uV resolution.And perhaps you even want to compensate the T.C. of the voltage divider by small steps of the DAC.PWM is not the solution of all problems.Ok you have no resistor T.C.But the other challenges are charge injection, temperature dependant switching times, and temperature + voltage dependant switch impedance to name a few.With best regardsAndreas

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 11, 2016, 09:04:52 pm
content: Quote from: Andreas on March 11, 2016, 07:01:02 pmI don't want to use 8bit DACs. I thought about a MCP4922 (2x 12bit 3ppm/K gain error) and use one for coarse and one for fine adjustment.

author: uncle_bob
author_rank: Unknown
timestamp: March 12, 2016, 12:46:43 am
content: Quote from: e61_phil on March 11, 2016, 09:04:52 pmHiAs mentioned above, the "big guys" pretty much have gone over to PWM for this sort of thing. Once you start tossing op amps into the mix, some of the PWM stuff becomes less of an issue. Because it's a "well known technique" there are a lot of schematics and app notes to give you a trail of bread crumbs to follow.A somewhat crude example:Start off with "sort of 20V" derived from who knows where. Divide it by two (your 10 V) and by 4 (your 5V) and maybe by some other integers in the 2 to 16 range. Yes this sounds a bit stupid, but there are some advantages:1) You can run things at reasonable speed (no 1 Hz lowpass filters).2) You can have consistent switching in all your outputs (except for the obvious odd / even issue).3) You can get useful voltages in most ranges without a lot of crazy effort.4) Your lowpass filters are not very crazy to do.So where is this going you wonder? (Yes, there is a very large ex-bottle of wine somewhere around here .. hmm ....).Part of your problem with PWM is working out what the TC of "this" compared to the TC of "that". Building lots of PWM's is dirt cheap. Feed them into a bank of 2400's and go to it. Average for tens of minutes if you need to. Feed back the results to a display. Let that tell you what you have on each of your outputs.Ok so back to the top (remember the 20V starting point?).Play with your same servo dac stuff to get "whatever" to produce 20V. Put that (after PWM) into one side of a 2400 (or multiple 2400's). Run the other side of the 2400 off of your magic super reference (after PWM). Your comparison is now good at the "who knows what" fraction of a ppm level. The real answer is going to depend on a bunch of averaging issues and firmware and those pesky op amps.Bob

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 12, 2016, 11:50:43 am
content: A PWM might be the way to go if you want a stable output for a very long time. But it isn't easy to build. My approch will get you only a new calibration from time to time for the amplifier. I think cheap precision resistors are good enough to create a stable divider for a few hours of measuring time. After this few hours you rerun this AutoCAL procedure. It is not meant to keep track on the tempco. It's only a cheap way to recalibrate the amplifier every time before you use it.

author: uncle_bob
author_rank: Unknown
timestamp: March 13, 2016, 02:49:41 am
content: Quote from: e61_phil on March 12, 2016, 11:50:43 amHiI guess a lot depends on what you consider cheap and what you are after in terms of accurate. A fixed width PWM is dead simple to build.Bob

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 13, 2016, 02:26:11 pm
content: Quote from: uncle_bob on March 13, 2016, 02:49:41 amA fixed PWM isn't tuneable to 10.00000V and a tuneable PWM with >20bit with at least 120dB filtering isn't that simple. Or I don't know how

author: fmaimon
author_rank: Unknown
timestamp: March 13, 2016, 02:43:45 pm
content: Try using this:https://www.eevblog.com/forum/projects/general-purpose-power-supply-design-7488/msg105509/#msg105509I've tested it (some results further down the thread above), and it works very well.

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 13, 2016, 05:06:01 pm
content: Quote from: fmaimon on March 13, 2016, 02:43:45 pmI think it is next to impossible to achieve 120dB of filtering with some kind of PWM dithering. The dithering will produce low frequency stuff which is next to impossible to filter with a DC accuracy needed for these reference circuit.The way I would go is something with combined PWMs like this 32bit PWM DAC from the EDN magazine or something like this. But that is all much more effort than tuning with an additional DAC which only correct for the last digits. Furthermore, for the DAC output no filtering is necessary.

author: uncle_bob
author_rank: Unknown
timestamp: March 14, 2016, 01:45:10 am
content: Quote from: e61_phil on March 13, 2016, 02:26:11 pmHiOk, going back to my original post on how to do this:You have a synthesized 20V it has no PWM on it's output so no 120 db questions.You have a steering circuit for the 20V with a D/A just like you originally proposed, so no 120 db questions on that either.So your output signal at 20V has no issues.====Now, you divide the 20 to 10, that's with a 2:1 PWM that likely is at a nice high frequency. You are doing "many samples" (as in minutes) on your ADC. The bandpass of the filter does *not* have to be very large.Pick a frequency, let's call it 100 MHz to have something to work with. You want 120 db? One ideal pole will get it for you at 100 Hz. Two poles at 100 KHz. Both are a long way away from "many minutes". Neither one is a very complex filter. If you need more poles, there is *lots* of room for improvement. If you really are concerned, it's a fixed frequency, put in a tuned trap.Yes indeed your grounding and board layout will have to be up to snuff to achieve good isolation. If you are after 120 db isolation from the "stuff" on the AC line, they will have to be pretty good for that as well. The audio crap on your AC is a *lot* closer to your intended output passband than *anything* coming from a PWM. Cascading high frequency filters is pretty trivial compared to isolating the 10, 15 and 20 (etc) line crud.Again, the guys who make the high dollar calibrators seem to have no problems doing it this way. It's generally easier to copy what is known already to work than to go off and find a whole bunch of issues doing it some other way.Bob

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 14, 2016, 07:39:17 am
content: Quote from: uncle_bob on March 14, 2016, 01:45:10 amPerhaps, I missed your point, but isn't that exactly what I want to do?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 14, 2016, 12:54:52 pm
content: What is wrong with the "standard " autocal approach? All we need is to keep the ratio between 7V and 10V constant and correct. A hi-res delta-sigma ADC, like LTC2400, is a good device to measure that ratio and it is fairly easy to (mostly) remove other influences but the ADC noise and long-term gain drift. Measure the 7V and 10V with the same 2:1 divider and the same reference voltage (for simplicity, 10V output divided by 2 by the second divider, as the LT5400 has two pairs of resistors) and, as long as the drift and noise of that divider and the reference is low enough for the duration of both sequential measurements, the result is accurate. The rest is pretty straightforward and a 12 bit multiplying DAC should be enough to correct the main 7 to 10V amplifier gain drift with temperature. And the ADC can sit on the ground rail, no need to float it.CheersAlex

author: uncle_bob
author_rank: Unknown
timestamp: March 14, 2016, 09:05:08 pm
content: Quote from: e61_phil on March 14, 2016, 07:39:17 amHiIn your approach, the ADC floats between 5 and 10V. In a more conventional approach, there is one ground and everything is referenced to it. In the conventional approach, PWM's are easy to build high accuracy dividers to get it all worked out.Bob

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 14, 2016, 09:37:11 pm
content: Quote from: uncle_bob on March 14, 2016, 09:05:08 pmIn the conventional auto-calibration approach, if your ADC is stable enough at ratio measurements, you don't need high accuracy dividers. You just need dividers with a good short-term stability and decent WW resistors should be good enough for that.CheersAlex

author: uncle_bob
author_rank: Unknown
timestamp: March 15, 2016, 12:44:24 am
content: Quote from: Alex Nikitin on March 14, 2016, 09:37:11 pmHiIf the OP had an ADC with a 10V input range, this all would be even simpler still.Bob

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 15, 2016, 01:42:59 am
content: Quote from: uncle_bob on March 15, 2016, 12:44:24 amSure, however there isn't one available and suitable for this application that I know of. Two WW resistor dividers and a CMOS switch is not too complicated.CheersAlex

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 15, 2016, 11:27:29 am
content: Quote from: uncle_bob on March 14, 2016, 09:05:08 pmHi,I think I'm still missing your point. In my approach there is no need for any PWM. The only stable divider by 2 is realized with a LTC1043 (and even this divider has to stable only. It is not necessary to divide by 2.00000). It is also possible to use two dividers (both in one LTC1043) as mentioned by Alex Nikitin and divide all voltages down and measure all against one common GND (one will lose ~1bit of resolution by doing this, but it shouldn't matter).I'm still thinking to use a floating GND for the LTC2400 is much more easy to handle in PCB layout and buffering than handling with tenth of MHz PWM frequency. Furthermore, there is no fixed PWM needed.I'm only interested in how "the guys who make the high dollar calibrators" create their stable 20.00000V which is used by the PWM to create all other voltages. Is this done with DACs only (and precision resistors) or is there any PWM involved, too?Philipp

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 15, 2016, 12:04:21 pm
content: Quote from: e61_phil on March 15, 2016, 11:27:29 amHi Philipp,In your configuration the divider quality (tempco and long-term drift) is still important. If you only measure the voltage ratio using the same divider for both voltages, only a very short term divider stability is required, so it is one uncertainty less. Even LT5400 has some drift.CheersAlex

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 15, 2016, 12:25:02 pm
content: Which divider do you mean? I don't want to use LT5400.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 15, 2016, 12:55:53 pm
content: Quote from: e61_phil on March 15, 2016, 12:25:02 pmYou can use just two plain metal film 5-10ppm/C resistors in this case, as long as the divider ratio is stable for a very short time required for every autocal cycle.CheersAlex

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 15, 2016, 01:01:59 pm
content: I think I will need a stable divider by 2 and the stable LTC2400 for long term stability. Otherwise you will need some switching between different divider, I think.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 15, 2016, 01:09:05 pm
content: If you want to use the LTC2400 to compare the 7 and 10 V directly towards zero, you need to use just one divider and a buffer amplifier to switch between the 7 and 10 V. Separate dividers just don't make sense as you need 2 stable dividers to replace one.The alternative to using a divider would be subtracting a certain voltage. If this voltage is well proprotional to the 10 V this is OK and can give that 1 extra bit of resolution. Still drift from the divider enters, and the ADCs gain error enters, unless extra switching is used.

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 15, 2016, 01:12:58 pm
content: Quote from: Kleinstein on March 15, 2016, 01:09:05 pmIsn't that exactly what I've described in my opening posting?I use the LTC1043 to subtract 5V from the 7V reference element. And I will only use one stable divider. The drift will by given by the 1:2 divider and the ADC only.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 15, 2016, 01:18:51 pm
content: Quote from: e61_phil on March 15, 2016, 01:01:59 pmThe idea is to switch the same divider between two input voltages 10V and 7V, in that case the divider accuracy and long-term drift does not matter, and even the long term stability and accuracy of the reference for the LTC2400 does not matter. What matters is the stability of the LTC2400 ratio measurements (should be very good) and the noise on the measurements (so there will be a compromise between the speed and accuracy - if you reduce the noise by averaging you increase the drift influence from the reference and the divider). For the best result a by-stable relay can be used for switching however a solid state switching should also be possible with a little bit of trickery to get it's impedance out of the loop.CheersAlex

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 15, 2016, 01:32:46 pm
content: Quote from: Alex Nikitin on March 15, 2016, 01:18:51 pmIn this approach an additional divider for the reference of the LTC2400 is needed or a second short term stable reference. But that shouldn't be a problem. Also a nice ideaYou eleminate the long term drift of the LTC2400 and the divider, but the LTC2400 should be very linear for this approach. Jim Williams and Andreas showed that is possible.A buffer (LTC2057 for example) at the output of the CMOS switch should be enough to get rid of the switch impedance, I think.I've drawn my idea on a whiteboad. Perhaps, it was not clear to everyone due to my crude english(the left part is only the LM399 1mA current source the right part behind the dashed line is the AutoCAL circuit).Another adavantage of the higher digital supply is the easy application of a 5V DAC on the 10V output.

author: Andreas
author_rank: Super Contributor
timestamp: March 15, 2016, 06:00:08 pm
content: Quote from: Alex Nikitin on March 15, 2016, 12:04:21 pmThe LTC1043 has virtually no drift over temperature and time.Practically you have something like 16nV/K or 0.006 ppm/K which is mostly due to the offset drift of the buffer amplifier.https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg528192/#msg528192Quote from: e61_phil on March 15, 2016, 01:32:46 pmI would make the 7->10V transfer with one single OP-Amp. (saving at least one Op-Amp if you need to buffer the 7V for the ADC).with best regardsAndreas

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 15, 2016, 08:16:08 pm
content: The two OP-Amps on the left side are only used for the current source. It should be no problem, to connect the "main" OP-Amp directly to the zener and use a "normal" precision dual opamp for the current source.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 15, 2016, 08:51:19 pm
content: The 2 critical OPs are the ones to buffer the 7 V and the 5 V output of the LTC1046 divider. For the 7 V the load from the LTC2400 input might be to much to connect it directly to the reference.The other OPs are not that critical. The one in the center to output the 10 V is corrected by the ADC/DAC, so like the resistive divider it only needs to be reasonably stable to keep the DAC range small.

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 15, 2016, 09:09:11 pm
content: Has anyone experiences with a good buffer OP-Amp for the LTC2400?My first guess would be an auto-zero opamp combined with a buffer stage.

author: Andreas
author_rank: Super Contributor
timestamp: March 15, 2016, 10:12:32 pm
content: Hello,I never tried a buffer stage.just a LTC2057 or a LTC1050 together with a R-C low pass of 820R + 470-820pF like in Schematics "Widerstandsteiler"https://www.eevblog.com/forum/projects/oshw-24bit-adc-measurement-system-for-voltage-references/msg434154/#msg434154the largest problem is that the LTC2400 generates transients at the input during conversion.without the low pass these transients are rectified at the output stage of the OP-Amp.This generates a negative offset near zero and a positive offset at full scale. (ADC seems to have too much gain).If the capacitor is too large you get non-linearities. There is a sweet spot where the gain is exactly 1.000000For the "current source" it is sufficient to put a 3K resistor from 10V output to the zener.with best regardsAndreas

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 16, 2016, 10:08:01 pm
content: Quote from: Andreas on March 15, 2016, 10:12:32 pmI read in the datasheet the long term specifications with 1mA +/- 0,1% and I thought it could be nice to be within this 1mA +/-0,1% even with different LM399 (voltages) for comparision.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: March 22, 2016, 09:46:08 am
content: I came up with another idea of using multiple LTC1043 however LT Spice seems to have huge problems in simulating the circuit.I'm sending Youzipped .asc fileso You can simulate the circuit (or make the simulation working).It seems that it would be not so easy to make the circuit stable.Finally, if someone has 2 or 3 pcs of LTC1043 he could check the concept in the real world.PS. The print screen is to small to fit all the schematic, but it explains the idea.Comments invited.

author: Andreas
author_rank: Super Contributor
timestamp: March 22, 2016, 07:33:00 pm
content: Quote from: doktor pyta on March 22, 2016, 09:46:08 amHello,the trick that you have to do with a LTC1043 as divider is in phase 1:put all flying capacitors in series to the output capacitor.in Phase 2: connect all flying capacitors in parallel to the output capacitor.Only in this way the tolerances of the capacitors play nearly no role.I cannot see this principle in your cirquit to generate the 1V part.With best regardsAndreas

author: doktor pyta
author_rank: Frequent Contributor
timestamp: March 22, 2016, 09:10:00 pm
content: QuoteWell, not exactly.Speaking of LTC1043, the 'inverting' topology introduce the same error as 'differential to single ended' topology so 2ppm.Both these topologies are unaffected by capacitors tolerances.Proposed circuit uses 'single ended to differential' topology stacked to obtain 10V from 1V input. The main problem I see is the pulsed current taken from the output on an opamp.

author: d-smes
author_rank: Regular Contributor
timestamp: March 23, 2016, 03:14:42 am
content: Does this accomplish objective: Use divide-by-two from 10V to form reference for LTC2400 (buffered, of course). Then use differential-to-single-ended circuit on cover of LTC1043 datasheet to translate the 10V - 7V differential to ground potential. Buffer this and use as Vin of LTC2400. Now everything is at ground potential and you're using only one LTC1043 plus two buffers. LTC2400 output ratio of 3V / 5V minus the reference (calibrated) ratio is your ACAL error term to the correction DAC.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 23, 2016, 11:06:32 pm
content: For the 7 V to 10 V step you have to distinguish 2 cases:1) you have some 6.9-7.1 V source and want to get exactly 10 V out (e.g. reference source). Here you need the fine adjustment. The LTC1043 alone can not provide this solution.2) you have about 7 V and need a stable about 10 V, e.g. as a reference for an ADC. In this case you might get away with a factor close to 10/7 , maybe even 3/2. Here the LTC1043 circuit could do the trick as not fine adjustment is needed.Using the LTC1043 to transfer the 10- 7V difference to ground would work, but the advantage is limited. There is no big deal having the LTC2400 positive tied to the 10 V output. The whole circuit of ADC, µC and DAC does not have to be ground referenced.

author: d-smes
author_rank: Regular Contributor
timestamp: March 24, 2016, 02:19:32 am
content: I don't understand Kleinstein response. For point #1, I agree the 7V has a wide initial tolerance but the main objective is to compensate for 7V to 10V amplifier drift. All the LT1043 does with this voltage is divide it by two so the resulting ~3.5V +/- 1ppm (from data sheet divide-by-two circuit) is within the range of the LTC2400 as a input signal.I don't understand point #2. By taking the differential 10V - ~7V = ~3V and translating the result to ground, the ~3V signal is within the LTC2400's 5V reference range, so a accurate ratio can be established. The LTC1043 isn't clear what accuracy of differential-to-single-ended conversion is, but the CMRR and Charge Injection discussions in the data sheet implies that's where this chip excels. While I agree the white-board concept posted by the OP would work, I submit having everything ground referenced is easier and has less hardware. The math of the ratio is different, but a correction term from the ratio to drive the DAC can still be derived.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: March 26, 2016, 11:45:22 pm
content: Quote from: doktor pyta on March 22, 2016, 09:10:00 pmI agree that you should be effectively free from capacitor tolerance issues, even tapping off 7V from the middle like that. Like you, I'm also suspicious of driving the 1u caps directly from LTC1051s (I get about 50mVpp oscillation in LTSpice. Easily fixed in spice with some resistance.) However, I'm also suspicious of your outer feedback loop; the 1043s take a long time to stabilize and don't do it smoothly--I would guess U12 oscillates. And it does exactly that in my stripped down LTSpice run. (Just one 1043+1051.)Simulating the 1043 in ltspice is super fiddly. I'd found some advice about it somewhere, involving changing the integration type to Gear and messing with some of the tolerance values, but I still would have to mess with the switching frequency and couldn't always get it to work.I've also played around a bit with doing LTZ1000 -> 10V with 1043s (I used 3, and wrote a program to solve for their arrangement) in LTSpice, but I don't have the switching knowledge to practically implement it without spoiling the precision DC output.