Electronics > Metrology

7.xV to 10.00000V AutoCAL circuit

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d-smes:
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.

Galaxyrise:

--- Quote from: doktor pyta on March 22, 2016, 09:10:00 pm ---
--- Quote ---Only in this way the tolerances of the capacitors play nearly no role.
--- End quote ---

Well, 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.

--- End quote ---

I 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.

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