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Analog optocoupler

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profdc9:
How about using a IL300 optocoupler with a op-amp as a voltage-to-current driver for the LED and another as the current-to-voltage driver for the photodiode?

T3sl4co1l:

--- Quote from: ricko_uk on June 20, 2020, 08:09:49 pm ---Hi Tim,
Do you mean accuracy in terms of output amplitude vs input amplitude? If so then not that bothered because I can amplify/attenuate before or after.

If you mean distortion or how "identical" the o/p is to the i/p then I have no idea how to quantify/specify. My background is mostly digital and have been getting into analog only recently. :)

--- End quote ---

Well... okay, what's the signal doing, what's it for?

Being able to adjust amplitude is a big gimme:
- Optos have terrible manufacturing accuracy (CTR 50-200% say)
- It changes over time (mainly the LED fades with use, over some years/decades)
- And of course temperature too

Distortion is where the gain depends on the signal's instantaneous amplitude (or some past history thereof, but that gets even more complicated).  We can express distortion in many useful ways: peak, average or RMS error from a best-fit line; distortion of a sine wave, or more complicated wave (THD); mixing products of two or more sine tones (IMD); etc.

Sine distortion is simply that, when a sine wave A sin wt is passed through a nonlinear, one-to-one transfer function f(x), we can substitute it into the Taylor series of the function, f(A sin wt) = f(0) + (A sin wt) f'(0) + (A sin wt)^2 f''(0) / 2 + ..., and reduce the powers of sin^n using trig identities.  Some terms will go to DC (e.g., sin^2 x = (1 - cos 2x) / 2 has a constant term 1/2), some will go to the fundamental frequency, others go to harmonics (like the cos 2x term).

If you've not taken calculus, Taylor series isn't going to mean anything to you, but suffice it to say, it is an equivalent form to represent a function (given some constraints, which amplifiers obey so we're good), f'(x) means the derivative of the function (well, also not going to mean much, but if nothing else, it's a basic operation we can apply to functions), and the "..." means it's an infinite series (actually N terms with an error term absorbing the difference; if the error term happens to go to zero as N-->infty, great, it's an exact representation).

And a function is simply the curve corresponding to pairs of input and output values, so, voltages or currents or whatever.

If you were using this for a DC application like setting a power supply's output, you'd most likely be interested in the statistical error approach: how closely does it fit a straight line?  Also, what is the zero intercept of that line, do we need to trim that out as well or is it fine as is?

For signals, audio for example, THD is the more useful measure.  It's not the easiest to calculate, but to be fair, it's not the easiest to measure the transfer curve anyway; at best you can sample some points on it.  Easier to measure it with a signal generator and, say, sound recorder, then let a computer do the hard work (Fourier transform perhaps). ;D

There's also frequency response, how flat it should be over time -- does the A factor vary with w (frequency) as well?  Or for a time domain response, say you apply a step, how fast does it settle down within some threshold bound of the target value?

Tim

Wolfgang:

--- Quote from: TimFox on June 20, 2020, 10:52:20 pm ---Mouser lists 81 VFC ICs from $2 to $32 in singles.  Obviously, higher spec units will cost more.  The VFC32 is a precision part with a good app note in the data sheet.

--- End quote ---

There are some IL300 circuits in the IL300 app notes. I built some of those, but they are
- not very accurate
- a bit noisy
They have better ICs today, I would guess.

Zero999:
Again, not exactly cheap but Analog Devices make some decent isolation amplifier ICs. The ADuM3190 is the most cost effective one I could find in RS Components.
https://uk.rs-online.com/web/p/isolation-amplifiers/7863151/
https://docs.rs-online.com/941a/0900766b814bac4c.pdf

exe:
I think I tried all the analog optocouplers with feedback on the market (such as il300, etc). They are only precises in ideal conditions. Under mechanical stress they start to noticeably drift. Also there is gain non-linearity*. "precision" versions are also expensive. If taking your requirement "without distortion" literally, then analog optocouplers are not for you. Output is also like 0-500mV, so you need to scale it up. I'd definitely wouldn't use them if accuracy is required.


*Datasheets mention "0.01 % servo linearity", and "High gain stability, ± 0.005 %/°C typically", which made me thinking those devices have quite some precision, but then they say "Transfer gain linearity" is +-0.5% typically, which is roughly what I got under ideal conditions (i.e., constant temperature and no mechanical stress).

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