Electronic – Need an optocoupler for getting directly proportional output galvanically isolated from the DC input


I want to use an optocoupler to linearly step down a DC voltage which willy vary from 0V to 400V. That is, I want the output to be directly proportional to the input and I want it to be galvanically isolated from the input

V1-0 to 400V, V2- 12V

Now, for this purpose I used IL300 and MCT2E earlier. The problem with IL300 was that the slope (linearity factor) varied across different samples of the IC. I can not allow that to happen. And the problem with MCT2E was that the linearity lived only up to a particular value of input ad the output tended to get saturated after that. I realize that changing Rs might have solved this problem, but the saturation couldn't be eliminated quite very properly. Now, I'm looking for some other optocoupler for this purpose.
I have shortlisted LTV 817 for this purpose but I'm not sure how it is going to perform. So, I want to select atleast two more ICs for this purpose and then test all of them to select the best one.

What I require from the IC:

  1. Directly proportional output for a DC input varying from 0-400V.
  2. Galvanic Isolation between input and output.
  3. Output to vary linearly with the input, between 0 to 12V.

Can you guys please suggest me some optocouplers that will help me solve this purpose?
Note: There might be devices other than optocouplers, available for this purpose, and I welcome suggestions regarding them as well, but I prefer optocouplers.

Best Answer

A standard way to do this is to use a second optocoupler (preferably on the same chip) to compensate the non-linearity of the first one. The example below is from Analog Devices AN-106. The principle is to put a current through the second optocoupler's primary such that it secondary gives the same result as the secondary of the first optocoupler. Then that current must be the current through the first optocoupler's primary.

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An IL300 optocoupler is a slight variation on this theme: it has two secondaries coupled to the same primary, so the primary side can do the compensation.

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As Dirk suggests, you could go digital at the primary side. A digital signal is much easier to get through an isolation barrier. You don't have to build this yourself, check for instance the HCPL-7510 linear isolation amplifier :

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