Difference Between Isolated Amplifier vs Linear Optocoupler?

Joined Oct 26, 2012
434
I understand that an isolated amplifier uses capacitors for isolation, and a linear optocoupler uses an LED and photodiodes.
But, assuming unity gain, why would i choose one over the other?

I'm guessing the main difference is that the isolated amplifier offers some gain/buffering.

Here's an example of an isolated amplifier:
TI AMC1311x High-Impedance, 2-V Input, Reinforced Isolated Amplifiers

Here's a linear optocoupler:
Ixys/LittelFuse LOC110 Linear Optocoupler
https://www.littelfuse.com/media?re...ttelfuse-integrated-circuits-loc110-datasheet

Personally, I'm interested in DC applications for the iso, but AC is also interesting.

Last edited:

Joined Oct 26, 2012
434
I found an answer on SE. Key points:
• Isolation amps are good for measuring accuracy over a wide dynamic range, with at least 1,500 volts isolation, so you can measure line voltage at 600 VAC with no problem, and measure the current as well.
• Optocouplers are faster and cheaper.

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Ian0

Joined Aug 7, 2020
10,035
? My application is secondary feedback on an isolated DC-DC converter. It's for a battery charger, so i think about 5 mV linearity over 5 volts full scale is acceptable. Not sure how to convert that to % (is it 5 mV/5, or 5 mV/1?).
so why don’t you do it like the rest of the world does, by comparing the output with the reference on the isolated side, then transferring the error signal across the isolation?
if you start introducing delays into the feedback loop, you will make it oscillate.

Joined Oct 26, 2012
434
so why don’t you do it like the rest of the world does, by comparing the output with the reference on the isolated side, then transferring the error signal across the isolation?
if you start introducing delays into the feedback loop, you will make it oscillate.
How to transfer the error signal across the isolation?

Ian0

Joined Aug 7, 2020
10,035
How to transfer the error signal across the isolation?
A PC817 or other cheap optoisolator does the job perfectly well, as non-linearities and variations in gain are removed by the error amplifier.

Joined Oct 26, 2012
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Ian0

Joined Aug 7, 2020
10,035
The FOD2742 is an optical error amplifier. This is what you mean, correct?
https://www.mouser.com/datasheet/2/308/1/FOD2742B_D-2313634.pdf

View attachment 311706

How can i know it's linearity? Is it related to CTR?

How about drift at higher temps? Datasheet states
temperature coefficient of 50 ppm/C
Yes, that will do, but it is nothing other than a combination of a fairly ordinary optoisolator and a TL 431.
It isn’t particularly linear, but had it ever bothered you how linear the open loop gain of an op-amp might be?

Joined Oct 26, 2012
434
had it ever bothered you how linear the open loop gain of an op-amp might be?
Why do you mention the open loop gain of an op-amp?

Any ideas about calculating linearity or drift at higher temps?

Ian0

Joined Aug 7, 2020
10,035
Why do you mention the open loop gain of an op-amp?
because the circuit is identical to an op amp. The TL 431 input is its inverting input, the non inverting input is connected to a 2.5V reference, and the output is the output of the opto.
Any ideas about calculating linearity or drift at higher temps?
look At the TL431 datasheet.
You might need to know the speed of the opto to calculate the phase shift for the feedback.

Joined Oct 26, 2012
434
had it ever bothered you how linear the open loop gain of an op-amp might be?
then it bothers me. Shouldn't it?

Ian0

Joined Aug 7, 2020
10,035
Take the standard op-amp gain equation:
gain = A/(1+AB)
where A=open loop gain and B=feedback factor
How much difference does the actual value of A make provided that it is large?

Joined Oct 26, 2012
434
Take the standard op-amp gain equation:
gain = A/(1+AB)
where A=open loop gain and B=feedback factor
How much difference does the actual value of A make provided that it is large?
Open loop gain is gain of the op amp without any feedback, correct?
Gain is calculated based on the formula you gave.
How do we calculate linearity from that?
Unclear what you're implying about linearity.

Ian0

Joined Aug 7, 2020
10,035
Open loop gain is gain of the op amp without any feedback, correct?
Gain is calculated based on the formula you gave.
How do we calculate linearity from that?
Unclear what you're implying about linearity.
The non linearity is about the same as the difference between gain calculated by the normal method (R1/R2) and the gain calculated from A/(1+AB) where B=R2/R1

If you want 10V output and have a 2.5V reference then the gains is 4, so B=0.25
If you assume that A=100000, then the actual gain is 3.9998.
Bearing in mind that you might be using 1% tolerance resistors at the best, the real gain due to resistor error will vary from 3.92 to 4.08.