Automotive signal isolation - alternatives to optocouplers

Question

I'm working on an automotive project, where I'd like to interface various digital signals to an MCU. The signals can either be ON/OFF (e.g. radiator fans) or measurements of pulse frequency, width, duration, etc. (e.g. tachometer or speed). The amplitude of the logic level high can be 5V, 9V or 12V. I would like to protect the MCU inputs from overvoltage conditions (e.g. voltages up to 40V) as well as reverse polarity conditions.

In testing and prototyping, I've used the below basic optocoupler circuit, selecting a suitable R1 value to ensure the current through D2 at 40V is within the specifications for the optocoupler:

This circuit works as intended, but I'm concerned about the longevity of the optocoupler LED and the degradation of the CTR over time. Is there an alternate method of providing isolated coupling for a wide range of voltage inputs, which has better long-term reliability and better high frequency characteristics than optocouplers?

Answer 1

There are off-the-shelf isolator chips. For example:

1. Inductively coupled from Analog
2. RF coupled from Silabs (yes! basically a small radio inside a chip!)
3. Capacitively coupled like ISO77xx from TI

Answer 2

but you only need a resistor.

^{simulate this circuit – Schematic created using CircuitLab}

the protection diodes in the microncontroller are sufficient to tame the less than 1 mA that the resistors will pass through to the micreocontroller pin.