# ADC Input Protection with 4-20mA

If you've got a 4-20mA transducer, powered by 14V, and you want to measure it, your analog front end might look something like this:

simulate this circuit – Schematic created using CircuitLab

... but now my thinking shifts toward failure mode considerations. My ADC has an absolute maximum rating that dictates that it's inputs must not be greater than 3.6V and must not be less than -0.3V.

What circuit do you usually put on the positive input to your ADC to make sure that it never sees more than 3.3V if the transducer fails to limit current to 20mA or if R1 doubles in resistance for some reason (or whatever). Is this input protection circuit a good idea (BAT42 Schottky diodes in a crowbar configuration) and a current limiting resistor:

simulate this circuit

• Yes, that‘s how you can do it. I don‘t know a better way. – Stefan Wyss Jun 16 at 19:18
• Is this input protection circuit a good idea yes. Additionally, you can add a capacitor (10~100n) across D2 to form an RC filter with R1. – Rohat Kılıç Jun 16 at 19:23
• @RohatKılıç good thinking! – vicatcu Jun 16 at 22:12