Electronic – Protect ADC input from over-voltage with Schottky diodes


Alan Walsh on this article has suggested using Schottky diodes to protect ADC input from over-voltage, the circuit looks like below:


simulate this circuit – Schematic created using CircuitLab

But it seems that the 3.3V regulator should have current sink capability in order to this circuit work, and the regulator I've chosen(LF33CDT) has not. One way I can think of is to add a load resistor on 3.3V rail to make a path for Schottky current to ground, But how can I make sure this path conducts enough current for Schottky to pull ADC input voltage down? (The Schottky is a BAT54).

Best Answer

You can do something like this:


simulate this circuit – Schematic created using CircuitLab

D2 is a shunt regulator capable of sinking about 100mA. R2/R3 set the regulator voltage to 3.3V nominal. R1 provides the bias current the regulator needs without loading the inputs (it needs 1mA minimum to regulate).

As shown you can protect multiple inputs with a single shunt regulator. The series resistors limit the op-amp output current and may not be required if the op-amp has a guaranteed maximum short-circuit current that is sufficiently low.

As shown it will protect as many as 5 inputs by adding diodes and resistors.

The voltage drop due to Schottky leakage and the series resistors should be negligible, but check the worst-case conditions (maximum temperature).