# 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).

• Why don't you power the buffer amplifier from the same reference.
– user16222
Apr 19, 2019 at 8:51
• This is not exactly the same as my circuit, I have an instrumentation amplifier and I'm limited to input common mode voltage, So I have to use higher Vdd for Op-Amps.
– MHTB
Apr 19, 2019 at 9:03
– user16222
Apr 19, 2019 at 9:40
• You can just put a 5.6V zener on your 5V rail (like the REF has). It's not the best way to do it though. I would put a series resistor before the schottky clamp. The ADC can typically handle a few kohms source impedance without issue (check datasheet).
– Jon
Apr 19, 2019 at 9:50
• @mohammad, this is the 1st time you have mentioned 3v3. Please post your EXACT circuit
– user16222
Apr 19, 2019 at 17:21

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).

The article you're referring to says:

For extra protection, if the reference has little to no sink current capability, a Zener diode or clamp circuit could be used on the reference node to guarantee the reference voltage cannot be pulled too high.

If a Zener is not accurate enough for your circuit, use a better shunt regulator, e.g., a TL431.

• Yes, as you know zeners are not ideal clamps around 3.3V. Does not placing a Shunt regulator on 3.3V rail interfere with series regulator I already have for 3.3V?(series LF33 regulator)
– MHTB
Apr 21, 2019 at 17:02
• Make the shunt regulator a little bit above 3.3 V so that it activates only for real overvoltages.
– CL.
Apr 21, 2019 at 20:17