protect op amp input from overvoltage from piezo sensor

Question

I use a piezo to register the moment of a knock via a micro controller's analog in (1.8V Max) So far I used the scheme in the figure without any diodes. This works so far, but depending on the knocks harshness, the op-amp cuts the sensor signal. This is why I want to protect the op-amps in+ from overvoltage.

I have two questions:

1. will two back back zener-diodes 5.1 V protect my op-amp sufficiently?
2. if yes: Does anyone know smt diodes which are quiet big, like 3 mm or so.

Answer 1

1. I don't know the specifics of the piezo, but I doubt it can give enough current to break the overvoltage protection of the op amp. The Zener is going to cut the voltage at somewhere above 5.1 V, but at that point the protective circuit is already conducting. The series resistor to op amp vin+ pin is probably going to be adequate protection and you might even do fine without it. Raising the value will give more protection.

The leakage current of the TVS might ruin your application. Maybe that's why you selected a value 5.1 V and not 1.8 V anyway.

If you decide to go with the diodes, you can remove the left hand side diode.

2. SMA/SMB/SMC are standard packages for surface mount diodes. They are reasonable size and it's easy to find alternatives. Easy to hand solder even without magnifying.

Answer 2

1. yes. That's commonly done. Diodes in such usage are called TVS diodes
2. um, asking for a diode of the right size makes little sense, as you're probably designing a board around this, anyway, if you're looking at SMT? Anyways, yeah, there's plenty of SMD diodes, you'll just have to go to any larger distributor's website (Farnell/element14/Newark, for example) and click through to the discrete diodes and just throw out the packages that are too small for you. You really want to think about simply buying a dedicated TVS diode pair, however.

Answer 3

The MCP606 has a maximum input current of

Current at Input Pins ....................................................±2 mA

This means that if your resistor potential divider can have sufficient resistance, you are protecting the op-amp input.

Given that the input resistance (in normal operation) of the +Vin input of the op-amp is 1000s of Gohm, leakage current is 80 pA and the input capacitance is only 6 pF, I'm fairly certain you could raise the 1.2 kΩ resistor to 120 kΩ without any loss of performance or functionality. To get 2 mA through a 120 kΩ resistor requires 240 volts from your piezo at the very least so, is this realistically a possibility?

If it is then you have the option of placing a single unidirectional TVS (or zener) diode where you have it shown in your schematic.