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How to design a circuit to protect against overvoltage for input?

Latest Updated:11/01/2005

Question:

How should I design a circuit an operational amplifier to protect against overvoltage for input?

Answer:

Assuming a case where the input voltage is higher than V+ or lower than V-, you should clamp the input pins to V+ and V- respectively using a diode with a small forward voltage, such as a Schottky barrier diode. The forward voltage of the parasitic diode is about 0.6 V, the same as a normal silicon switching diode. To protect the circuit, therefore, a diode with a small forward voltage must be attached to stop current flowing through this parasitic diode.



When externally attaching a normal silicon switching diode as a protection diode, the device can be protected from damage by inserting a series resistor at the input to limit the current flowing through the parasitic diode to no more than about 5 mA. However, as this may cause malfunction of the device, we recommend using a diode with a small forward voltage, such as a Schottky barrier diode.

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