I will change the position of resistor and capacitor and get back to this thread.
Its a DC input varying from 0 to 10V. As the output reach the 3.3 volt, processor is program to shutdown the particular part of the circuit. Hence I dont need output above 3.3V also it may damage the processor.hi,
What is the source and range of the input signal.?
You have shown +/- inputs , but one input is connected to 0V.?
Thanks for your detailed explanation.The 3.3 V Zener there is a bad idea, even with a resistor.
1) The opamp will basically see a short when the output tries to push beyond 3.3V, this will cause maximum current/power dissipation and quick failure.
2) Zeners are not terribly accurate, the clamping voltage will not be very predictable.
3) Zeners don't typically have a super sharp threshold, they start to conduct more gradually than you might expect.
Reasons 1, 2 and 3 mean that even with a resistor to limit the current flow, the upper ADC values close to the threshold may become non-linear.
Use a simple diode/resistor to clamp the output voltage to the 3.3V rail, with an additional resistor before the ADC input to limit any current flowing into the MCU's input protection diodes.
Essentially the external diode is in parallel with the internal one, the extra resistor ensures that the external diode sees most of the clamp current.
This network will never conduct any current within the full ADC voltage range- no chance of non-linearity.
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