I am analyzing the following circuit used as an analogue input protection for an ADC ( image source: http://www.analog.com/library/analogDialogue/archives/43-04/process_control.html ):
I don't understand the purpose of c5, c6, and D3. So my questions:
1- I'm guessing c5 can reduce spikes on the input. Is this correct?
2- I have no idea what D3 (1n4148) does. If a negative voltage is placed at the input of the circuit this diode would immediately fry (defeats the purpose of the circuit).
3- I also have no idea what c6 does?
4- Also the ref input is connected to 0.5 volts. Why? The op-amp is rail-to-rail, why is there any need to shift the output by 0.5 volts?
P.S. Anti aliasing filter is placed on the ADC input and is not shown in this circuit.