Post #5 - It will not work and it will probably damage the PIC.
Is it safe to use the AC Neutral with PIC controller and other IC'S GND directly..?
Yes, provided you are careful and can ensure correct connection, it will work but only if you use a potential divider. Resistors in series with the PIC input will never work simply because the division ratio is "(2 * 330K):almost infinity". The input impedance of the PIC is the lower part of the divider and being a MOS device that impedance will be hundreds of Megohms.
A far better solution is to use a resistive divider. At this point you have to decide whether you want to measure the AC by sampling the waveform or you just want a representation of the voltage.
1. if you want to sample the waveform you do it by taking many samples of the whole AC cycle and mathematically calculating the effective RMS voltage. This method gives best accuracy but adds a problem: the PIC cannot measure negative voltages at all. The way to solve that problem is to make sure the AC can not go negative by adding a positive voltage to it. You first use a potential divider across the live to neutral to drop the voltage down to less than 2.5V peak to peak, remember that the peak of a sine is 1.414 times its RMS value. Then you add 2.5V DC to the result so the waveform is centered between 0V and 5V so the ADC can read it. This is where the op-amp comes in handy, it can be used to add the offset and it also helps to make the ADC more accurate by providing it with a low impedance voltage source to measure.
2. if you just want a rough indication of the AC voltage, rectify it (a bridge rectifier would be appropriate) so the negative component of the waveform is removed, then filter it with as small as possible reservoir capacitor (~1uF) and finally divide it down to the peak voltage is less than 5V. Buffering with an op-amp will again improve accuracy. This method is simpler to implement but has two drawbacks, firstly it ignores the shape of the waveform so it doesn't see distortion or spikes, secondly it is slower to react to voltage changes because it relies on the charge and discharge of the reservoir capacitor to track the AC voltage.
Your choice....
Brian.