1. I have been reading this:
http://www.linear.com/product/LTC1966

2. Is it possible that single LTC1966 can be used for both ac & dc voltmeter(one at a time).
Suppose I have two measuring points (like in voltmeter, red & black probe), I connect those to differential input to LTC1966 & then to diff adc.

3. Suppose I connect dc 2V across diff probe, then it will pass through LTC1966 which will pass its true dc value (although signal is already dc)
again now if connect 2V ac across it, then it will pass through LTC1966 whose actual rms to dc value is passed.

So can I say the point on voltmeter is now used for both ac & dc voltage measurement without have to change any setting.

 

Yes, that chip will measure an AC or DC input.
Note that if the AC has a DC offset it will measure the combined value of both. If you want only the AC component you will need to switch in a series DC blocking capacitor.

 

I have completed details of project here:
http://forum.allaboutcircuits.com/threads/project-help-4-channel-voltmeter-application.115088/