Measure both positive and negative voltages using ADC
My input voltage is in range from -55 to +55V


My first observation is that you must mean a slow moving signal; in other words, you have an input that can vary between -55 V and +55 V. I say "slow moving" because the ADC you have chosen is nominally aimed at low sampling rates. Yes it can do 240 Sps but this wouldn't be great at sampling a 50/60 Hz AC voltage because you can easily miss the peaks if not synchronized.

The range is 110 volts and this needs reducing to 2.048 volts, so a simple attenuation will do the trick (two resistors). Next, you need to bias (or offset) the range -1.024 V to +1.024 V up by +1.024 V and a simple op-amp summing circuit can do this. 



All resistor values identical with point (a) fed with -1.024 V and point (b) fed with the attenuated signal. There will be a little bit more attenuation due to the loading effect of the resistor in the (b) line but this can be managed and can be a bit of a problem solver too given that ADCs don't have a perfect reliable input range as specified on the front sheet of the data sheet.

You will end up with an inverted output i.e. +1.024 V is represented by -1.024 V but that is trivial once digitized.

You need to also watch out for the internal reference accuracy and drift in the ADC - it's pretty crappy if you want accurate and reliable measurements.

For the op-amp I'd be considering a rail-to-rail type so that it can reach its output down towards 0V when the input signal is at one extreme.