short ADC: 1) have a reference voltage (0v-5v for a ATmega). 2) have a precission (10Bit/1024 points for a ATmega). 3) and calculate sensors voltage with this conditions. no more. against the laws of physics can not do anything. if you sensor send only "2.0v - 2.5v - 3.0v" what to do?
want more points. 1) try decrease "reference voltage" - mcp3204/mcp3208 accept it(differential mode). 2) increase "precission" - mcp320x 12Bit/4096 points. 3) make more powerfull signal from sensor. mcp601 - electrical amplifier. gears - kinematics amplifier.
tle5011 cost ~4$, and dont want try calculate another solutions price.