ADC IC with positive and negative reference input voltages

Hello,

i am looking for an ADC IC which is capable of two reference voltages, an upper and a lower one.

The MSP430 has exatly those inputs available, but with external ADCs I could not find any yet, am I doing soemthing wrong? do people use differential measurments with GND connected to the negative reference?

For example my lowes signal would be 1,5 V and my upper limit 2,5 V, so with a 2,5 V reference to GND I would lose 1,5 V of resolution, in my MSP430 concept i simply connect 1,5 V to the neg reference and now I can measure 1 V with a resolution of 12 bit, this is what I would like to achieve.

Thank you.