I am looking to measure very low voltage signal (0-0.5V range) and was wondering if it is possible without amplification. I am a bit confused about what the minimum references of ADC can be. Looking at datasheets, for all ADCs I looked at so far, the range of Vref seems to be from 1.5V. Are references as low as 0.5V not possible? Also, are negative reference voltages possible? Like if I use a bipolar ADC instead which has a negative to positive range for ADC references, can I use a reference of 0.5V then?
Read the datasheet. No, distributors won't catalog this information, and yes you have to look at a lot of datasheets. We do too. That's just how it is.
Most continue to work at low voltages, with corresponding degradation of performance (INL/DNL being due to fixed internal offset voltages / charge injection, thus being relatively more significant at low VREF). Put another way, below the minimum recommended value, you aren't gaining any SNR or ENOB, the reading is equivalent to one taken at VREF(min) and scaled up numerically.
Some ADCs have preamplification available, like, ADS1220 I think is such a case. This has a differential input, so can still be used single-ended by tying one to the common/ground reference (but do mind input voltage range limits). Even some MCUs have features like this, like the XMEGA family of AVRs provide selectable gain, without impairing INL/DNL much (as far as I know).
It is difficult to go under 1.5V of reference due to process limits… for example a typical schottky diode would cause a 0.3V loss; noise is an issue too. Using micropower techniques there are maybe ways to do that but usually you lose some other good quality (like speed or precision); it's always a tradeoff. In fact even the MSP430 (which is a micropower MCU) has the minimum to 1.4V for external Vref (internal is still 1.5V, by the way).
Depending on your bandwidth requirements they make really cheap fixed gain amps: current shunt amplifiers have pretty decent specifications and often cost less than a similar general purpose amp.
Another way is to simply use a lot of effective bits and throw away the upper range of your ADCs (usually not doable with the junky internal ADCs of most MCU)