The 1.5 bit DAC exist in pipeline ADCs. The MDAC stages provides N and half bit resolution (due to less no of refrence levels than 2^N) and after some digital corrections M MDAC stages provides the right resolution of whole ADC. For example 10bit pipeline ADC has 10 1.5bit MDAC stages and digital correction circuit which calculate residuas of each MDAC stage to provide correct conversion. You need to chceck this at literature ;-)