In digital circuit :
Generally , the bulk of nmos connetcs GND!;
the bulk of Pmos connetcs VDD!;
In analog circuit :
Generally , Depending process, we can connect the bulk of a kind of mos .
But in a mixed-signal process , because of Bandgap which needs the PNP , the process is p type substrate. Then , we can make the N well , the bulk of pmos can be connected the source which is the same potential as N-WELL . This WELL is named floating well which potention of is changing as the source voltage.
Because the source connects the bulk , the voltage difference between the source and the bulk is zero , So the threshold of transistor is not modulted by the bulk voltage . In the mean time , the noise of substrate is not coupling into the MOS .
But the source level is change , this can effect the MOS performance.