it's a resistor and capacitor mixed dac. i don't quite understand how to switch on and off those switches. if possible, plz give me the expression of the output Vs.
It seems like a charge steering architecture. Depending on the input code the charge is accumulated in the capacitors by the decoding logic and they are shared to give the output voltage accordingly. To understand the basics you can refer to any of the data converter books e.g., razavi or plassche or even the CMOS analog book by allen could be of good help.