An ultra-low power bio-impedance sensor for wrist watch-type blood pressure monitoring system is implemented in 0.18 mu m CMOS technology under 1.8 V supply. The duty-cycled current generator (DCG) enables low power and high amplitude current injection with 93.58% power reduction in a current generator block. A fast-settling readout circuit with DC balanced amplifier reduces settling time to 276 mu s, allowing bio-impedance signal acquisition even when the motion artifact induced circuit transient occurs. The simulated 2.125 mm(2) single chip bio-impedance sensor consumes only 55.77 mu W.