This paper describes a micropower instrumentation amplifier that enables chronic biopotential sensing in battery powered applications. The amplifier is chopper-stabilized to eliminate excess noise from 1/f or popcorn processes, insuring the highest fidelity of signal measurement for diagnostic analysis. The circuit consumes 2.0 μW of power from a 1.8V supply, with a noise floor of 0.94μVrms in a bandwidth from 0.05 to 100Hz; the resulting noise-efficiency factor of 3.6 is the lowest published to date. The specific implementation of chopper stabilization also provides railto-rail inputs and 100dB CMRR at low frequency. A digitally programmable on-chip high pass filter (0.05Hz, 0.5Hz and 2.5Hz) is used to suppress front-end electrode offsets while maintaining relevant physiological data. Although the focus of this paper is on biopotential sensing, the circuit architecture is also useful for a variety of micropower sensor interfaces using synchronous demodulation.