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We present a new circuit topology for potentiostats that interface with three-electrode amperometric electrochemical sensors. In this new topology, a current-copying circuit, e.g., a current mirror, is placed in the sensor current path to generate a mirrored image of the sensor current. The mirrored image is then measured and processed instead of the sensor current itself. The new potentiostat topology consumes very low power, occupies a very small die area, and has potentially very low noise. These characteristics make the new topology very suitable for portable or bioimplantable applications. In order to demonstrate the feasibility of the new topology, we present the results of a potentiostat circuit implemented in a 0.18-mum CMOS process. The circuit converts the sensor current to a frequency-modulated pulse waveform, for which the time difference between two consecutive pulses is inversely proportional to the sensor current. The potentiostat measures the sensor current from 1 nA to 1 muA with better than 0.1% of accuracy. It consumes only 70 muW of power from a 1.8-V supply voltage and occupies an area of 0.02 mm2.