An integrated potentiostat designed for amperometric electrochemical sensors is presented in this paper. The analog input current is digitized using an A/D converter which employs a current-mode, first-order single-bit sigma-delta (ΣΔ) A/D architecture. Compared with traditional potentiostat, 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 applications. The potentiostat circuit is implemented in a 0.18 um CMOS process. The circuit converts the sensor current to digital code and the results indicated that the potentiostat can measure the current as low as several nA, and the digital output has a good linearity.