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This paper describes a micropower CMOS integrator with an extremely large time constant for use in a variety of low-frequency signal processing applications. The specific use of the integrator in an implantable biomedical integrated circuit is described. The integrator is based on the OTA-C approach and a very small transconductance of 100 pA/V was achieved by cascading a short chain of transconductance-transimpedance stages. The time constant of the integrator is tunable between about 0.2 and 10 s, and any offset voltages at the output terminal can be trimmed out. The circuit was fabricated in a 0.8-μm CMOS process, dissipates 230 nW from ±1.5 V power supplies (excluding the bias circuitry and output buffers) and has a core area of 0.1 mm2. The integrator offers superior performance in terms of power consumption, die area and time constant when compared to previously published work.