Abstract: For Giant Magnetoresistance (GMR) Biosensor application an Analog Front-End (AFE) circuit is presented in this paper, which consists of readout circuitand 10 bit/1 MHz Successive Approximation Analog-to-Digital Converter (SAR ADC). The readout circuit provides 300 mV bias voltage for GMR biosensor array, which transfers the resistor change into current change. Then the current is transferred into voltage and amplified by Trans-Impedance Amplifier (TIA). Finally the digital code is output by SAR ADC and processed by DSP. The AFE is implemented in SMIC 0.18 μm 1P6M CMOS process. The power supply is 3.3 V.The measurement results show that, in 1 MHz clock frequency, the ENOB of circuit is 7.8 bit, meets the requirement for GMR biosensor detection.