There is a growing demand among scientists and clinicians for low-noise low-power small-size and mobile biopotential acquisition system, which can be not only applied for long time medical monitoring, but also extended to home health care, sports and entertainment applications. Low frequency filter is one of the basic building blocks for biomedical systems, wherein analog preprocessing blocks, such as low noise preamplifier and filters for the acquisition of biopotential signals are employed. Low pass filter (LPF) is usually adopted in order to eliminate the out-of-band interference. This thesis presents the analysis and implementation of nano-watt class subthreshold-biased source-follower-based (SFB) LPF for biopotential signal acquisition system. The generic gain-loss problem of SFB LPF is alleviated by utilizing a gain-compensation technique. The tradeoffs between power consumption, linearity and noise are investigated in detail. Finally, two design examples of 4th-order SFB LPFs for Electroencephalogram (EEG) acquisition system are implemented and compared. Comparative results have validated that the analytical results are consistent with the simulation results. The simulated results show that the proposed LPF possesses the third harmonic distortion (HD3) of -64.4 dB, DC gain of 0 dB and the power consumption of 5 nW under a 3-V power supply. The integrated in-band noise voltage is 188.5 µV with optimized total capacitor value of 45.5 pF. Therefore the proposed filter can be applied to the acquisition system of EEG whose basic signal bandwidth is located in 40 Hz.