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In this paper, a micropower current sample-and-hold front-end is designed for weak current bio-medical applications in a 0.35-µm standard CMOS process. The design reduces the distortion of the current-mode sample-and-hold stage by exploiting the exponential I-V relationship of weakly-inverted MOS transistors. The design and optimization process for the stage is introduced in the paper. With the optimized design, SPICE simulation shows that the sample-and-hold stage can achieve over 60dB SNDR at the sampling rate of 25kS/s, with the input signal at the Nyquist frequency of 12.4kHz. The input current range is 100nA. The power consumption of the stage is about 3.6µW.