This paper presents an electrochemical sensing front-end circuit with a cyclic voltammetry potentiostat. To accommodate a large and bidirectional current sensing range for the current-mirror based potentiostat, a constant offset current is added. With the technique of chopper-stabilization, the flicker noise of the readout circuit is successfully reduced. A charge-based oscillator is also proposed in this paper to convert the current into a frequency for further data readout. The proposed readout IC was fabricated in a 0.18μm, 1.2V CMOS process and the current range is -2μA 12.2μA, the R2 linearity of 0.992, with a minimum detectable current of 3.1nA. The dynamic current range is 73.2dB whereas the chip consumes 40.5μW at a maximum sensing current of 12.2μA.