An open-loop VCO-based ADC with quasi-chopping for bio-sensor applications is reported in this paper. The VCO is implemented as a trans-conductor (Gm) stage followed by a current-controlled oscillator (CCO). The proposed Gm circuit is designed to accommodate a large input range. This work also adopts a quasi-chopping technique that suppresses low-frequency noise while performing demodulation in the digital domain. The off-chip non-linearity cancellation (NLC) algorithm further improves the overall linearity. The proposed chip is fabricated in the TSMC 180-nm process. The core area of the chip is 0.26 mm². The total power consumption is 0.65 mW. With an input of 1.52 Vpp, the ENOB is 9.3 with a 100-kHz bandwidth. The ADC achieves a FoMs of 140 dB and FoMw of 5.1 pJ/conv.