A VCO-based delta-sigma modulator (ΔΣM) is proposed for bio-potential signal acquisition. The VCO quantizer is placed in a loop with 1-bit feedback to improve modulator linearity. Furthermore, frequency calibration employing gated VCO technique is proposed to improve the operation range. Fabricated in a 0.18µm CMOS, this chip consumes 379nW and occupies an active area of 0.06mm2. Over a 2kHz signal bandwidth, the proposed ΔΣM achieves a peak SNDR of 58.5dB, which results in an FoM of 137fJ/conv.-step.