A 40 fJ/c-s, 1 V, 10-bit SAR ADC is presented for energy constrained wearable body sensor network application. The proposed 10-bit dual sampling capacitive DAC topology reduces switching energy by 62% compared with 10-bit conventional SAR ADC. Also, it is more robust to capacitor mismatch than the conventional architecture due to its cancelling effect of each capacitive DAC. The proposed SAR ADC is fabricated in 0.18 ${mu}m$ 1P6M CMOS technology and occupies 1.17 $mm^2$ including pads. It dissipates only 1.1 ${mu}W$ with 1 V supply voltage while operating at 100 kS/s.