To meet the stringent spectral mask requirement of IEEE 802.15.6, a novel spectral shaping buffer and a new automatic calibration are proposed in this paper. By introducing a floating current into buffer, second order intermodulation product (IM2) is cancelled at output. To calibrate buffer, analog signal processing methods for average power extraction of residual IM2 and accurate quantization are presented, along with error-tolerant decision making logic. A sample human body communication (HBC) transmitter is designed in 0.13 mu m CMOS process. Simulation results show spectral mask is suppressed below -120 dBr, satisfying the IEEE 802.15.6 standard specification.