This paper presents a fast mixed-signal automatic gain control (AGC) method for zero-IF/sliding-IF receivers used in wireless personal and body-area networks (WPAN/BAN). The preamble defined in Bluetooth low energy (BLE)/802.15.4 /802.15.6 specifications are as low as 1 Byte. It is a tough challenge for the zero-IF/sliding-IF receivers to perform AGC training, frequency synchronization and symbol timing estimation. By detecting the RF input of the quadrature mixer, IF input of analog filter and ADC output, the proposed method can achieve a desired IF amplitude within 1 bit when interference signal is small and 3 bits when carrier-to-interference ratio (CIR) is -27dB.