An analog front circuit for a medical device includes an automatic gain control loop and a 2-order-3-bit-quantization Sigma-Delta analog-to-digital converter. The automatic gain control loop is configured to implement automatic control of loop gain and output an analog signal to the 2-order-3-bit-quantization Sigma-Delta analog-to-digital converter. The 2-order-3-bit-quantization Sigma-Delta analog-to-digital converter is configured to convert the analog signal output from the automatic gain control loop into a digital code and output the digital code to a DSP for processing.