Audio processing had been in demand throughout the electronic era. Recent advances in neural networks increased the demand on audio processing for speech recognition applications. In this work, a rigorous study on the dynamic range and system complexity optimization is presented for a mixed-signal keyword spotting system. The proposed system consists of an analog feature extractor and a neural network based keyword classifier. The results showed that with the proposed method, more than an order of magnitude power saving can be achieved in the analog feature extraction compared to the digital state-of-the-art counterpart.