Two in-pixel encoding mechanisms to convert analog input to spike output for vision sensors are modeled and compared with the consideration of feedback delay: one is feedback and reset (FAR), and the other is feedback and subtract (FAS). MATLAB simulations of linear signal reconstruction from spike trains generated by the two encoders show that FAR in general has a lower signal-to-distortion ratio (SDR) compared to FAS due to signal loss during the reset phase and hold period, and the SDR merit of FAS increases as the quantization bit number and input signal frequency increases. A 500 µm2 in-pixel circuit implementation of FAS using asynchronous switched capacitors in a UMC 0.18µm 1P6M process is described, and the post-layout simulation results are given to verify the FAS encoding mechanism.