Pixel structure for asymmetry removal in ToF 3D camera

Most of time-of-flight (ToF) cameras have a 2-tap pixel structure for demodulating a reflected near infrared (NIR) from objects. In order to eliminate the asymmetry between two taps in the pixel, a ToF camera needs another measurement, which collects photo-generated electrons from reflected NIR by inverting the phase of clock signals to transfer gates. This asymmetry removal needs additional frame memories and suppresses the frame rate due to the additional timing budget. In this paper, we propose novel asymmetry removal scheme without timing and area overheads by employing 2x2 shared 2-tap pixels with cross-connected transfer gates. The 2-tap pixel is shared with neighbor pixels and transfer gates in the pixel are cross-connected between upper and lower pixels. In order to verify the proposed pixel architecture, an electron charge generated in floating diffusion is simulated. And then we try to calculate a depth from camera to objects using simulated electron charge and measure a linearity of depth. In simulation result, proposed pixel architecture has more linear graph than conventional pixel structure along the real distance of objects.