A high-speed digitally programmable CCD transversal filter

Abstract

A 32-stage programmable transversal filter is described which has 6-bit digitally programmable tap weights and has been operated at a 25-MHz clock rate. The device has a linear dynamic range of more than 60 dB and occupies a chip area of 24 mm/SUP 2/. Pipe-organ architecture made it possible to use a simple floating diffusion output circuit. The tap weight values are set by a 6-bit multiplying D/A converter (MDAC) at each delay-line input. The MDAC is a multiple CCD input structure with binary-weighted input gate areas and logic-controlled gates to multiply each charge packet by 0 or 1. The conversion speed of this structure is as high as that of a CCD input structure, but careful control of threshold voltage variations is required to achieve high accuracy. Experiments are described which show that threshold offsets can be reduced to about 2 mV RMS for a fill-and-spill input indicating that MDACs of this type with 8-bit accuracy are feasible.