A 50 MHz 10-bit CMOS digital-to-analog converter with 75 Omega buffer

A trimless 10-b, 50-MHz digital-to-analog converter based on resistor strings is discussed. The voltage dependence and the mutual matching of large-area polysilicon resistors allow the design of a converter with high integral and differential linearity. However, in a single 1024-tap-resistor ladder, output settling requires such low tap resistors that accurate resistor matching, and consequently linearity, becomes a problem. The solution to this problem is the combination of a dual ladder with a matrix organization for the fine ladder, a full decoding scheme, an on-chip 75- Omega output buffer, and an additional ladder for the reduction of distortion at high signal frequencies. The coarse ladder consists of two ladders, each with 16 large-area 250- Omega resistors connected antiparallel to eliminate the first-order resistivity gradient. A 1024-resistor fine ladder is arranged in a 32*32 matrix, where every sixty-fourth tap is connected to the coarse ladder taps.<<ETX>>