A custom data conversion subsystem for personal computers

A `full custom' data conversion integrated circuit is described. it is used in a personal computer as an interface to the cassette recorder and the audio sound channel, and interfaces the joysticks to the computer. The chip contains a six-bit digital-to-analog converter (DAC), a three-channel multiplexer, a four-channel operational amplifier, and a comparator. The IC's unique features include the following: it uses a merged 96-collector p-n-p to produce the weighted current sources for the DAC; the bipolar op amp has a full rail-to-rail input capability; and the hysteresis in the comparator is accomplished using a cross-coupled current mirror technique. The chip operates with a single 5 V supply at 50 mW.