Schematic Volt

The circuit of a 9-bit Digital to Analog Convener (DAC) is shown. This system combines the concepts of multiple·switch COS/MOS lCs, a low-cost ladder network of discrete metal-oxide film resistors, a CA313O op-amp connected as a follower, and an inexpensive monolithic regulator in a simple single power-supply arrangement.  An additional feature of the DAC is that it is readily interfaced with COS/MOS input logic, e.g. 10- volt logic levels are used in the circuit.

 The circuit uses an R/2R voltage- ladder network, with the output potential obtained directly by terminating the ladder arms at either the positive or the negative power-supply terminal. Each CD4007A contains three "inverters", each "inverter" functioning as a single-pole double- throw switch to terminate an arm of the R/2R network at either the positive or negative power-supply terminal. The resistor ladder is an assembly of one per cent tolerance metal-oxide film resistors. The five arms requiring the highest accuracy are assembled with series and parallel combinations of 806,000 ohm resistors from the same manufacturing lot. A single 15-volt supply provides at positive bus for the CA313O follower amplifier and feeds the CA3085 voltage regulator. A "scale-adjust" function is provided by the regulator output control, set to a nominal 10- volt level in this system. The line voltage regulation (approximately 0.2%) permits a 9-bit accuracy to be maintained with variations of several volts in the supply. The flexibility afforded by the COS/MOS building blocks simplifies the design of DAC systems tailored to particular needs.