Discrete Dac

A digital-to-analogue converter (DAC) that is easy to build from a handful of readily available parts. The 8-bit digital input for the circuit is applied to resistors R17-R24 incl., each of which drives an associated current source composed of two series- connected diodes, a transistor and a current defining resistor fed from the positive supply rail. A logic high level at the input causes the relevant cur- rent source to be switched on, a logic low level switches it off. The sum of currents from T, -Ts incl. is arranged to pass through preset Pi, which thus drops a voltage U. in accordance with the magnitude of the 8-bit word written to the circuit. The current supplied by each current source is about 700/R. [mA], where R. is the value of the associated resistor between the emitter and the +V rail. In order to ensure satisfactory linearity of the analogue output voltage, resistors R1-R8 incl. must be dimensioned to obtain a current ratio of 1:2 between any two adjacent sources. I...