Microcomputer interfacing Interfacing a lO-bit DAC

IN THIS COLUMN we shall discusshow to interface an Analog De-vices AD7522 monolithic CMOS lO-bit multiplying digital-to-analog converter (DAC), a 28-pin chip which is a recent example of a new generation of inexpensive DACs that can be interfaced directly to 8-bit mi-crocomputers. A digital-to-analog converter is an electronic device that converts digital signals into analog signals. A typical converter consists of an arrangement of weighted resistors, each controlled by a single bit of input data that de-velops varying output analog volt-ages or currents in accordance with the digital input code. ' A DAC is used to provide a <mall analog error signal from a microcomputer that is used in a feedback control circuit; to convert a sequence of bytes in mem-ory into analog-vs-time data and thus simulate the output from an analog instrument such as a gas chro-matograph or uv-visible spectro-Table 1 photometer; to provide analog data for the two channels of an X-Y recorder; or, in general, to operate any device that requires an analog voltage or current and is interfaced to a digital device, such as a micro-computer. For a general discussion of the principles of analogidigital con-version, we refer you to the excellent Analog Devices conversion hand-book ' or to the series of small pam-phlets distributed by National Semi-conductor Corporation. ' Important concepts associated with DACs in-clude resolution, accuracy, scale er-ror,,gain error, offset error, linearity, differential linearity, monotonicity, settling time, slew rate, overshoot and glitches, tem-perature coefficient, supply re-jection, conversion rate, and output drive capability. 2 Definitions for a few such terms are summarized i