DAC Specifications

The fundamental function of a D/A converter (DAC) is to transform a digital word M _in into an analog (or continuous) output signal N _out (strictly speaking, the output is not truely continuous but has a finite number of distinct values), and the input-output relationship is given by

(5.1)

K _ref is an analog reference whose dimension may be electric voltage, current, or charge. The digital input M _in may be represented by either a binary (also called binary-weighted) code or a thermometer code, depending on the application's requirements; however, note that these two coding formats are interchangeable. The magnitude of M _in can be expressed as

(5.2)

Here, A _i's are the binary-code bits while B _j's are the thermometer-code bits. In general, to represent 2 ^M different digital input values, a binary code only needs M bits, whereby a thermometer code needs up to (2 ^M - 1) bits. Table 5.1 compares a 3-bit binary code and the equivalent 7-bit thermometer code.