Digital Electronics: Principles, Devices and Applications

480 Digital ElectronicsSome D/A converters even provide four-quadrant operation by allowing the use of both positive andnegative reference. Multiplying D/A converters are particularly useful when we are looking for digitallyprogrammable attenuation of an analogue input signal.12.3.2 Bipolar-Output D/A ConvertersIn bipolar-output D/A converters the analogue output signal range includes both positive and negativevalues. The transfer characteristics of an ideal two-quadrant bipolar-output D/A converter are shownin Fig. 12.7.12.3.3 Companding D/A ConvertersCompanding-type D/A converters are so constructed that the more significant bits of the digital inputhave a larger than binary relationship to the less significant bits. This decreases the resolution ofthe more significant bits, which in turn increases the analogue signal range. The effect of this is tocompress more data into more significant bits.12.4 Modes of OperationD/A converters are usually operated in either of the following two modes of operation:1. Current steering mode.2. Voltage switching mode.12.4.1 Current Steering Mode of OperationIn the current steering mode of operation of a D/A converter, the analogue output is a current equal tothe product of a reference voltage and a fractional binary value D of the input digital word. D is equalto the sum of fractional binary values of different bits in the digital word. Also, fractional binary valuesof different bits in an n-bit digital word starting from the LSB are 2 0 /2 n ,2 1 /2 n ,2 2 /2 n , ,2 n−1 /2 n .+FSAnalogO/P-FS+FSDigitalI/P-FSFigure 12.7Bipolar-output D/A converter transfer characteristics.