Digital Electronics: Principles, Devices and Applications

478 Digital ElectronicsActualIdealAnalog outputOffset ErrorDigital Input(b)Figure 12.4(continued).time of a few nanoseconds. The settling time specification for D/A converter type number AD 9768from Analog Devices USA, for instance, is 5 ns.12.2.4 Dynamic RangeThis is the ratio of the largest output to the smallest output, excluding zero, expressed in dB. Forlinear D/A converters it is 20 × log2 n , which is approximately equal to 6n For companding-type D/Aconverters, discussed in Section 12.3, it is typically 66 or 72 dB.12.2.5 Nonlinearity and Differential NonlinearityNonlinearity (NL) is the maximum deviation of analogue output voltage from a straight line drawnbetween the end points, expressed as a percentage of the full-scale range or in terms of LSBs.Differential nonlinearity (DNL) is the worst-case deviation of any adjacent analogue outputs from theideal one-LSB step size.12.2.6 MonotonocityIn an ideal D/A converter, the analogue output should increase by an identical step size for everyone-LSB increment in the digital input word. When the input of such a converter is fed from the outputof a counter, the converter output will be a perfect staircase waveform, as shown in Fig. 12.5. In suchcases, the converter is said to be exhibiting perfect monotonocity. A D/A converter is considered asmonotonic if its analogue output either increases or remains the same but does not decrease as thedigital input code advances in one-LSB steps. If the DNL error of the converter is less than or equalto twice its worst-case nonlinearity error, it guarantees monotonocity.