Digital Electronics: Principles, Devices and Applications

Data Conversion Circuits – D/A and A/D Converters 477have a percentage resolution of (1/255) × 100 = 0.39 % or simply an eight-bit resolution. A 12-bit D/Aconverter would have a percentage resolution of (1/4095) × 100 = 0.0244 %. In general, for an n-bitD/A converter, the percentage resolution is given by (1/2 n − 1) × 100. The resolution in millivolts forthe two cases for a full-scale output of 5Visapproximately 20 mV (for an eight-bit converter) and1.2 mV (for a 12-bit converter).12.2.2 AccuracyThe accuracy of a D/A converter is the difference between the actual analogue output and the idealexpected output when a given digital input is applied. Sources of error include the gain error (orfull-scale error), the offset error (or zero-scale error), nonlinearity errors and a drift of all these factors.The gain error [Fig. 12.4(a)] is the difference between the actual and ideal output voltage, expressedas a percentage of full-scale output. It is also expressed in terms of LSB. As an example, an accuracyof ±0.1 % implies that the analogue output voltage may be off by as much as ±5 mV for a full-scaleoutput of 5 V throughout the analogue output voltage range. The offset error is the error at analoguezero [Fig. 12.4(b)].12.2.3 Conversion Speed or Settling TimeThe conversion speed of a D/A converter is expressed in terms of its settling time. The settling timeis the time period that has elapsed for the analogue output to reach its final value within a specifiederror band after a digital input code change has been effected. General-purpose D/A converters havea settling time of several microseconds, while some of the high-speed D/A converters have a settlingIdealActualAnalog outputGainErrorDigital Input(a)Figure 12.4(a) Gain error and (b) offset error.