Digital Electronics: Principles, Devices and Applications

500 Digital Electronics12.10.6 Command Register, Status RegisterThe command register is an internal register of the ADC that can be programmed by the user toselect various modes of operation such as unipolar or bipolar mode selection, range selection, dataoutput format selection, etc. The status register indicates the current status of the analogue-to-digitalconversion with a ‘busy’ or ‘conversion complete’ signal.12.10.7 Control LinesDigital input/output pins that activate/monitor and control ADC operation are called control lines.Some examples are chip select, write, start convert, conversion complete, etc.Example 12.5Determine the resolution of a 12-bit A/D converter having a full-scale analogue input voltage of 5 V.Solution• A 12-bit A/D converter resolves the analogue input voltage into (2 12 − 1) levels.• The resolution = 5/2 12 − 1 = 5000/4096 − 1 = 5000/4095 = 122mV .Example 12.6The data sheet of a certain eight-bit A/D converter lists the following specifications: resolution eightbits; full-scale error 0.02 % of full scale; full-scale analogue input +5 V. Determine (a) the quantizationerror (in volts) and (b) the total possible error (in volts).Solution(a) The eight-bit A/D converter has 2 8 − 1 = 255 steps. Therefore, the quantization error = 5/255 =5000/255 = 19.607 mV.(b) The full-scale error = 002%offullscale = 002 ×5000/100 = 1 mV. Therefore, the total possibleerror = 19607 + 1 = 20607mV .12.11 Types of A/D ConverterAnalogue-to-digital converters are often classified according to the conversion process or the conversiontechnique used to digitize the signal. Based on various conversion methodologies, common typesof A/D converter include flash or simultaneous or direct-conversion A/D converters, half-flash A/Dconverters, counter-type A/D converters, tracking A/D converters, successive approximation type A/Dconverters, single-slope, dual-slope and multislope A/D converters and sigma-delta A/D converters.Each of the above-mentioned types of A/D converter is described in the following paragraphs.12.11.1 Simultaneous or Flash A/D ConvertersThe simultaneous method of A/D conversion is based on using a number of comparators. The numberof comparators needed for n-bit A/D conversion is 2 n −1. One such system capable of converting an