ADCAnalog and Digital Converter Circuits Part 1 Fourth

ADC-Analog and Digital Converter Circuits Part 1 Fourth year Digital control Prepared by Dr. Eng. Abdel Gayed Fathy 1

Analog Signals every where Examples of A/D • Microphones - take your voice varying pressure waves in the air and convert them into varying electrical signals • Seat Belt • Thermocouple – temperature measuring device converts thermal energy to electric energy • Voltmeters • Digital Multimeters • ADSL

Need to Sample an analog signal Most signals are analog Then convert to digital by A/D converter Are sensor outputs Analog ? Eg. Seatbelt ? EEG, oil temp Figure 36 The process of periodically sampling an analog signal. (a) Sample-and-hold (S/H) circuit. The switch closes for a small part (t seconds) of every clock period (T). (b) Input signal waveform. (c) Sampling signal (control signal for the switch). (d) Output signal (to be fed to A/D converter). 3

A/D converter and D/A Converters Analog to Digital What parts of your i. Phone operation are Analog ? / Digital to Analog Your internet access: Analog ? Digital ? Figure 37 The A/D and D/A converters as circuit blocks. 4

A/D converter Converts analog signals into binary words

D/A Conversion Normal Output from digital domain is staircase Filtered to produce smooth Analog output Figure 38 The analog samples at the output of a D/A converter are usually fed to a sample-and-hold circuit to obtain the staircase waveform shown. This waveform can then be filtered to obtain the smooth waveform, shown in color. The time delay usually introduced by the filter is not shown. 6

Conversion accuracy: eg 2 -bits Blue line ? • Analog is continuous • But digital is discrete • Limited by number of bits Red ? 7

3 -bit conversion example Quantization levels Eg 5 V divided into 8 levels – each 0. 625 Each binary representation is a “range” 8

D/A conversion implementation Weighted Summing Circuit in = ? i =? vo = - [(Rf / R 1) * v 1 + (Rf / R 2) * v 2 +…. + (Rf / Rn) * vn] Figure 10 A weighted summer. 9

N-bit D/A Converter Implementation Binary weighted resistive ladder digital values control switches S 1 - Sn Figure 39 An N-bit D/A converter using a binary-weighted resistive ladder network. 10