Principles & Applications Digital Electronics Principles & Applications Fifth Edition Roger L. Tokheim Chapter 13 Connecting with Analog Devices ©1999 Glencoe/McGraw-Hill

CHAPTER 13 PREVIEW Hybrid System (analog-to-digital-to-analog) A/D and D/A Converters (block diagrams) Simple 4-bit D/A Converter Simple D/A Converter Operation Counter-ramp-type A/D Converter Ramp-type A/D Converter Successive-approximation A/D Converter Flowcharting- Successive Approximation A/D Converter Specifications

HYBRID SYSTEM (BOTH ANALOG AND DIGITAL) input Encoder A/D Converter Digital Processing Unit Decoder D/A Converter Analog output Converts from analog input to digital output Converts from digital input to analog output

A/D AND D/A CONVERTERS Digital-to-analog converter Processing Unit Decoder D/A Converter Encoder A/D Analog output input Digital-to-analog converter A/D Analog-to-digital converter 0 1 1 0 0 1 1 0 Resistor Network Summing Amplifier Analog Voltage INPUT

SIMPLE D/A CONVERTER 2N = 16 Vreference 150 k 75 k 37.5 k 18.7 k Binary inputs N = 4 3 V Feedback resistor Vreference 10 k 2N = 16 18.7 k 37.5 k 75 k 150 k Summing amplifier (inverting) Input resistors This circuit can produce 16 different analog output voltages.

D/A CONVERTER OPERATION Binary input = 0000 0001 3 V 10 k = 0.2 V = 0 V 18.7 k 37.5 k 150 k 75 k Vout = x 3 V = 0.2 V 10 k 150 k The smallest step size for this circuit is 0.2 V.

4-BIT D/A CONVERTER As the binary input steps from 0000 to 1111 Vout(max) = (2N - 1) x 0.2 V = 3 V 3 V Vout steps from 0 V to 3 V 0 V 3 V 10 k The total span for this circuit is from 0 to 3 V.

COUNTER-RAMP TYPE A/D CONVERTER Clock Binary output Vin Comparator HIGH LOW D/A Converter ramp Vin When the ramp exceeds Vin the comparator output goes LOW and the clock is disabled. ramp

RAMP-TYPE A/D CONVERTER 0 1 0 1 Counter Binary output Clock Comparator Vin LOW Ramp Generator Vin = + 5 V Comparator output Clock pulses to counter

SUCCESSIVE-APPROXIMATION TYPE A/D CONVERTER Binary output 0 1 1 1 Clock Successive Approximation Logic 7V input Vin Voltage Comparator D/A Converter

FLOWCHARTING SUCCESSIVE APPROXIMATION Assume an input of 7V to a 4-bit binary output A/D converter using successive approximation Start 7 V input Set MSB to 1 Is 0100 high or low? HIGH LOW Set 2s place to 1 Is 1000 high or low? LOW HIGH Is 0110 high or low? HIGH LOW Clear 8s. Set 4s to 1. Set 1s place to 1 STOP Final result= 0111

A/D CONVERTER SPECIFICATIONS Type of output Binary (microprocessor-type) Decimal (digital meter type) Resolution Binary output- number of bits (such as 4, 6, 8, 12, 14 or 16) Decimal output- number of digits on display (such as 3 1/2 or 4 1/2) Accuracy Binary output- (ranges from +/- 1/2 LSB to about +/- 2 LSB) Decimal output- (ranges from about 0.01 to 0.05 percent) Conversion time is the time it takes for the IC to convert the analog input voltage to digital output. Binary output A/D converters commonly have faster conversion times than decimal output types.