Chapter 1 Basic Principles of Digital Systems outlin e 1

11.67. For each circuit, state the logic level at the output

of gate 1.

FIGURE 11.65

Problem 11.3

Waveforms

FIGURE 11.66

Problems 11.4 and 11.5

Logic Circuit

I1

3

I3

I4

I6

2

4

5

I1

3

I3

I4

I6

2

4

5

NAND: 74LS00

NOR : 74LS02

XOR : 74LS86A

Problem 11.10

Current Calculations

Problems 561

gates of a 74LS08 AND device. Calculate the power

dissipation of the circuit for the following input logic

levels:

Calculate the maximum total power dissipation of the

circuit when its input state is ABCDE _ 01100. Include

all unused gates. (Connect unused gate inputs so that they

will dissipate the least amount of power.)

11.15 a. Calculate the no-load power dissipation of a single

gate at 1 MHz for a 74HC00A quad 2-input

NAND gate (VCC _ 5 V). (Neglect quiescent

current.)

the gate in part a of this question is operated with a

new value of VCC _ 3.3 V. (f _ 1 MHz)

Section 11.5 Noise Margin

11.16 Calculate the maximum noise margins, in both HIGH and

LOW states, of:

not a 74HC? Show calculations.

74HC gate. Explain briefly how it works.

NAND gate drive? Use data sheet parameters to

support your answer. Assume VCC _ 4.5 V. Show all

calculations.

inverter shown in Figure 11.20. What is the purpose of

the diode?

11.22 Briefly explain the operation of a multiple-emitter input

transistor used in a TTL NAND gate. Describe how the

transistor responds to various combinations of HIGH and

LOW inputs.

NAND gates and an output pull-up resistor. The

gate inputs are as follows:

Gate 1: Inputs A, B

Gate 2: Inputs C, D

Gate 3: Inputs E, F

Write the Boolean function of the circuit output.

11.24 Calculate the minimum value of the pull-up resistor if the

circuit drawn in Problem 11.23 is to drive a logic gate

having input current IIL _ 0.8 mA and the NAND gates

can sink 12 mA in the LOW output state. (Assume that

Problems 11.11 to 11.13

Logic Circuit

11.12 The gate outputs in Figure 11.68 are switching at an average

frequency of 100 kHz, with an average duty cycle of

60%. Calculate the power dissipation if the gates are all

74S08 AND gates.

CMOS gates.

state is ABCDE _ 010101. (VCC _ 4.5 V, TA _

25°C)

b. Calculate the circuit power dissipation if the outputs

are switching at a frequency of 10 kHz, 50% duty

cycle.

c. Repeat part b for a frequency of 2 MHz.

11.14 The circuit in Figure 11.69 consists of two 74LS00

NAND gates (gates 4 and 5) and three 74LS02 NOR

gates (gates 1, 2, and 3). When this circuit is actually

built, there will be two unused NAND gates and one unused

NOR gate in the device packages.

FIGURE 11.69

Problem 11.14

Logic Circuit

562 C H A P T E R 1 1 • Logic Gate Circuitry

11.25 Draw a circuit consisting only of open-collector gates

whose Boolean expression is the product-of-sums

expression

(A _ B)(C _ D)(E _ F)(G _ H).

shorted to ground? Why or why not?

shorted to VCC? Why or why not?

(ideally)?

output? How can the problem be controlled?

connected together.

For each circuit, calculate the current flowing when

the LED is ON. Calculate the ratio between the LED

ON current and IOL or IOH of the inverter, whichever

is appropriate for each circuit. State which is the

best connection for LED driving and explain

why.

Section 11.8 Internal Circuitry of CMOS Gates

11.33 State several precautions that should be taken to prevent

electrostatic damage to MOSFET circuits.

enhancement-mode MOSFET.

MOSFET in the cutoff and ohmic regions.

in the cutoff and ohmic regions.

Derive the truth table of the gate by analyzing the

operation of all the transistors under all possible input

conditions.

analog signals to an automotive speedometer/tachometer.

Each sensor produces an analog voltage proportional

to its measured quantity. Briefly explain how these

analog signals are switched to the display output

circuitry.

the performance of a transistor in a TTL circuit.

switching frequency of its output? Explain.

the following gates:

a. 74LS00

b. 74S00

d. 74AS00

e. 74HC00A (quiescent and 10 MHz)

f. 74HCT00A (quiescent and 10 MHz)

g. 74F00

330

_

_

Vcc

Vf _ 2 V

_

74LS04

Problem 11.31

LED drivers

LAMP

_ 24 V

Problem 11.32

Lamp Driver

11.32 Calculate the current flowing when the lamp in Figure

11.71 is illuminated. Choose one of the following devices

as a suitable driver: 74LS04, 74LS05 74LS06, 74LS16.

Explain your choice. (Data sheets for these devices are

found in Appendix C.)

Answers to Section Review Problems 563

the current is leaving the gate. See Figure 11.2.)

switching. Since the majority of current is drawn when the

outputs switch, the more often the outputs switch, the more

current is drawn from the supply. This is the same as saying

that power dissipation increases with frequency.

Section 11.5

11.5 VNH _ 1.98 V, VNL _ 0.66 V

Section 11.6

11.6 2.5 V. The interface buffer and load should have the same

supply voltage so that the output voltage of the buffer and

input voltage of the load are compatible.

Section 11.7a

11.7 a. Provision of logic HIGH when output transistor is OFF

b. Limitation of IOL when output transistor is ON

a low-impedance path, causing IOH to exceed its rating.

Problem 11.37

Speedometer/Tachometer Switching Circuit

A N S W E R S T O S E C T I O N R E V I E W P R O B L E M S

CMOS logic families: 74HCNN, 74HC4NNN,

74HCTNN, and 74HCUNN.

11.42 Assume that the power dissipation of a metal-gate or

high-speed CMOS gate increases in proportion to the

switching frequency of its output. Calculate the speedpower

product of the following gates at 2 MHz, 5 MHz,

and 10 MHz:

a. 4011B

c. 74HCU04

both output transistors OFF in the high-impedance state

and allows them to be in opposite states when the output is

enabled.

They require an extra transistor stage to cancel the inversion

introduced by NAND or NOR transistor logic.

Section 11.8a

11.12 The thin oxide layer in the gate region can be damaged by

overvoltage, such as that caused by electrostatic discharge.

If the oxide layer is damaged, it may no longer insulate

the gate terminal from the MOSFET substrate,

which causes the transistor to malfunction.

Section 11.8b

11.13 It allows complementary operation with an n-channel

MOSFET. Specifically, a gate voltage of 0 V turns OFF

an n-channel device having a grounded source. The same

voltage turns ON the p-channel device whose source is

tied to VCC. It does so by making the p-channel gatesource

voltage more negative than the required threshold.

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C H A P T E R 12

Interfacing Analog

and Digital Circuits

O U T L I N E

Signals

Conversion

Conversion

C H A P T E R O B J E C T I V E S

Upon successful completion of this chapter, you will be able to:

• Define the terms “analog” and “digital” and give examples of each.

• Explain the sampling of an analog signal and the effects of sampling frequency

and quantization on the quality of the converted digital signal.

• Draw the block diagram of a generic digital-to-analog converter (DAC) and

circuits of a weighted resistor DAC and an R-2R ladder DAC.

• Calculate analog output voltages of a DAC, given a reference voltage and a

digital input code.

• Configure an MC1408 integrated circuit DAC for unipolar and bipolar output,

and calculate output voltage from known component values, reference

voltage, and digital inputs.

• Describe important performance specifications of a digital-to-analog

converter.

• Draw the circuit for a flash analog-to-digital converter (ADC) and briefly

explain its operation.

• Define “quantization error” and describe its effect on the output of an ADC.

• Explain the basis of the successive approximation ADC, draw its block diagram,

and briefly describe its operation.

• Describe the operation of an integrator with constant input voltage.

• Draw the block diagram of a dual slope (integrating) ADC and briefly explain

its operation.

• Explain the necessity of a sample and hold circuit in an ADC and its

operation.

• State the Nyquist sampling theorem and do simple calculations of maximum

analog frequencies that can be accurately sampled by an ADC

system.

can be remedied.

• Interface an ADC0808 analog-to-digital converter to a CPLD-based state

machine.

digital converter and a CPLD-based state machine.

Electronic circuits and signals can be divided into two main categories: analog and digital.

Analog signals can vary continuously throughout a defined range. Digital signals

take on specific values only, each usually described by a binary number.

Many phenomena in the world around us are analog in nature. Sound, light, heat, position,

velocity, acceleration, time, weight, and volume are all analog quantities. Each of

these can be represented by a voltage or current in an electronic circuit. This voltage or current

is a copy, or analog, of the sound, velocity, or whatever.

We can also represent these physical properties digitally, that is, as a series of numbers,

each describing an aspect of the property, such as its magnitude at a particular time.

To translate between the physical world and a digital circuit, we must be able to convert

analog signals to digital and vice versa.

We will begin by examining some of the factors involved in the conversion between

analog and digital signals, including sampling rate, resolution, range, and quantization.

We will then examine circuits for converting digital signals to analog, since these have

a fairly standard form. Analog-to-digital conversion has no standard method. We will study

several of the most popular: simultaneous (flash) conversion, successive approximation,

and dual slope (integrating) conversion.

no instantaneous changes.

values.

by a proportional continuous voltage or current. An analog voltage or current

can have any value within a defined range.

Digital A way of representing a physical quantity by a series of binary numbers.

A digital representation can have only specific discrete values.

to a digital code. (Also called an A-to-D converter, A/D converter, or ADC.)

an analog voltage or current. (Also called a D-to-A converter, D/A converter, or

DAC.)

Electronic circuits are tools to measure and change our environment. Measurement instruments

such as “How hot is this water?”, “How fast is this car going?”, and “How many electrons

are flowing past this point per second?” These data can correspond to voltages and currents

in electronic instruments.

If the internal voltage of an instrument is directly proportional to the quantity being

measured, with no breaks in the proportional function, we say that it is an analog voltage.

Like the property being measured, the voltage can vary continuously throughout a defined

range.

movements mathematically as a sum of sine waves of various frequencies. The patterns of

magnetic domains on an audio tape are analogous to the sound waves that produce them

and electromagnetically represent the same mathematical functions. When the tape is

played, the playback head produces a voltage that is also proportional to the original sound

waves. This analog audio voltage can be any value between the maximum and minimum

voltages of the audio system amplifier.

K E Y T E R M S

If an instrument represents a measured quantity as a series of binary numbers, the representation

is digital. Since the binary numbers in a circuit necessarily have a fixed number

of bits, the instrument can represent the measured quantities only as having specific

A compact disc stores a record of sound waves as a series of binary numbers. Each

number represents the amplitude of the sound at a particular time. These numbers are decoded

and translated into analog sound waves upon playback. The values of the stored

numbers (the encoded sound information) are limited by the number of bits in each stored

digital “word.”

The main advantage of a digital representation is that it is not subject to the same distortions

as an analog signal. Nonideal properties of analog circuits, such as stray inductance

and capacitance, amplification limits, and unwanted phase shifts, all degrade an analog

signal. Storage techniques, such as magnetic tape, can also introduce distortion due to

the nonlinearity of the recording medium.

Digital signals, on the other hand, do not depend on the shape of a waveform to preserve

the encoded information. All that is required is to maintain the integrity of the logic

HIGHs and LOWs of the digital signal. Digital information can be easily moved around in

a circuit and stored in a latch or on some magnetic or optical medium. When the information

is required in analog form, the analog quantity is reproduced as a new copy every time

it is needed. Each copy is as good as any previous one. Distortions are not introduced between

copy generations, as is the case with analog copying techniques, unless the constituent

bits themselves are changed.

Digital circuits give us a good way of measuring and evaluating the physical world,

with many advantages over analog methods. However, most properties of the physical

world are analog. How do we bridge the gap?

We can make these translations with two classes of circuits. An analog-to-digital converter

accepts an analog voltage or current at its input and produces a corresponding digital

code. A digital-to-analog converter generates a unique analog voltage or current for

every combination of bits at its inputs.

Sampling an Analog Voltage

Sample An instantaneous measurement of an analog voltage, taken at regular

intervals.

signal.

number.

codes. Analog step size.

Before we examine actual D/A and A/D converter circuits, we need to look at some of

the theoretical issues behind the conversion process. We will look at the concept of

of the digital representation. We will also examine quantization, or the number of bits in

the digital representation of the analog sample, and its effect on the quality of a digital signal.

Figure 12.1 shows a circuit that converts an analog signal (a sine pulse) to a series of

4-bit digital codes, then back to an analog output. The analog input and output voltages are

shown on the two graphs.

There are two main reasons why the output is not a very good copy of the input. First,

the number of bits in the digital representation is too low. Second, the input signal is not

K E Y T E R M S

568 C H A P T E R 1 2 • Interfacing Analog and Digital Circuits

sampled frequently enough. To help us understand the effect of each of these factors, let us

examine the conversion process in more detail.

The analog input signal varies between 0 and 8 volts. This is evenly divided into 16

ranges, each corresponding to a 4-bit digital code (0000 to 1111). We say that the signal is

quantized into 4 bits. The resolution, or analog step size, for a 4-bit quantization is 8 V/16

steps _ 0.5 V/step. Table 12.1 shows the codes for each analog range.