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Digital-to-Analog

Analog-to-DigitalInterface Part IVMicroprocessor

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Data Collection and Control

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Digital-to-Analog Conversion

[DAC]

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Digital to Analog(D/A) Converters Digital to analog converters can be broadly classified in three categories:

current output, voltage output, and multiplying type.

The current output DAC provides current as the output signal.

The voltage output DAC internally converts signal into the voltage signal.

The voltage output DAC is slower than the current output DAC because of

the delay in converting the current signal into the voltage signal.

The multiplying DAC is similar to the other two types except its output

represents the product of the input signal and the reference source, and

product is linear over a broad range.

D/A converters are available as integrated circuits. Some are designed to

be compatible with the microprocessor. Typical applications include digitalvoltmeters, peak detectors , panel meters, programmable gain and

attenuation, and stepping motor drive.

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Figure shows a block

diagram of a 3 bit D/A

converter; it has threedigital input lines (D2, D1,

D0) and one output line for

the analog signal.

The three input lines can

assume eight input

combinations from 000 to

111.

If the input ranges from 0 to

1 V, it can be divided into

eight equal parts; each

successive input is 1/8V

higher than the previous

combination

D/A

Conveter

Digital

Input

LSB

MSB

D0

D1

D2

Analog Output

3 Bit D/A converter

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Digital-to-Analog Conversion 2 Basic Approaches

Weighted Summing Amplifier R-2R Network Approach

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Weighted Sum DAC One way to achieve D/A conversion is to use a

summing amplifier.

This approach is not satisfactory for a large

number of bits because it requires too much

precision in the summing resistors.

This problem is overcome in the R-2R network

DAC.

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Weighted Sum DAC

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R-2R Ladder DAC

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R-2R Ladder DAC

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R-2R Ladder DAC The summing amplifier with the R-2R ladder of

resistances shown produces the output where the

D's take the value 0 or 1.

The digital inputs could be TTL voltages which close

the switches on a logical 1 and leave it grounded

for a logical 0.

This is illustrated for 4 bits, but can be extended toany number with just the resistance values R and2R.

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Q1. Write a program to generate a triangular

wave?

Q2. Write a program to generate a square wave

of particular frequency?

Q3. Write a program to generate a saw tooth

wave?

Q4. Write a program to generate a ramp wave?

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Analog to Digital Conversion[ADC]

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ADC Basic Principle The basic principle of operation is to use the

comparator principle to determine whether or

not to turn on a particular bit of the binary

number output.

It is typical for an ADC to use a digital-to-

analog converter (DAC) to determine one of

the inputs to the comparator.

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ADC Various Approaches 3 Basic Types

Digital-Ramp ADC

Successive Approximation ADC

Flash ADC

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Digital-Ramp ADC Conversion from analog to digital form

inherently involves comparator action where

the value of the analog voltage at some pointin time is compared with some standard.

A common way to do that is to apply the

analog voltage to one terminal of acomparator and trigger a binary counterwhich drives a DAC.

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Digital-Ramp ADC

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Digital-Ramp ADC The output of the DAC is applied to the other

terminal of the comparator.

Since the output of the DAC is increasingwith the counter, it will trigger thecomparator at some point when its voltageexceeds the analog input.

The transition of the comparator stops thebinary counter, which at that point holds thedigital value corresponding to the analog

voltage.

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Successive approximation ADC

Illustration of 4-bit SAC with 1 volt step size

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Successive approximation ADC Much faster than the

digital ramp ADCbecause it uses digital

logic to converge onthe value closest to theinput voltage.

A comparator and a

DAC are used in theprocess.

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Flash ADC It is the fastest type of ADC available,

but requires a comparator for each

value of output.

(63 for 6-bit, 255 for 8-bit, etc.) Such ADCs are available in IC form up to

8-bit and 10-bit flash ADCs (1023

comparators) are planned.

The encoder logic executes a truthtable to convert the ladder of inputs tothe binary number output.

Illustrated is a 3-bit flash ADC with resolution 1 volt

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ADC080x Features Compatible with 8080 P

derivativesno interfacinglogic needed

-

access time-

135 ns.

Easy interface to allmicroprocessors, oroperates stand alone

Differential analog voltageinputs Logic inputs andoutputs meet both MOS andTTL voltage levelspecifications

Works with 2.5V (LM336)voltage reference On-chipclock generator 0V to 5Vanalog input voltage rangewith single 5V supply

No zero adjust required

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ADC080x, interfacing

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Interfacing Application

To control the temperature of heater

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Temperature control using 8085

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PROJECT DISCRIPTION

It sense the temperature and on/off theheater accordingly.

The sensing circuitry may consist of resistance

thermometer, thermocouples, thermisters,pyrometers place in the arm of wheatstonebridge.

A change in temperature causes a change inresistance giving a voltage that is proportionalto the change in temperature

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Thermocouples are widely used transducers to

measure the temperature.

The o/p of thermocouple is proportional to

the temperature of heater in microvolts.

This voltage is amplified by multistage

amplifier and then A/D converter.

8085 sends a SOC signal to A/D converter via

8255.

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After the conversion an EOC signal is given to

the microprocessor. Then the microprocessor

reads the o/p as digital quantity proportional

to the temperature to be measured.

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HARDWARE

tthermocouple

3stage

amplifier ADC8255

EOC

SOC

PORTA

PC7

PC3

DB0-DB7

MPU

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FLOWCHART

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PROGRAM

MVI A, 91H

OUT CWR ;INITIALIZE 8255

L2:CALL CONVERSION

CPI 80H ; COMPARE WITH SET POINT

JC L1

MVI A,0EH ; RESET PC7,HEATER OFF WITH BSR MODE

OUT CWR

JMP L2 L1: MVI A,0FH ; SET PC7, HEATER ON WITH BSR MODE

OUT CWR

JMP L2

CONVERSION MVI A 00H

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CONVERSION: MVI A,00H

OUT PB ; SEND ADDRESS TO SELECT I/P 0

MVI A,08H ;LATCH ADDRESS BY ALE=1

OUT PB

MVI C,0AH

L3: DCR C

JNZ L3;DELAY

MVI A,18H

OUT PB ; MAKE SOC HIGH

MVI A,08H ; MAKE SOC LOW

OUT PB

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MVI A,00H

OUT PB ; MAKE ALE LOW

L4: IN PC

ANI 01H

JZ L4 ; WAIT FOR EOC

IN PA

RET

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