Digital Measurements

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Submitted By:

Mr. Santosh Kumar Gupta

Assistant Professor

EE Department, SIT Sitamarhi

Contents

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Advantages of Digital meters Digital Components Digital Voltmeter(DC) Digital Voltmeter(AC) Measurement of Current Measurement of Resistance A complete DMM Measurement of hfe A digital voltmeter and ohmmeter using

Arduino.

Advantages of Digital meters

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Very high accuracy (5 to 0.05%)

Has very input impedance (2MΩ)

which ensures less loading effect on

the input.

The numeric display of digital meters

provide zero parallax error.

Digital Components

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Crystal Oscillator

Counter

A/D Converter

Voltage-Frequency Converter

Phase Locked Loop

Crystal Oscillator

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A piezoelectric crystal is used.

Generally, a1MHz crystal is used

which produce a clock pulse of 1MHz.

Used to provide the clock pulses.

Counters

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A/D Converters

Converts Analog data to Digital data

using a dual slope converter.

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Voltage-Frequency Converter

Produces a frequency signal which is

proportional to the input voltage.

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Digital Voltmeter(DC)

The voltage is first attenuated and

then converted into a digital word

which is displayed.

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Role of ADC

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The lines supplied by the ADC to the

display consists of

◦ The digital word

◦ A latch

◦ Start bit

◦ Polarity information

◦ Floating point information

Features

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Polarity detection

Auto ranging

Auto zeroing

Polarity detection:

The information is given by the ADC.

When the voltage is +ve, the characteristic

curve of the converter is in first quadrant.

The characteristic curve will be in the fourth

quadrant, if the voltage is –ve.

Auto Ranging

Two comparators are used.

The typical ranges are,

◦ 0 to 100 mV

◦ 100 mV to 1 V

◦ 1 V to 100 V

◦ 100 V to 1000V

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Auto Zeroing

Zero is displayed during the start of

conversion.

This is achieved by discharging the

capacitance to the ground properly.

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DVM (AC)

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The AC is processed first i.e., phase

compensated attenuation with

rectification and filtering is done.

The dc voltage obtained is converted

into a digital word, which is displayed.

For high frequencies, a precision

diode with small reverse recovery time

is required.

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DVM(Low voltage AC)

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For measurement of voltages in the

range between 1 nV and 10 nV, the

voltage is converted into a frequency

signal by a voltage-frequency

converter or phase locked loop.

The frequency is measured by proper

networks.

This reduces the effect of noise

through ADC.

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Phase locked loop

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Measurement of current by DMM

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Typical ranges

◦ 0 to 10 mA

◦ 10 mA to 100 mA

◦ 100 mA to 1A

◦ 1 A to 10A

Auto ranging is available.

Auto zeroing is also available.

Two ways of measurement

◦ Measuring the voltage drop across a

resistance.

◦ Current frequency converter.

In the first method, current is passed

through a constant resistance. The

voltage drop across it is measured

using a DVM.

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In the second method, a current-

frequency converter is used.

The output of comparator is a

frequency which is proportional to the

current.

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Measurement of Resistance using DMM

A constant current is passed through

the unknown resistance and the drop

across it is measured by a DVM.

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Complete circuit of a DMMThe switches are coupled to each other.

The display is generally 3 digits (-1999 to 1999)

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Measuring hfe of a transistor

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Hfe measurement

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A small project: Digital Voltmeter

using ATMEGA328P

ATMEGA328P

A0 GND

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Direct voltage measurement:

RANGE EXTENDED VOLTMETER

ATMEGA328P

A0 GND

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CODE:

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float voltage, previousvoltage=0;

void setup()

{

serial.begin(9600);

}

void loop()

{

voltage=analogRead(A0)*0.00488758553;

if(voltage!=previousvoltage)

{

Serial.println("the voltage is");

Serial.println(voltage);

previousvoltage=voltage;

}

delay(500);

}

Ohmmeter using arduino

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Arduino code

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int analogPin= 0;

int a;

int Vin= 5;

float Vout= 0;

float R1= 1200;

float R2= 0;

float buffer= 0;

void setup()

{

Serial.begin(9600);

}

void loop()

{

a= analogRead(analogPin);

if(a)

{

buffer= a * Vin;

Serial.println(a);

delay(1000);

}

}

Challenges

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If you use a small value of known

resistance to measure a large value of

resistance the value gives up an error,

So use known resistance which is

some what closer to the measuring

resistance to give an output with

greater accuracy.

References

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Sawhney A K, “A Course in Electrical and

Electronic Measurement and Insrtumentation”,

Dhanapat Rai & Sons, New Delhi, 2011.

Thank You!!

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