This document summarizes a student project on common mode rejection. It includes: 1) An introduction defining common mode signals, common mode rejection, and common mode rejection ratio (CMRR). 2) Sections covering the overview of operational amplifiers and how they relate to CMRR, different sources and types of common mode signals, applications of differential amplifiers, and methods for measuring CMRR. 3) The importance of high CMRR is discussed for reducing noise in applications like audio/video and transmission lines. Advantages are noise cancellation while disadvantages include complexity and resistor ratio matching issues.

1. CHINHOYI UNIVERSITY OF TECHNOLOGY
. DEPARTMENT OF INDUSTRIAL ELECTRONICS
LEVEL 5.1 CUIEE 501 MINI PROJECT
TOPIC: COMMON MODE REJECTION
Presented by Supervised by
Willard Marambakuwanda Eng A Janga
Anyway T muchangwara
Valentine R Manhando

2. INTRODUCTION
The common mode refers to signals or noise that flow in the same direction
in a pair of lines. the differential (normal) mode refers to signals or noise
that flow in opposite directions in a pair of lines
Common-mode signal is the voltage common to both input terminals of an
electrical device
In telecommunication, the common-mode signal on a transmission line is
also known as longitudinal voltage
Common-mode rejection is the ability of the differential amplifier to
eliminate the common-mode voltage from the output
The CMRR instrument determines the attenuation applied to the noise 2

3. AN OVERVIEW OF AN OP-AMP
CMMR = differential mode gain / common-mode gain
The ideal op amp will have the infinite CMRR and with the finite
differential gain and zero common mode gain
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4. TYPES OF COMMON MODE
The common-mode voltage can be ac, dc, or a combination of ac and dc.
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5. TYPES OF COMMON MODE
THREE SOURCES OF COMMON-MODE VOLTAGE ARE REPRESENTED IN
FIGURE 1 ABOVE AS EGD, ELC, AND EOS:
EOS IS TYPICALLY A DC OFFSET INTRODUCED BY A DIFFERENTIAL-
MODE DRIVER OPERATING FROM A SINGLE SUPPLY, AS
REPRESENTED IN FIGURE 3.
EGD IS A NOISE SIGNAL REPRESENTING THE DIFFERENCE IN
GROUND POTENTIALS AT THE TRANSMITTING AND RECEIVING
LOCATIONS.
ELC IS A LONGITUDINALLY COUPLED NOISE SIGNAL OCCURRING
EQUALLY ON BOTH TRANSMISSION LINES DUE TO CAPACITIVE,
ELECTROMAGNETIC, OR INDUCTIVE COUPLING FROM EXTRANEOUS
SOURCES.
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6. AN OVERVIEW OF THE DIFFERENTIAL
AMPLIFIER
THE DIFFERENTIALAMPLIFIER AMPLIFIES THE VOLTAGE
DIFFERENCE PRESENT ON ITS INVERTING AND NON-
INVERTING INPUTS
IS A VOLTAGE SUBTRACTOR CIRCUIT WHICH PRODUCES AN
OUTPUT VOLTAGE PROPORTIONAL TO THE VOLTAGE
DIFFERENCE OF TWO INPUT SIGNALS
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7. TYPES OF DIFFERENTIAL AMPLIFIERS
1. WHEATSTONE BRIDGE DIFFERENTIAL AMPLIFIER- a differential
voltage comparator by “comparing” one input voltage to the other
2. LIGHT ACTIVATED DIFFERENTIAL AMPLIFIER - a light-activated
switch which turns the output relay either “on” or “off” as the light level
detected by the ldr resistor exceeds or falls below some pre-set value
3. INSTRUMENTATION AMPLIFIER- . instrumentation amplifiers are
mainly used to amplify very small differential signals from strain gauges,
thermocouples or current sensing devices in motor control systems
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8. APPLICATIONS OF DIFFERENTIAL
AMPLIFIER
The main application of the differential amplifier is to amplify the balanced
differential signal.
Differential amplifier circuits are used in the audio amplifier for accurate
and noiseless volume control.
In analog and digital data transmission system differential amplifiers are
used for noise cancellation.
Differential amplifiers are used for audio and video processing.
Used as an automatic gain control circuit. 8

9. ADVANTAGES AND DISADVANTAGES
ADVANTAGES
DIFFERENTIAL AMPLIFIER HAS NOISE CANCELLATION
PROPERTY.
DIFFERENTIAL AMPLIFIER CAN REDUCE EXTERNAL
INTERFERENCE.
THESE AMPLIFIERS HELP TO INCREASE CMRR (COMMON MODE
REJECTION RATIO) WHICH FURTHER HELPS TO AVOID
UNWANTED SIGNAL
DISADVANTAGES
COMPLEXITY 9

10. WHY COMMON MODE REJECTION
1.NOISE IS GENERATED IN THE WIRES AND CABLES, DUE TO
ELECTROMAGNETIC INDUCTION, ETC., AND IT CAUSES A
DIFFERENCE IN POTENTIAL (I.E., NOISE) BETWEEN THE SIGNAL
SOURCE GROUND AND THE CIRCUIT GROUND.
2.CURRENT FLOWING INTO THE GROUND OF A CIRCUIT FROM
ANOTHER CIRCUIT CAUSES A GROUND POTENTIAL RISE (NOISE).
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11. COMMON MODE REJECTION RATIO
AD = 1/2[R3/(R1+R3)] [(R4 + R2)/R2 + R4/R2]
CMRR = 20 LOG (AD/ACM)
TO eliminate the effect of the common mode component, we can either
1. MAKE THE INPUT COMMON MODE COMPONENT EQUAL TO ZERO,
I.E. MAKE V2 = -V1
2.CHOOSE THE RESISTOR VALUES OF R1 TO R4 IN SUCH A WAY THAT
ACM IS ZERO.
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12. OFFSET ERROR OF A CMRR OF THE OP-
AMP
CMRR can build parallel out offset voltage in op amps configured in
the non-inverting amplifier which is shown in the below figure
ERROR (RTI) = VCM / CMRR = VIN / CMRR
VOUT = [1 + R2/R1] [ VIN + VIN/ CMRR]
ERROR (RTO) = [1+R2/R1] [VIN/CMRR]
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13. MEASURING COMMON MODE REJECTION
RATIO
There are different ways to measure the common mode rejection ratio
1. Four-precision resistor
2. CMRR without using precision resistors
Four-precision resistor
. A signal is applied to the both inputs and changes in the output are
measured
An amplifier with infinite CMRR also no changes in the output
The equation of the circuit is given by
ΔVOUT = ΔVIN / CMRR (1 + R2/R1)
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14. MEASURING COMMON MODE
REJECTION RATIO
CMRR WITHOUT USING PRECISION RESISTORS
. BY SWITCHING THE POWER SUPPLY VOLTAGE, THE
COMMON MODE REJECTION RATIO IS CHANGED
THE CIRCUIT CAN BE IMPLEMENTED EASILY IN PRACTICAL
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15. APPLICATION OF COMMON MODE
REJECTION
The CMRR is a very important specification, as it indicates how much of
the common-mode signal will appear in measurement
It can be applied in the following
1.Biomedical instrumentation
2.Audio amplifiers
3.Instrumentation amplifiers
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16. ADVANTAGES AND DISADVANTAGES OF
CMRR
ADVANTAGES
IT INDICATES THE PRESENCE OF COMMON-MODE SIGNALS
AT THE OP-AMP INPUTS, WHICH EVENTUALLY
DETERMINES THE OP-AMP’S ABILITY TO MINIMIZE THE
NOISE IN AUDIO, VIDEO AND COMMUNICATION DESIGNS.
REDUCING NOISE ON TRANSMISSION LINES
DISADVANTAGES
THE RATIO MATCH OF THE RESISTORS IS AS IMPORTANT AS
THE CMRR OF THE OPAMP.
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17. RESULTS SHOWING A
SIMULATION OF CMRR
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