real time system

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
3

4. TYPES OF COMMON MODE
The common-mode voltage can be ac, dc, or a combination of ac and dc.
4

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.
5

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
6

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
7

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).
10

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.
11

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]
12

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)
13

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
14

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
15

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.
16

17. RESULTS SHOWING A
SIMULATION OF CMRR
17