LECTURER PROF.Dr. DEMIR BAYKA AUTOMOTIVE ENGINEERING LABORATORY I

NOISE CONTROL IN LOW LEVEL DATA SYSTEMS A COMMON PROBLEM IN CONTROL AND DATA SYSTEMS IS HANDLING LOW LEVEL ELECTRICAL INPUTS GROUNDED AT THE SOURCE

TRANSDUCERS SELECTOR SWITCHES AMPLIFIER A/D CONVERTER COMPUTER GOUNDED MULTIPLE TRANSDUCERS WITH LOW LEVEL OUTPUTS

TO DESIGN AN OPTIMUM SYSTEM PROPERLY ; 1. YOU NEED A SOUND TECHNICAL UNDERSTANDING OF THE PROBLEM 2. YOU NEED TO PAY ATTENTION TO DETAIL 3. YOU NEED TO HAVE COURAGE TO APPLY JUDGEMENT BASED ON EXPERIENCE

GENERAL GUIDELINES THE EXACT NATURE OF THE DISTURBING NOISE SOURCE IS NOT ALWAYS WELL DEFINED AND PRECAUTIONS MUST BE TAKEN AGAINST ALL POSSIBLE SOURCES

GENERAL GUIDELINES THE AMOUNT OF CONTROL THAT CAN BE EXERTED IS LIMITED BY THE STATE OF ART IN CIRCUIT AND NETWORK DESIGN SPURIOUS SIGNALS CANNOT BE COMPLETELY ELIMINATED, ONLY ATTENUATED

GENERAL GUIDELINES THE SAME END CAN OFTEN BE ACHIEVED BY A VARIETY OF MEANS THE DECISION AS TO THE BEST SOLUTION DEPENDS ON THE ECONOMICS OF THE SITUATION AND YOUR OWN JUDGEMENT AS A DESIGNER

NOISE SOURCES 2. COMMON MODE CURRENT DUE TO GROUND POTENTIAL 3. COMMON MODE CURRENT DUE TO ELECTRICAL FIELDS 1. NORMAL MODE CURRENT

NORMAL MODE CURRENT e cm isis ZLZL A B C 1 2 ARE THOSE CURRENTS CAUSED BY A SIGNAL GENERATOR IN SERIES WITH THE LOAD IN A CLOSED ELECTRICAL LOOP e nm eses

NORMAL MODE CURRENT e cm isis ZLZL A B C 1 2 SWITCH THE CONTACT MATERIALS IN THE SWITCH MAY EXHIBIT PELTIER EFFECT AND CAUSE i nm i nm

COMMON MODE CURRENT THESE ARE CAUSED BY EITHER A POTENTIAL DIFFERENCE BETWEEN DIFFERENT GROUNDS IN A NETWORK OR BY ELECTRICAL PICKUP IN A PAIR OF CONDUCTORS

EXAMPLE e cm isis ZLZL A B C 1 2 eSeS THE THERMOCOUPLE IS A SIGNAL GENERATOR THAT CAUSES THE NORMAL CURRENT i s TO FLOW IN THE LOOP SHOWN

EXAMPLE e cm isis ZLZL A B C 1 2 eSeS THERE IS A POTENTIAL DIFFERENCE BETWEEN THE EARTHINGS AT 1 & 2

EXAMPLE e cm isis ZLZL A B C 1 2 eSeS IF A CONNECTION IS MADE BETWEEN ( B ) AND ( C ) i cm i cm WILL BE ADDED TO OR SUBTRACTED FROM i s

NORMAL MODE SPURIOUS SIGNALS ARE INTERMIXED WITH THE DESIRED SIGNAL THESE SIGNALS OFTEN HAVE FREQUENCY CHARACTERISTICS WHICH DIFFERENTIATE THEM FROM THE DESIRED SIGNAL SOME SEPARATION CAN BE AFFECTED ON THIS BASIS

PRECAUTIONS AGAINST COMMON MODE CURRENTS 1) IMPEDENCE CONTROL BY ISOLATION 2) IMPEDENCE CONTROL BY BALANCING THESE PRECAUTIONS MAY BE APPLIED SIMULTANEOUSLY

COMMON MODE SOURCES e cm eses ZLZL ZsZs ZcZc ZcZc ZgZg i cm A B eoeo e s & Z s TRANSDUCER WITH INTERNAL IMPEDENCE

COMMON MODE SOURCES e cm eses ZLZL ZsZs ZcZc ZcZc ZgZg i cm A B eoeo Z c IMPEDENCE OF CABLE

COMMON MODE SOURCES e cm eses ZLZL ZsZs ZcZc ZcZc ZgZg i cm A B eoeo Z L INPUT IMPEDENCE OF RECEIVER

COMMON MODE SOURCES e cm eses ZLZL ZsZs ZcZc ZcZc ZgZg i cm A B eoeo Z g IMPEDENCE OF GROUND CONNECTION

COMMON MODE SOURCES e cm eses ZLZL ZsZs ZcZc ZcZc ZgZg i cm A B eoeo PROBLEM ARISES IF RECEIVER IS DISTANT FROM THE TRANSDUCER

COMMON MODE SOURCES e cm eses ZLZL ZsZs ZcZc ZcZc ZgZg i cm A B eoeo THEN THE GROUNDS AT THE TWO LOCATIONS WILL PROBABLY BE AT DIFFERENT POTENTIALS

COMMON MODE SOURCES e cm eses ZLZL ZsZs ZcZc ZcZc ZgZg i cm A B eoeo TYPICAL VALUES Z L = 10 M  Z s = 500  Z c = 10  Z g = 1 

COMMON MODE SOURCES e cm eses ZLZL ZsZs ZcZc ZcZc ZgZg i cm A B eoeo REDUCING Z c WITH RESPECT TO Z g MAY BE TOO EXPENSIVE FOR LONG TRANSMISSION LINES

COMMON MODE SOURCES e cm eses ZLZL ZsZs ZcZc ZcZc ZgZg i cm A B eoeo BALANCING OR / AND ISOLATION METHODS ARE CHEAPER

BALANCING e cm eses Z L1 ZsZs ZcZc ZcZc ZgZg i cm A B eoeo Z L2 i cm

i cm1 i cm2 e cm eses Z L1 ZsZs ZcZc ZcZc ZgZg i cm A B eoeo Z L2 Z L1 & Z L2 IS A DIFFERENTIAL GROUNDED AMPLIFIER

i cm1 i cm2 e cm eses Z L1 ZsZs ZcZc ZcZc ZgZg i cm A B eoeo Z L2 IF i cm1.Z L1 = i cm2.Z L2 THEN ERROR IN OUTPUT WILL BE ZERO

i cm1 i cm2 e cm eses Z L1 ZsZs ZcZc ZcZc ZgZg i cm A B eoeo Z L2 IN PRACTICE THE PERCENTAGE ERROR IN THE OUTPUT WOULD BE 0.1 % IF Z S + Z c = 1000 

i cm2 e cm eses ZLZL ZsZs ZcZc ZgZg i cm A B eoeo ISOLATION ZcZc Z LG

i cm2 e cm eses ZLZL ZsZs ZcZc ZgZg i cm A B eoeo ZcZc Z LG Z LG ISOLATES THE LOAD Z L FROM GROUND

i cm2 e cm eses ZLZL ZsZs ZcZc ZgZg i cm A B eoeo ZcZc Z LG Z L & Z LG IS A SINGLE ENDED FLOATING AMPLIFIER

i cm2 e cm eses ZLZL ZsZs ZcZc ZgZg i cm A B eoeo ZcZc Z LG Z LG ~ 10 M  TO 1000 M 

i cm2 e cm eses ZLZL ZsZs ZcZc ZgZg i cm A B eoeo ZcZc Z LG IN PRACTICE THERE WILL BE DIFFICULTIES

i cm2 e cm eses ZLZL ZsZs ZcZc ZgZg i cm A B eoeo ZcZc Z LG THERE WILL BE LEAKAGE IN THE CABLES

i cm2 e cm eses ZLZL ZsZs ZcZc ZgZg i cm A B eoeo ZcZc Z LG THERE WILL BE LEAKAGE IN THE CABLES

BALANCING AND ISOLATION e cm eses Z L1 ZsZs ZcZc ZcZc ZgZg i cm A B eoeo Z L2 i cm Z LG

BALANCING AND ISOLATION IT IS POSSIBLE TO DESIGN A MORE STABLE AND DIRT-FREE AMPLIFIER USING DIFFERENTIAL INPUT. FOR THESE REASONS MOST AMPLIFIERS FOR CRITICAL LOW-LEVEL DATA SYSTEMS ARE DIFFERENTIAL FLOATED INPUT

DIFFERENTIAL INPUT AMPLIFIER BALANCING AND ISOLATION SMALL GAIN FILTERING OFFSET ZERO ADJUSTMENT UP TO 1000 AMPLIFICATION

NORMAL CURRENTS CAUSED BY ELECTRICAL FIELDS INDUCTANCE

THE CABLES THAT CARRY THE LOW LEVEL SIGNAL FROM THA TRANSDUCER MUST BE SHIELDED. IN EXTRENME CASES ( STRONG ELECTRICAL FIELD) DOUBLE SHIELDING IS USED. 100 pF 0.2  F TRANSDUCER END AMPLIFIER END

ANALOGUE TO DIGITAL CONVERSION IN MOST MODERN DATA ACQUISITION SYSTEMS ANALOGUE DATA FROM TRANSDUCERS IS CONVERTED INTO DIGITAL DATA AND STORED IN A STORAGE DEVICE SUCH AS A DATA LOGGER OR COMPUTER

ANALOGUE TO DIGITAL CONVERSION CAN BE DESCRIBED IN TWO PHASES : 1. SAMPLING 2. QUANTISATION SAMPLING PROCESS IS THE TIME POINTS TAKEN AT EQUAL INTERVALS QUANTISATION IS THE ASSIGNMENT OF NUMBERS TO THE OBSERVED DATA POINTS

ACTUAL DATA DIGITIZED DATA

THE ERROR IS DUE TO ALIASING TO AVOID ALIASING : SAMPLE FREQUENCY SHOULD BE GREATER THAN TWICE THE HIGHEST FREQUENCY OF THE OUT PUT MINIMUM SAMPLING RATE IS KNOWN AS THE NYQUIST CRITERION

ACTUAL DATA DIGITIZED DATA