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Various noise sources and noise reduction techniques in instrumentation
 

Various noise sources and noise reduction techniques in instrumentation

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Tutorial paper covering Various noise sources and noise reduction techniques in instrumentation

Tutorial paper covering Various noise sources and noise reduction techniques in instrumentation

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    Various noise sources and noise reduction techniques in instrumentation Various noise sources and noise reduction techniques in instrumentation Document Transcript

    • R. Attri Instrumentation Design Series (Electronics), Paper No. 1, June 19981 Copyright © 1998 Raman K. AttriVARIOUS NOISE SOURCES & NOISE REDUCTIONTECHNIQUES IN INSTRUMENTATIONRAMAN K. ATTRICentral Scientific Instruments Organization, Chandigarh, Indiarkattri@rediffmail.com1. INTRODUCTIONNoise is the biggest environmental factor, whichdetermines if a system will operate reliably in practice.Noise can be random or repetitive, occurringcontinuously or in isolated burst. It may affect currentor voltage and may occur at any frequency from DC tovery high frequencies. A particular source may generatenoise over narrow or wide band of frequencies. Sometime the effect of noise on the system performance canbe very drastic. The overall performance of the circuit isentirely dependent on its noise characteristics. A highlycritically designed system may prove to be a failure if itdoes not conforms to the noise characteristicsdemanded by the application. It should be understoodthat the effect of noise is usually application dependent.Noise fetches the extra amount of attention especially inthe instrumentation and measurements systems. Theoverall accuracys of such systems is highly affected bythe noise and the interference. It is absolutely necessaryto find out the remedy techniques for noise because itprevails almost in every electronics system.Once the noise can be observed in a system it is alreadya composite signal from many sources. So the first stepis to isolate and identify each noise source. Withknowledge of mechanisms, which produce noise, designrules can be introduced to ensure that systems are bothtolerant to received noise and can meet the statutoryrestrictions on emitted noise. This paper discussesvarious practical techniques to remove noise frominstrumentation systems.2. NOISE SOURCESThe strategy for minimizing the effects of noise dependsupon the location of its source. The noise problems cangenerate either in the outer world or it can be causedand communicated within the system or it can be thelocal problem with particular circuit or connection. Inabove view noise is classified at bellow:a) Internal noiseb) External noisec) Local noise2.1 INTERNAL NOISE SOURCESThese noises are generally internal to the system,generated internally into the components of theelectronics systems. Following are the majorcomponents of source of such noise.2.1.1 White NoiseIt is uniform noise over entire frequency spectrum andhas Gaussian amplitude distribution. It evades generallyall the Frequencies equally. There are two mechanismscausing white noise.Shot noise (Shottky Noise)It is a noise current caused by the fact current flow isnot a continuous process but is due to the movementsof individual electrons, which are discrete chargedparticles. The rms value of shot noise over a chosenfrequency `f is given bys= 2qIbfWhere Ib is the bias current flowing and `q is the chargeon the electron.Thermal Noise (Johnsons noise)It is due to the random motion of thermally chargedparticles in any resistive path. The charges will occur inrandom manner at the two ends giving rise to a noisevoltage, which increases with temperature. Its rmsvalue over chosen bandwidth is given byVt = 4 K T R fWhere k is Boltzmann’s constant, R is resistance, Tabsolute temperature. This noise comes in picture dueto resistances used at the source and amplifiers.2.1.2 Fliker noise (1/F Noise)This noise is superimposed on the shot noise in a verylow range of frequency. This additional source of noisehas been observed both in valves and transistors. Thisnoise increases when the frequency decreases. This isnormally called flicker noise or 1/f noise. The flickernoise r.m.s value is given byV = K ln (fh / f1)Where K= noise content in a decade which has negligible
    • R. Attri Instrumentation Design Series (Electronics), Paper No. 1, June 19982 Copyright © 1998 Raman K. AttriWhite noise in 0.1 to 1 Hz Fh is corner frequency whichseparates the dominant White and pink noise region.The flicker noise occurs when current in the circuit iscontrolled by localized barrier, whereas shot noise is dueto random way electrons surmount the barrier. Flickernoise is due to fluctuations in the effectiveness of thebarrier.2.1.3. Popcorn NoiseThese internal noise sources are commonly caused byimperfections in the semiconductor production processor materials. For example, a low frequency burst biascurrent change is produced by faults in the surface ofprocessed wafers. This is normally called popcorn noise.2.1.4. Barkhausen NoiseThis gets introduced if an instrumentation systememploys some magnetic Sensor. This occur due to thefinite size of domains in the ferromagnetic material andthe random manner in which directions of orientation ofsuch domains are changed during magnetization2.1.5. Contact NoiseContact noise is due to the breakdown of contactbetween adjacent particles in the path of the current. Itmanifests itself in the excess noise which occurs in agreat many carbon resistors when current is passedthrough them. Such noise can be greatly reduced byimproved design of the component.2.1.6. Transit Time NoiseIf the time taken by an electron to travel from(say)emitter To the collector of a transistor becomescomparable to the period of the signal being amplified,so called transit time effect take place and noise inputadmittance of the transistor increases. This occurs atfrequency in upper VHF range and beyond. Once thishigh freq. noise makes its presence felt, it goes onincreasing with freq. at a rate that soon approaches 6dBper octave and this random noise then quicklypredominates over the other noise.2.1.7. Partition NoiseIt occurs only in multi-electrode valves which are nomore in use. It exists where ultimate destination of theany single electron is extremely random because of theavailability of two or more electrodes to which anelectron can travel.2.2 EXTERNAL NOISE SOURCESThe external noise is the effects of the externalatmosphere or majority effects of other electricalsystems. Following are the some of the external sourcesof noise.2.2.1 Switching Current & Voltages• High current load, which is, switched on/off causesTransients in its power supply lines due to theirinductance or capacitance.• SMPS, which generates signal noise between 1 KHzand 10 kHz. Switching supplies, shows a discreteset of pulses at the fundamental switchingfrequency or harmonics of it.2.2.2 Power Lines Interference• Phase control circuits using thyristors and similardevice show a continuous interference spectrum,which is a bit harder to filter. In a high currentflowing in a line parallel to the signal line willproduce noise it even if it is some distance away.Over only a meter parallel run of lines a meterapart, a mill volt of noise will cause from thecurrent supplying a 3kw load.• Fluorescent lighting system produces noise at 100Hz or 120 Hz depending upon the local mainfrequency.2.2.3 Sparking and radiation• Anything which causes a spark or arching willradiate an electromagnetic wave as noise. Relay andswitch arcing and motors with the commentators orslip rings falls into this category.• Similar pattern of noise is produced by radio andTV stations which deliberately pollute the ether.• Noise in 30 MHz to GHz region gets radiated in formof electromagnetic wave. The varying electric andmagnetic field produce noise in the other systems.• Noise can be radiated at low frequency of 100 kHzeven from the free wires. Long unshielded lines andopen conductors.2.2.4 Environmental and atmospheric noise• Spurious radio waves causes by lighteningdischarge in thunderstorms and other naturalelectric disturbances occurring in atmosphere. It isin form of impulses and spread over all radiospectrum normally used for the broadcasting. It isless severe in the freq above 30MHz.• Solar noise due to the variability of sun radiations,sun cycle and sun spots.• Cosmic noise due to thermal or black body noisedistributed uniformly over the entire sky by thestars, quasars and galaxies. Above noise have littleeffects on instrumentation systems.2.2.5 Electrostatic DischargeIt is the serious problem in areas with continentalclimate or building with an air conditioner and muchdryer air. A buildup is possible on almost any material.Rate of charging make the ESD very damaging. Thecurrent pulse from an ESD can rise at four ampere pernanosecond.
    • R. Attri Instrumentation Design Series (Electronics), Paper No. 1, June 19983 Copyright © 1998 Raman K. Attri2.3 LOCAL NOISE SOURCESThis noise is internal to the system but external to someparticular set of circuits. It is mainly associated withinterconnection of circuits, interfacing of boards,transmission cables and power supply effects. It shouldbe understood that this type of noise warrants specialdesign attention which shall be discussed later.Following are the sources of local noise.2.3.1 Coupling NoiseThe following figure shows the mechanism by which thenoise gets introduced in the signal paths. At lowfrequency the impedance of concern are either resistiveor capacitive, while at higher frequencies the inductanceof a short length can become an important factor.Capacitive couplingCapacitive coupled interference is associated with thepresence of a varying electrostatics field, or difference inpotential, between two conductors coupled by somestray capacitance. For example two pins in a connectorcan be the `plates’ of the capacitor, while the mountinginsulator is the `dielectricMagnetic couplingIt is associated with the presence of a varying field in thevicinity of the signal paths. For example, if one of thetwo pins considered above is conducting a varyingcurrent, there exist a varying magnetic field around it.The second pin, if it is the part of an electrically closedcircuit, can have a voltage induced by the varying field,resulting in noise.Current CouplingCurrent coupled noise occurs where the signal andother currents use a common path, intentionally orotherwise. Any impedance in this path causesinterfering currents to develop extraneous signals,which the following device cannot distinguish from thedata. For example, if a conductor were used as the`low side of the signal path from the transducer to itsassociated amplifier, and as `low side of an acexcitation for another device, the common resistancewould develop a component of ac that would appear assignal.2.3.2 CrosstalkCrosstalk is the noise when one signal line pickup thesignal from the signal line running parallel to it.Crosstalk is mostly by E-field coupling. Faraday screen(discussed later) is used to reduce it but the screenefficiency reduces as frequency of signal increases.Though PCB may exhibit the problem, crosstalk becomeacute when many lines must run in parallel over asignificant distance. Main factor affecting the amount ofcrosstalk are -signal rise time, path length, geometry ofpath and termination of path.2.3.3 Power Supply Induced NoiseThe power supply coupling is caused by the use of theTransformers in the power supply circuits whichinduces certain inductive coupling in the circuit. Spikesand sags in the currents are some type of noise evadingthe system internally.2.3.4 Interfacing & Cable noiseThis is the noise from interfacing of circuits throughcables. The cable or the interconnection wires build up apotential along the length and also get exposed to theexternal fields. Improper ground loops do create someproblems.2.4 INTERNAL NOISE REDUCTION TECHNIQUESThis noises the noise which is the unavoidable noiseand there are some noises which cannot be removed,still some preventive measures can be taken as follows.• Since the internal noises are usually uniformlydistributed over the entire frequency spectrum,the bandwidth of analog section of a systemshould be limited to that really needed.• Source resistance and the bias currents should beas small as other consideration permits.• For VHF range operation RF transistors should beused which are remarkably low noise2.5 EXTERNAL NOISE REDUCTION TECHNIQUESAs discussed above, noise gets radiated from the open &free wires and close running signal wires and from theexternal sources. Capacitive coupling is resultedbecause of this. This should be minimized by avoidingthe free wires and close run of parallel signalwires. Radiated noise causes some problems related toinductive coupling as explained above.2.5.1 Faraday Shield:An alternate route may be provided for noise by anearthed shield put between the source and the victim ofthe noise. It is called a faraday shield. It restricts E-fieldpropagation. Shield which fully encloses components iscalled faraday cage.2.5.2 ESD protection:ESD protection can be given by connecting zener diodefrom line to ground. Connecting two zeners back to backin series gives protection against surges of either sign.Its voltage limiting effect restricts voltage from ESD orEMP to a safe amount by diverting the excess current toground.
    • R. Attri Instrumentation Design Series (Electronics), Paper No. 1, June 19984 Copyright © 1998 Raman K. Attri2.5.3 Enclosure shield:• Plastic enclosure shield is best to protect fromradiated noise. The enclosure should be continuous& should have minimal of holes and passages.When a box must have a door, continuity can beretained by using a conductive gasket to give nearlycontinuous contact. Joints & seams should bereduced. According to electrostatic theoryconductive shield is the best shield. Now manyconductive coating of acrylic resin based paintscontaining nickel loading with thickness of 0.025 to0.05 mm can done on the plastic enclosures. Thisconductive layer would give resistivity of less than1.5 ohm per square.• Power lines and high voltage sources should eitherbe kept away from the signal lines or otherwise bothshould be properly shielded. Sensitive circuitshould not be allowed to work near fluorescenttube.• The external noise can be reduced mainly by properenclosure shielding of the system. Many of the rulesof shielding, screening, grounding and cables aswell as the enclosures of power supply arediscussed in the next section.2.6 LOCAL NOISE REDUCTION TECHNIQUESThe external noise has been found to become minimumby proper overall Shielding, coating and sometime byproper layout of components. The most severe noisearises due to the local noise which is internal to thesystem. This topic needs elaborate considerationbecause it is only noise in the hand of the designer. It isdiscussed in detail as follows.2.6.1 Shielding for Capacitive Coupling:• Amplifier is particularly susceptible to electrostaticcoupling. To reject electrostatic voltages, input of acharge amplifier must be entirely shielded, with thisshield connected to the input common of theamplifier.• Besides shielding special printed circuit tracklayout called a Guard is used to surrounds theinput pins and is connected to the shield andground. The track itself forms a shield and preventsany stray currents crossing the surface to theinputs.• In some systems where interface (connector, forexample) in the input cable is being used, inputleads must be well shielded in the interface area. Itis better to use coaxial cable connectors.• Use of good quality cable is important to stabilizeshunting capacitance and minimize leakage .Highinsulation resistance cable that should enter thereceiver chassis by means of an enclosed connector.2.6.2 Shielding for Magnetic Coupling:• Electromagnetic field sources such as powertransformers, solenoids, motors and otherelectrical devices to be a sufficient distance awayso that the induced voltage is negligible.• Minimizing signal cable lengths and ensuring wideseparations.• Twisting the conductor carrying currents tightlywith the return current conductors. This results inthe cancellation of foregoing and incoming emf andresultant reduction.• Shielding should be grounded in such a way toreduce the loop area. Grounding the shield at oneend only provide electric field shield but nomagnetic shield .If shield is grounded at both theends, difference in ground potential at two pointsmay result into noise current. The rule is toprovide ground at one end at freq less than 1 MHzand ground at both ends above this frequency.There are certain rules for shielding of cables and thesystem which are enumerated below for reference:1 An electrostatic shield enclosure, to be effective,should be connected to the zero-signal referencepotential of any circuitry contained within theshield.2 The shield conductor should be connected to zerosignal reference potential at the signal earthconnection.3 When shield is broken into segments, shield isrequired to be tied in tandem as one conductorand then connected to zero signal referencepotential at the signal earth point.4 The number of separate shields required in asystem is equal to the number of independentsignals being processed plus one for each powerentrance.2.6.3 Techniques to Reduce ground loops• Uniform cable and in particular, employing equalresistors in series with each cable lead asterminations should be used. Otherwise commonmode signals shall be produced at the output ofinput amplifier in case of transducer-amplifiercombination• As shown in the figure igl should be as small aspossible to minimize the secondary effects due toimbalances in the cable losses and the finite CMRRof the amplifier. To this end loop impedance shouldbe made as high as possible• Is essential to provide low impedances between thecable screen and ground at the transducer terminalor at the amplifier terminal.2.6.4 Techniques To Reduce High frequencyEffects• A conductor of a low resistance path at 50Hz canhave significant inductive impedance at 500kHz.Inthis case high frequency source (viz.computers,transmitters, switching ckts, fluorescent tubesshould be identified and it should be suppressed atthe source itself.
    • R. Attri Instrumentation Design Series (Electronics), Paper No. 1, June 19985 Copyright © 1998 Raman K. Attri• Conducted interference present at highfrequencies because of stray capacitance fromprimary to secondary of the power transformercan be removed by highly shielded transformer.• One technique of reducing high frequency groundloop currents consists of increasing the impedanceof the cable screen by winding the cable on theferite core, thereby creating a coil of greatlyincreased series inductance.• Well designed high frequency amplifier for removalof modulation of the desired signal by unwantedradio frequency noise.2.6.5 Techniques to reduce Interconnecting CableNoise• Triboelectric effect( noise associated with therelative motion, localized separation between thecable dielectric & outer shield across thedieletric) is removed by the low noise cable havingits dielectric coated with a conductive powder tomaintain better contact between outer conductorand the dielectric.• Cable should be securely tied down to avoid motionand bends and it should be held to a gentle radiusto avoid strains.2.6.6 Techniques to Reduce Power Supply Noise• Use of main filter which reduce spikes, transientsand RFI by 60 to 65 dB. The simplest filter puts acapacitor in parallel with the supply to act as lowpass filter.• Adding an inductor (choke) in series with thesupply gives a further damping effects by absorbinghigh frequency energy in the magnetization core.• Use of torroidal wound inductor with balancedwinding connected in both live and neutral linesopposing fluxes in the core. This preventssaturation and gives improved attenuation of thesymmetrical noise.• Protection is improved by using a ferro resonantisolating transformer. This improves sags andsurges if supply frequency is constant. Thetransformer has a tuned resonant secondarywinding which produces a current limited regulatedsinusoidal voltage output. It reduces spikes,transients and RFI and can hold the supply up forabout half a cycle in the event of a very shortinterruptionAbout the AuthorRaman K. Attri is a Certified EngineeringManager and Global Scientific and TechnicalConsultant with 16 years of scientific andproduct development experience ininstrumentation system design, scientificapplications and semiconductorinstrumentations equipments. He has servedas Scientist for 10 years for national researchlaboratory. He has also worked as ProductDevelopment Manager and Systems Engineering Manager for leadingMNC product development companies in US, UK and India. Hisconsulting attachments spans across UK, USA, APAC and India.E-mail: rkattri@rediffmail.comBlog: http://ramankumarattri.blogspot.comLinkedIn: http://www.linkedin.com/in/rkattriWebsite: http://sites.google.com/site/ramankumar.attri/