Chapter 5: Signals and Noise • Signal to Noise Ratio • Types of Noise • Signal to Noise Ratio Enhancement – Signal Averaging – FilteringSignal and Noise • The signal is what you are measuring that is the result of the presence of your analyte • Noise is extraneous information that can interfere with or alter the signal. - It can not be completely eliminated, but hopefully reduced! – Noise is considered random!!!• indeterminate!SIGNAL with NOISESIGNAL with NOISE averaged outThe reason for a Signal to Noise Ratio? • The average strength of the noise N is constant and independent of the magnitude of the signal. !Signal vs. NoiseWhy is a Signal to Noise Ratio? • Signal-to-noise ratio (S/N) - Parameter describing quality of data - often referred to as “figure of merit”!!• This is mathematically the inverse of the RSD, or we can say that……… sX deviation standardmean NoiseSignal==RSD1 NS=What is a Signal to Noise Ratio?S/N = 4.3:1 S/N = 43:1 Very little confidence in ability to determine peaksSignal and Noise 1. What are signal and noise? 2. Why is using a signal-to-noise ratio? 3. The relationship between signal-to-noise ratio and RSD.S/N Objective? • A!S/N!ra(o!of!3!is!usually!the!minimum!that!is!acceptable.!!• Increase!the!signal! to! noise!ra(o!!– More!signal!vs.!noise!means!a!lower!STDEV!!!!!More!precise!measurement!– Lower!STDEV!means!a!be=er!LOD!!!!!!!!!!!!!!!!!!!!Lower!limits!of!detec@on!• Reduce!as!much!of!the!noise!as!possible!by!carefully!controlling!condi(ons!– Temperature,!power!supply!varia@ons,!etc.!etc.!etc.!Noise!Types of Noise in Chemical Analysis • 1.!Chemical!Noise!• 2.!instrumental!noise!Types of Noise….. • 1.!Chemical!Noise!– Undesired!chemical!re ac ( ons!in!the !System!being!analyzed!– Examples:!!Temperature!Pressure!Humidity!!Vibra(ons!Light!intensity!!!Fluctua(ons!=!uncontrolled!variables!!Position of chemical equilibria Moisture content of sample Stratification of powdered solids Photosensitive materialsTypes!of!Noise…..!!• 2.!Instrumental!Noise!!– Noise!from!instrumental!components!–!a!complex!composite!(several'types,'sources…)!– Affects!all!types!of!instruments!!– Can!oSen!be!controlled!physically!(e.g.!temp)!or!electronically!(hardware!and!soSware)!Energy Source Sample Analytical Signal Input Transducer (Detector) Signal Processor Output Transducer (Readout) Types!of!Noise…..!• 2.!Instrumental!Noise!Energy Source Sample Analytical Signal Input Transducer (Detector) Signal Processor Output Transducer (Readout) Types!of!Noise…..!• 2.!Instrumental!Noise!Energy Source Sample Analytical Signal Input Transducer (Detector) Signal Processor Output Transducer (Readout) Types!of!Noise…..!• 2.!Instrumental!Noise!Energy Source Sample Analytical Signal Input Transducer (Detector) Signal Processor Output Transducer (Readout) Types!of!Noise…..!• 2.!Instrumental!Noise!Instrumental Noise • a.!Thermal!(Johnson)!Noise:!Electronic!noiseV!Thermal!agita@on!of!electrons!in!load!resistor.!Due!to!different!veloci@es!and!movement!of!electrons!in!electrical!components.!!''''''''''''''root0mean0square'noise'voltage'''''''resistance'of'the'resis8ve'element'!!!''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''temperature'!'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''frequency'bandwidth''– Dependent!on!both!temperature!and!frequency!bandwidths!(not!frequency).!– Can!be!reduced!by!reducing!temperature!of!electrical!components,!narrowing!bandwidth!and!minimizing!the!resistance.!• Disappear!only!at!absolute!zero.!– Independent!of!physical!size!of!the!resistor.!!– Considered!“white!noise”!occurs!at!all!frequencies.!– Thermal!noise!in!the!resistance!of!the!signal!source!is!the!fundamental!limit!on!achievable!signal!sensi@vity.!Vrms= 4kTRΔf• b.!Shot!Noise:!!– Occurs!when!there!is!a!poten@al!barrier!–!wherever!electrons!or!charged!par@cles!cross!junc@ons!(different!materials!such!as!PN!junc@on!diode,!a!transistor,!vacuum!tube,!etc.)!!''''''''''''''''''''''''root0mean0square'current'fluctua8on'!!''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''average'direct'current'''''''frequency'bandwidth''– Considered!“white!noise”!occurs!at!all!frequencies.!– Due!to!the!sta@s@cal!varia@on!of!the!flow!of!electrons!(current)!across!some!junc@ons!• Some!of!the!electrons!jump!across!the!junc@on!right!away!• Some!of!the!electrons!take!their!@me!jumping!across!the!junc@on!– Shot!noise!in!a!current!measurement!can!be!minimized!only!reducing!bandwidth.!– Note!that!if!the!ac@ve!device!provides!amplifica@on,!the!noise!also!gets!amplified!along!with!the!signal.!irms= 2IeΔf• c.!Flicker!Noise!–!1/f!(oneVoverVf)!Noise!!– Associate!with!crystal!surface!defects!in!semiconductors;!the!oxide!coa@ng!on!the!cathode!in!vacuum!tubes.!– Frequency!dependent!WWW!Magnitude!is!inversely!propor@onal!to!frequency!– Significant!at!frequencies!less!than!100!Hz!– Not!a!white!noise!–!referred!to!as!pink!noise.!– Results!in!longWterm!dri\!in!electronic!components!such!as!dc!amplifiers,!light!sources,!voltmeters!and!current!meters.!– Can!be!reduced!by!using!resisters!that!are!metallic,!wire!wound!one.!• d.!Environmental!Noise:!Room!shoul d!b e!cold? ?!– A!composite!of!different!forms!of!noise!that!!arises!from!the!surroundings!W!Unlimited!possible!sources!– Can!o\en!be!eliminated!by!elimina@ng!the!source!• Other!noise!sources!can!not!be!eliminated!!!!!!!– Methods!of!elimina@ng!it…!• Moving!the!instrument!somewhere!else!• Isola@ng!/condi@oning!the!instruments!power!source!• Controlling!temperature!in!the!room!– Control!expansion/contrac@on!of!components!in!instrument!• Elimina@ng!interferences!!– Stray!light!from!open!windows,!panels!on!instrument!– Turning!off!radios,!TV’s,!other!instruments!Environmental noise spectrum 1kHz – 500 kHz 3 Hz -60 HzNoise 1. Types of noise. 2. Instrumental noise. a. Thermal (Johnson) Noise; b. Shot Noise; c. Flicker Noise; d. Environmental Noise;Improve S/N ratio