9/27/11 1 Lecture 12 – Sep 30 Chem 4101 – Fall 2011 Digital Signals & Noise 1- Signal-to-noise ratio definition 2. Sources of noise 3. Electronic reduction of noise 4. Lock-in amplifiers 5. Software (digital) methods -Sections 4A, 4B, 4C6, 5A – 5D Suggested exercises: , 4-2, 4-4, 4-8, 5-3, 5-8, 5-10 Lecture 12 – Sep 30 Chem 4101 – Fall 2011 Digital versus analog domains Figure 4-1, Section 4A Lecture 12 – Sep 30 Chem 4101 – Fall 2011 Decimal to binary numbers Section 4B9/27/11 2 Lecture 12 – Sep 30 Chem 4101 – Fall 2011 Binary-coded-decimal scheme Section 4B Lecture 12 – Sep 30 Chem 4101 – Fall 2011 Digital-to-analog converter On your own, confirm data in Table 4-3 Figure 4-6, Section 4C € vDAC= −VrefD1+C2+B4+A8 Equation 4-1 Vref = 5 V A = {0, 1} B = {0, 1} C = {0, 1} D = {0, 1} Possible values Lecture 12 – Sep 30 Chem 4101 – Fall 2011 Analog-to-digital converter Figure 4-7, Section 4C New components Clock Gate (AND) n-bit counter n-bit DAC9/27/11 3 Lecture 12 – Sep 30 Chem 4101 – Fall 2011 Signal-to-noise ratio Figure 5-10, Section 5C € SN=XsEq. 5-1 € SN n= nSN iEq. 5-11 Signal averaging after n scans Lecture 12 – Sep 30 Chem 4101 – Fall 2011 Sources of noise in instrumental analysis Figure 5-3, Section 5B Chemical noise (related to the chemistry of the system) Instrument noise (associated with each component of the instrument) Thermal noise or Johnson noise Shot noise: Flicker noise Environmental noise € νrms= 4kTRΔf € noise = signal € noise ∝1/ fk = Boltzman constant; T = Temperature; R = Resistance Δf = frequency bandwidth; f = frequency Lecture 12 – Sep 30 Chem 4101 – Fall 2011 Environmental noise spectrum Figure 5-3, Section 5B9/27/11 4 Lecture 12 – Sep 30 Chem 4101 – Fall 2011 Noise reduction with circuits - Instrumentation amplifiers Figure 5-4, Section 5C The circuit subtracts the noise between v2 and v1 R1: Gain control Op Amp (C): Rejects signals in the ground Ion selective electrodes Electrocardiograms Lecture 12 – Sep 30 Chem 4101 – Fall 2011 Noise reduction with circuits - Low-pass filters (Analog filtering) Figure 2-11b, Section 2B Figure 5-5, Section 5C If we want to block baseline drift, what filter type do we use? Lecture 12 – Sep 30 Chem 4101 – Fall 2011 Noise reduction with circuits - Modulation – Lock In Amplifier Figure 5-8, Section 5C Analyte signal Blank Only when high! Filter out high frequency noise9/27/11 5 Lecture 12 – Sep 30 Chem 4101 – Fall 2011 Software methods: unweighted moving average 1st average 2nd average Other methods Ensemble averaging Smoothing Correlation Fourier
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