Embedded Microcomputer Systems Lecture 9.1

Transcription

1 Embedded Microcomputer Systems Lecture 9. Recap from last time Aalog circuit desig Noise Microphoe iterface Objectives Active low pass filter Nyquist Theorem ad aliasig Speaker amplifier Lookig at oise, observe system with kow iput Magitude Type ) DVM AC mode => good for quatitative level 2) Scope Peak-peak => rough measure of quatitative level Shape => type of oise 3) Spectrum aalyzer Sigal to oise ratio Type

2 Embedded Microcomputer Systems Lecture 9.2 db FS = 20log 0 (V/.5) Filter types Aalog LPF BPF HPF Digital Extremely flexible Butterworth Filters 2 pole Butterworth aalog filter ) select the cutoff frequecy, f c 2) divide the two capacitors by 2πf c (let C A, C 2A be the ew capacitor values) C A = 4.4μF/2πf c C 2A = 70.7μF/2πf c 3) locate two stadard value capacitors (with the 2/ ratio) with the same order of magitude as the desired values let C B, C 2B be these stadard value capacitors, let x be this factor C B = C A /x C 2B = C 2A /x 4) adjust the resistors to maitai the cutoff frequecy R = x 4.4µF C V out V V = V i i + f 4 out ( f c ) R R C µF Two pole Butterworth low pass aalog filter. Capacitor specificatio low leakage (impedace), accuracy (tolerace) low temperature coefficiet temperature rage, voltage rage, frequecy rage, Extremely High Quality Capacitors Teflo, polystyree Polypropylee,,2,3,5%. Medium Quality Capacitors 5%, 0%, 20% tolerace Class, C0G ceramic, 5%, 30ppm/ o C, ±0.3% over -55 to 25 o C Class 2, X7R ceramic, 0%, ±5% over -55 to 25 o C Class 3, Z5U ceramic, 20%, 22 to -56% over 0 to 86 o C Performace Tip: If you choose stadard value resistors ear the desired values, you will save moey ad the circuit will still be a Butterworth filter. The oly differece is that the cutoff frequecy will be slightly off from the origial specificatio. ***********Show LPF.XLS**************** Show TI FilterPro

3 Embedded Microcomputer Systems Lecture 9.3 Geeral Istrumetatio/Cotrol System Real world Measurad x(t) Trasducer Primary sesig Variable coversio y(t) Aalog Preamp Aalog Filter ad Calibratio Sigal Amplificatio Actuator applies eergy Electromagetic Electrical Thermal Soud Optical z(t) ADC timer Microcomputer Quatitative DAS (thermometer i EE445L) rage (r x ) resolutio (Δx) precisio ( x i alteratives) frequecies of iterest (f mi to f max ) Qualitative DAS (soud recordig i EE345M) souds good looks pretty feels right Other qualitative DAS s ivolve the detectio of evets. true positive (TP) baby stops breathig ad apea moitor detects it false positive (FP) baby is breathig OK but apea moitor alarms false egative (FN) baby stops breathig but moitor does ot alarm Prevalece Sesitivity Specificity PPV NPV = (TP + FN) / (TP + TN + FP + FN) = TP / (TP + FN) = TN / (TN + FP) = TP / (TP + FP) = TN / (TN + FN) Usig Nyquist Theory to Determie Samplig Rate. Voltage quatizig precisio z = 2 Time quatizig Discrete digital sigal Cotiuous aalog sigal Time (s)

4 Embedded Microcomputer Systems Lecture 9.4 Nyquist theory states that if the sigal is sampled at f s, the the digital samples oly cotai frequecy compoets from 0 to ½f s. Coversely, if the aalog sigal does cotai frequecy compoets larger tha ½f s, the there will be a aliasig error. Aliasig is whe the digital sigal appears to have a differet frequecy tha the origial aalog sigal. ***********Show FFT6.XLS**************** The choice of samplig rate, f s, is determied by the maximum useful frequecy cotaied i the sigal. fs > 2 fmax A low pass aalog filter may be required to remove frequecy compoets above 0.5fs. A digital filter ca ot be used to remove aliasig. Aalog Filter Let the gai of the aalog filter be G3 = H3(s). The the system should pass, with little error as see by the ADC, for sigal frequecies betwee fmi ad fmax G Filter with o A/D error Ideal Z 2 properly represeted aliased 0 0 f c f mi f max f c Frequecy Ideal ad practical filter resposes. 0.5 f s Δz udetectable frequecy 2 f s To prevet aliasig => o measurable sigal above 0.5fs. Speaker iterface +3.3V 4.7 μf 0. μf R SSI0 Vdd SSI0Fss CS MAX μF MC349 SSI0Clk SClk R0 4 V- 6 SSI0Tx Di Out Vcc SSI0Rx c 0 kω 20kΩ C5 3 5 REF Gd FB V+ 0 kω VO μf 4.7μF V VO2 2 FC 5. kω +.233V 7 CD Gd LM404CILP +5V EE445Lbook, figure 0.36 Need a audio amp to coect DAC output to 32Ω speaker Be careful to limit voltage ad power to speaker

5 Embedded Microcomputer Systems Lecture < V < 5V P < 200 mw 0.2 W > (V O -V O2 ) 2 /32Ω (V O -V O2 ) < 2.5V 0.2 W > (V O -V O2 ) 2 /8Ω (V O -V O2 ) <.2V Look up maximum curret for your board Measure it with a exteral +5V supply ad a curret meter Mout the speaker i a box Choose Rf, Ri so 2*Rf/Ri is less tha Desig choices ADC bits Samplig rate Cheaper cables ad electroics cause more oise Aalog filters Cheaper microphoe ad speakers itroduce more oise Refer errors to the system specificatio Quatitative system errors cause iaccuracy Qualitative system errors cause it to soud bad