Lesson 7: Anti- Aliasing Filtering

Transcription

1 /5/06 Leon 7: nti- liaing Filtering ET 38b Sequential Control and Data quiition Department o Tehnology Leon 7_et38b.pptx Learning Objetie ter thi preentation you will be able to: Explain why anti-aliaing ilter are ued. Deign a irt order anti-aliaing ilter uing an OP MP Deign a nd order anti-aliaing ilter uing an OP MP Veriy the perormane o an anti-aliaing ilter uing imulation otware. Leon 7_et38b.pptx

2 mplitude mplitude /5/06 nti-liaing Filter Dealing with liaing in pratial ytem Exat requeny omponent o a ampled ignal are unknown. Can not determine i ignal omponent i alia or real Sampling rate limited by hardware eletion. Lab DQ ard rate i 00 khz-50 khz. nyquit = 00 khz 5 khz et input requeny limit. Bandwidth limit input ignal uing anti-aliaing ilter to eliminate requenie aboe nyquit. Leon 7_et38b.pptx 3 nti-liaing Filter Goal- redue amplitude o all requenie aboe nyquit to zero leel Ideal low pa ilter Bode plot Cut-o Frequeny, Stop band Set = nyquit or peret ignal elimination Pa band Pratial (Butterworth) ilter hae loping harateriti t Order: -0 db/deade nd Order: -0 db /deade 3 rd Order: -60 db/deade Frequeny Cut-o Frequeny, Frequeny Leon 7_et38b.pptx

3 /5/06 nti-liaing Filter: OP MP Ciruit Firt order Butterworth ilter R Deign ormula R C -V U U7 V Vo To DQ Channel R Voltage gain in the paband R Cut - o requeny (Hz) RC Deign Proedure.) Determine the aeptable leel o ignal gain,, at ampling requeny.) Ue the ormula below to determine the alue o baed o the deigned ignal gain or 3. Selet a alue o C rom tandard alue and ompute alue o R Leon 7_et38b.pptx 5 nti-liaing Filter: OP MP Ciruit Deign Proedure (ontinued).) Set R=R to gie pa band gain o - (o db). mpliy ignal ater iltering to redue noie and unwanted ignal omponent. (e.g. 60 Hz) Deign Example: data aquiition ytem ample an analog ignal at T =0.000 eond. Deign a t order anti-aliaing ilter that will redue the oltage leel o all ignal aboe the Nyquit requeny to 0. Solution: Determine the ampling requeny rom T then ind the nyquit T Hz nyquit 500 Hz 50 Hz Find the alue o or the gien leel o aboe nyquit 500 Hz Hz 56 Hz 5.5 Leon 7_et38b.pptx 6 3

4 /5/06 Deign Example (ontinued) R R C 0. mf -V U U7 V Set R=R to gie gain o - Vo Deign omplete. Chek reult with iruit imulation. To DQ Channel Selet a apaitor alue o 0. mf. Ue larger alue or lower to keep alue o reitor in range o k to 80k R C 0.0 R R C Selet tandard alue o 700 ohm and ompute alue o C 590 Hz khz =-7. db db=0log( ) 0 (db/0) = 0 (-7./0) = =0.3 Leon 7_et38b.pptx 7 nti-liaing Filter- nd Order Filter Seond order Butterworth ilter R Deign ormula R5 C C3 V U U7 -V Voltage gain in the paband Vo To DQ Channel Cut - o requeny (Hz) RR5C3 C With R R5 and C3 C Unity gain i ixed by negatie eedbak loop in thi deign. Ue ame deign proedure a t order ilter but ue ollowing ormula to ind ut-o requeny. or nd order ilter produe -0 db/deade rollo in top band Leon 7_et38b.pptx 8

5 /5/06 nti-liaing nd Order Filter Deign Deign Example : Repeat the deign o the t order ilter example uing a nd order ilter T =0.000 eond ampling. Deign a nd order anti-aliaing ilter that will redue the oltage leel o all ignal aboe the Nyquit requeny to 0. Same and nyquit a preiou ae T Hz nyquit 500 Hz 50 Hz Find the alue o or the gien leel o uing nd order ilter ormula 500 Hz Hz 86 Hz 5.5 Leon 7_et38b.pptx 9 Deign Example (ontinued) R.5 k R5.5 k C 0. mf 0. mf C3 V U U7 -V Vo To DQ Channel Selet alue o C and ompute C3. Let C=0. mf o.. C3= C= (0. mf)= 0. mf Ue deign ormula to ind alue o R=R5 With R R5 RR5C3C R C RC Sole or R gien C and R RC and C3 C C R R C 0.0 R 363 R5 F86 Hz Leon 7_et38b.pptx 0 6 Ue tandard alue o.5k. Repone ery enitie to R alue 5

6 /5/06 Deign Example (ontinued) Deign omplete. Chek reult with iruit khz =-9. db db=0log( ) 0 (db/0) = 0 (-9./0) = =0.3 Flatter repone in pa band harper roll o. Leon 7_et38b.pptx End Leon 7: nti- liaing Filtering ET 38b Sequential Control and Data quiition Department o Tehnology Leon 7_et38b.pptx 6