2.996/6.971 Biomedical Devices Design Laboratory Lecture 9: Analog Signal ProcessingActive FiltersFilter Roll-off CharacteristicsThe Sallen-Key LPF (Equal component value form)2nd Order HPF and BPFLTC1563-x Series Pre-packaged LPFLTC1062 – DC Accurate 5th order LPFSynchronous DetectionSynchronous DetectionAnalog MultipliersAll Pass FilterMicroprocessor-based Sync. Det.Analog-to-Digital Converters (ADC)Analog-to-Digital ConvertersComparatorsADC TopologiesSAR ADCsSigma-Delta ADCs2.996/6.971 Biomedical Devices Design LaboratoryLecture 9: Analog Signal ProcessingImportant DatesIn-class presentations: Dec. 5 and 10Journal articles due: Dec. 12Presentations at MGH: March 4 and 11 (4-6pm)Active Filters• Common Filter Tasks– LPF – Reduce high frequency noise– LPF – Smooth out an envelop– HPF – Block DC signal– BPF – Remove out-of-band noise– Notch filter – Remove interference signal• Ideal filter characteristics– Flat passband gain– Infinite stop band attenuation•1storder filter Æ 20 dB per decade attenuation• What if we want more?Filter Roll-off Characteristics• Butterworth– Maximum passband flatness•Bessel– Constant phase dispersion– Some pass-band attenuation– Slow roll-off• Tschebyscheff– Maximum roll-off steepness– Passband ripples– Tables available for 1, 2, and 3dB passband rippleBessel Butterworth Tschebyscheff (3dB)R4/R3 0.268 0.568 0.234Q 0.58 0.71 1.32ndOrder filter coefficients (Ref: Op Amps for Everyone, R. Mancini, pp 277)The Sallen-Key LPF(Equal component value form)()()()022013CCAAsRC A s RC sωω=+−+1CRCω=4031RAR=+2ndOrder HPF and BPF• Sallen-Key HPF • Multiple Feedback BPFLTC1563-x Series Pre-packaged LPF• Two internal opamps• Integrated capacitors Æ better accuracy• ~$2 in quantityImages removed due to copyright restrictions.LTC1062 – DC Accurate 5thorder LPF•Switched cap filter; ~$3 each in quantityImages removed due to copyright restrictions.Synchronous Detection• What is it for? – Measuring small signals in a noisy environment• How does it work?– Modulate the excitation signal– Demodulate the sensor output– Isolate signal from noise using the frequency and phase• When to use it?– Avoiding interference– Sensor noise at DC (1/f noise)– AC excitation• Remember: DC-coupled sensors are susceptible to drift errorsSynchronous Detection• Phase shift attenuates the signal by a cosine factor!Analog Multipliers• ~$5 each• ±15V Supply• More expensive versions with better BW and accuracyAll Pass Filter• Unity gain for all frequencies• Phase shift = 180° for• Phase shift = 90° for• Phase shift = 135° at • Switch R and C to get 0-90° phase shiftω→∞0ω→1/RCω=Microprocessor-based Sync. Det.•+1/-1 buffers must have low settling time!Analog-to-Digital Converters (ADC)• Fundamental tradeoff between resolution and speed (samples per second)Analog-to-Digital Converters•Comparators– Differences between a comparator and an Op Amp– Hysteresis• Architectures–Flash– SAR–Sigma-Delta– Pipelined• How to use SAR ADC– Flywheel capacitor• How to use a sigma-delta ADCComparators• A high gain amplifier without feedback• HysteresisADC TopologiesSAR ADCs• Free-wheel capacitorSigma-Delta