EECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 1Administrative• Project :– Discussions & report submission on Frid. Dec. 4th(make appointment via sign-up sheet)– Student presentations Dec. 3rd& Dec. 8th• Office hours @ 567 Cory :– Tues. Dec. 8th, 4 to 5pm– Wed. Dec. 9th, 10 to 11am• Questions can also be asked via emailEECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 2EE247Lecture 26• Bandpass ΣΔ modulators• ADC figures of merit• Term project student presentations • Examples of systems utilizing analog-digital interface circuitry (not part of final exam)• AcknowledgementsEECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 3Bandpass ΔΣ Modulator+_vINdOUT∫DAC• Replace the integrator in 1storder lowpass ΣΔ with a resonatorÆ 2ndorder bandpass ΣΔ Resonator∫∫EECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 4Bandpass ΔΣ ModulatorExample: 6thOrder Measured output for a bandpass ΣΔ (prior to digital filtering)Key Point:NTFÆ notch type shapeSTFÆ bandpass shapeRef: Paolo Cusinato, et. al, “A 3.3-V CMOS 10.7-MHz Sixth-Order Bandpass Modulator with 74-dB Dynamic Range “, ΙΕΕΕ JSSCC, VOL. 36, NO. 4, APRIL 2001 Input SinusoidQuantization NoiseEECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 5Bandpass ΣΔ Characteristics• Oversampling ratio defined as fs /2B where B= signal bandwidth• Typically, sampling frequency is chosen to be fs=4xfcenterwhere fcenterÆ bandpass filter center frequency• STF has a bandpass shape while NTF has a notch or band-reject shape• To achieve same resolution as lowpass, need twice as many integratorsEECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 6Bandpass ΣΔ Modulator Dynamic RangeAs a Function of Modulator Order (K)• Bandpass ΣΔ resolution for order K is the same as lowpass ΣΔ resolution with order L= K/2K=415dB/OctaveK=621dB/OctaveK=29dB/OctaveEECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 7Example: Sixth-Order Bandpass ΣΔ ModulatorRef: Paolo Cusinato, et. al, “A 3.3-V CMOS 10.7-MHz Sixth-Order Bandpass Modulator with 74-dB Dynamic Range “, ΙΕΕΕ JSSCC, VOL. 36, NO. 4, APRIL 2001 Simulated noise transfer functionSimulated signal transfer functionEECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 8Example: Sixth-Order Bandpass ΣΔ ModulatorRef: Paolo Cusinato, et. al, “A 3.3-V CMOS 10.7-MHz Sixth-Order Bandpass Modulator with 74-dB Dynamic Range “, ΙΕΕΕ JSSCC, VOL. 36, NO. 4, APRIL 2001 Features & Measured Performance Summaryfs=4xfcenterBOSR=fs /2BEECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 9SummaryOversampled ADCs• Noise shaping utilized to reduce baseband quantization noise power• Reduced precision requirement for analog building blocks compared to Nyquist rate converters• Relaxed transition band requirements for analog anti-aliasing filters due to oversampling• Takes advantage of low cost, low power digital filtering • Speed is traded for resolution• Typically used for lower frequency applications compared to Nyquist rate ADCsEECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 10ADC Figures of Merit• Objective: Want to compare performance of different ADCs• Can use FOM to combine several performance metrics to get one single number• What are reasonable FOM for ADCs?EECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 11ADC Figures of Merit• This FOM suggests that adding a bit to an ADC is just as hard as doubling its bandwidth• Is this a good assumption? ENOBsfFOM 21⋅=Ref: R.H. Walden, "Analog-to-digital converter survey and analysis," IEEE J. Selected Areas Comm., April 1999EECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 12Survey Data1bit/OctaveRef: R.H. Walden, "Analog-to-digital converter survey and analysis," IEEE J. Selected Areas Comm., April 1999EECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 13ADC Figures of Merit• Sometimes inverse of this metric is used• In typical circuits power ~ speed, FOM2captures this tradeoff correctly• How about power vs. ENOB?– One more bit 2x in power?]/[22convJfPowerFOMENOBs⋅=Ref: R.H. Walden, "Analog-to-digital converter survey and analysis," IEEE J. Selected Areas Comm., April 1999EECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 14ADC Figures of Merit• One more bit means...– 6dB SNR, 4x less noise power, 4x larger C– Power ~ Gm ~ C increases 4x• Even worse: Flash ADC– Extra bit means 2x number of comparators– Each of them needs double precision– Transistor area 4x, Current 4x to keep same current density– Net result: Power increases 8xEECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 15ADC Figures of Merit•FOM2seems not quite appropriate, but somehow still standard in literature, papers• "Tends to work" because:– Not all power in an ADC is "noise limited"– E.g. Digital power, biasing circuits, etc.• Avoid comparing different resolution ADCs using FOM2!EECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 16ADC Figures of Merit• Compare only power of ADCs with approximately same ENOB• Useful numbers: – 10b (~9 ENOB) ADCs: 1 mW/MSample/secNote the ISSCC 05 example: 0.33mW/MS/sec!– 12b (~11 ENOB) ADCs: 4 mW/MSample/secSpeedPowerFOM =3EECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 1710-Bit ADC Power/SpeedYoshioko ISSCC 05EECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 1812-Bit ADC Power/SpeedLoloee(ESSIRC 2002)EECS 247- Lecture 26 Bandpass Oversampled ADCs- Systems © 2009 Page 19Material Covered in EE247• Filters – Continuous-time filters• Biquads & ladder type filters• Opamp-RC, Opamp-MOSFET-C, gm-C filters•
View Full Document
Unlocking...