Lecture 20 Analog to Digital Converters continued Comparator design continued Comparator architecture examples Techniques to reduce flash ADC complexity Interpolating Folding Interpolating folding Residue Type ADCs Two Step flash Pipelined ADCs Architecture basics Effect of sub ADC sub DAC gain stage non idealities on overall ADC performance EECS 247 Lecture 20 Data Converters Nyquist Rate ADCs 2010 Page 1 CMOS Comparator Example Flash ADC Preamp Latch Flash ADC 8bits 1 2LSB INL fs 15MHz Vref 3 8V LSB 15mV No offset cancellation Ref A Yukawa A CMOS 8 bit High Speed A D Converter IC JSSC June 1985 pp 775 9 EECS 247 Lecture 20 Data Converters Nyquist Rate ADCs 2010 Page 2 Comparator with Auto Zero Note Reference input both differential Ref I Mehr and L Singer A 500 Msample s 6 Bit Nyquist Rate ADC for Disk Drive Read Channel Applications JSSC July 1999 pp 912 20 EECS 247 Lecture 20 Data Converters Nyquist Rate ADCs 2010 Page 3 Flash ADC Comparator with Auto Zero Voffset VC VC VRe f VRe f VOffset Ref I Mehr and D Dalton A 500 Msample s 6 Bit Nyquist Rate ADC for Disk Drive Read Channel Applications JSSC July 1999 pp 912 20 EECS 247 Lecture 20 Data Converters Nyquist Rate ADCs 2010 Page 4 Flash ADC Comparator with Auto Zero Voffset Vo Vo AP1 AP2 VIn VIn VC VC VOffset Substituting for VC VC from previous cycle Vo AP1 AP2 VIn VIn VRe f VRe f Note Offset is cancelled difference b etween input reference established Ref I Mehr and D Dalton A 500 Msample s 6 Bit Nyquist Rate ADC for Disk Drive Read Channel Applications JSSC July 1999 pp 912 20 EECS 247 Lecture 20 Data Converters Nyquist Rate ADCs 2010 Page 5 Flash ADC Using Comparator with Auto Zero Ref I Mehr and D Dalton A 500 Msample s 6 Bit Nyquist Rate ADC for Disk Drive Read Channel Applications JSSC July 1999 pp 912 20 EECS 247 Lecture 20 Data Converters Nyquist Rate ADCs 2010 Page 6 Auto Zero Implementation Ref I Mehr and L Singer A 55 mW 10 bit 40 Msample s Nyquist Rate CMOS ADC JSSC March 2000 pp 318 25 EECS 247 Lecture 20 Data Converters Nyquist Rate ADCs 2010 Page 7 Comparator Example Variation on Yukawa latch used w o preamp Good for low resolution ADCs in this case 1 5bit stage for a pipeline we will see later are tolerant of high offset Note M1 M2 M11 M12 operate in triode mode x M11 M12 added to vary comparator threshold Conductance at node X is sum of GM1 GM11 Ref T B Cho and P R Gray A 10 b 20 Msample s 35 mW pipeline A D converter IEEE Journal of Solid State Circuits vol 30 pp 166 172 March 1995 EECS 247 Lecture 20 Data Converters Nyquist Rate ADCs 2010 Page 8 Comparator Example continued M1 M2 M11 M12 operate in triode mode with all having equal L Conductance of input devices G1 G2 Cox L Cox L G W1 VI1 Vth W11 VR Vth VVo1o1 Vo2 W1 VI 2 Vth W11 VR Vth CoxW1 V V W11 V V R R I1 I 2 L W1 To 1st order for W1 W2 W11 W12 Vthlatch W11 W1 x VR G1 G2 where VR VR VR VR fixed W11 12 varied from comparator to comparator Eliminates need for resistive divider Ref T B Cho and P R Gray A 10 b 20 Msample s 35 mW pipeline A D converter IEEE Journal of Solid State Circuits vol 30 pp 166 172 March 1995 EECS 247 Lecture 20 Data Converters Nyquist Rate ADCs 2010 Page 9 Comparator Example Used in a pipelined ADC with digital correction No offset cancellation required Differential reference input M7 M8 operate in triode region Preamp gain 10 Input buffers suppress kick back f1 high Cs charged to VR f2B is also high current diverted to latch comparator output in hold mode f2 high Cs connected to S Hout comparator input VR S Hout current sent to preamp comparator in amplify mode Ref S Lewis et al A Pipelined 5 Msample s 9 bit Analog to Digital Converter IEEE JSSC NO 6 Dec 1987 EECS 247 Lecture 20 Data Converters Nyquist Rate ADCs 2010 Page 10 Bipolar Comparator Example Used in 8bit 400Ms s 6bit 2Gb s flash ADC Signal amplification during f1 high latch operates when f1 low Input buffers suppress kick back input current Separate ground and supply buses for frontend preamp kickback noise reduction Preamp Latched Comparator Ref Y Akazawa et al A 400MSPS 8b flash AD conversion LSI IEEE International Solid State Circuits Conference vol XXX pp 98 99 February 1987 Ref T Wakimoto et al Si bipolar 2GS s 6b flash A D conversion LSI IEEE International Solid State Circuits Conference vol XXXI pp 232 233 February 1988 EECS 247 Lecture 20 Data Converters Nyquist Rate ADCs 2010 Page 11 Reducing Flash ADC Complexity E g 10 bit straight flash Input range 0 1V LSB 1mV Comparators 1023 with offset 1 2 LSB Assuming Cin for each comparator is 0 1pF power 3mW Total input capacitance 1023 100fF 102pF Power 1023 3mW 3W High power dissipation large area high input cap Techniques to reduce complexity power dissipation Interpolation Folding Folding Interpolation Two step pipelining EECS 247 Lecture 20 Data Converters Nyquist Rate ADCs 2010 Page 12 Interpolation Idea Reduce number of preamps instead interpolate between preamp outputs Reduced number of preamps Reduced input capacitance Reduced area power dissipation Same number of latches 2B 1 Important side benefit Decreased sensitivity to preamp offset Improved DNL EECS 247 Lecture 20 Data Converters Nyquist Rate ADCs 2010 Page 13 Flash ADC Preamp Output Vin A2 A1 Preamp Output V 0 5 A2 Vref2 A2 A1 A1 Vref1 0 Zero crossings to be detected by latches at Vin 0 5 0 0 5 1 Vref1 EECS 247 Lecture 20 1 5 2 Vref2 2 5 3 Vin Vref1 1 Vref2 2 Data Converters Nyquist Rate ADCs 2010 Page 14 Simulink Model Vin 1 Vin Vi A2 Preamp2 2 Delta Vref2 2 Y A1 Preamp1 1 Delta Vref1 EECS 247 Lecture 20 Data Converters Nyquist Rate ADCs 2010 Page 15 Preamp Output Differential Preamp Output 0 5 Differential output crossings Vin Vref1 1 Vref2 2 A2 A2 A1 A 1 0 0 5 0 0 5 1 1 5 2 2 5 3 A1 A2 0 5 A1 A2 0 0 5 0 0 5 1 EECS 247 Lecture 20 1 5 Vin 2 2 5 3 Note Additional crossing of A1 A2 A2 A1 A1 A2 A1 A2 cross zero at Vref12 0 5 1 2 1 5 Data Converters Nyquist Rate ADCs 2010 Page 16 Interpolation in Flash ADC Half as many reference voltages and preamps Interpolation factor x2 Vin A2 Example For 10bit straight Flash ADC need 2B 1024 preamps compared 2B 1 512 for x2 interpolation A1 Compare A2 A1 Comparator output is sign of A1 A2 EECS 247 Lecture 20 Possible to accomplish higher interpolation factor Interpolation at the output of preamps Data Converters Nyquist Rate ADCs 2010 Page 17 …
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