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By: Ali Mesgarani Electrical and Computer Engineering University of Idaho 1

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Outline Motivation and goals Background New ADC topologies proposed for high speed, low power and medium resolution Asynchronous binary search ADC Pipeline binary search ADC Conclusion 2

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Motivation ADCs are key design blocks in modern microelectronic systems. More signal-processing functions are implemented in the digital domain. Noise immunity Low power Easy to design using CAD tools Reproducibility, … Today’s high speed communication systems have increased the demand for increased data rates, small area, and low power consumption. High speed ADCs have significant importance in today’s digital signal processing and communication systems. Designing energy efficient A/D converters by developing new architectures and circuits to take full advantage of what the modern process technologies have to offer. 3

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Outline 4 Motivation and goals Background New ADC topologies proposed for high speed, low power and medium resolution Asynchronous binary search ADC Pipeline binary search ADC Conclusion

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Why do we need A/D converters? The real world is analog, but easier to process digital data. Speech, image, … Digital data needs to be carried on an analog signal Signal received at the antenna must be digitized. Analog signals contain too much unnecessary amount of data ADC samples the data and splits it into finite information ADC converts analog information to digital information 5

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ADC at receiver in a link 6 ADC DAC

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Analog to digital converter (ADC) 7

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Quantization Time Digital Output

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High speed ADC applications Ultra Wide Band (UWB) communication High Speed Serial Links Digital Oscilloscope Hard Disk Drive Read-Channel Digital TV Wireless Personal Area Network (WPAN) Software Defined Radio 9

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High speed ADC applications ReferenceResolution (bits)Speed (GS/s)Power (mW)Application Park, CICC’ UWB Chan, JSSC’ UWB Verbruggen, JSSC’ UWB Harwood, ISSCC ‘ Serial links Cao, JSSC ‘ Serial links Uyttenhove, JSSC ‘ Hard disk drive read-channel Cao, ISSCC ‘ Hard disk drive read-channel Poulton, ISSCC ’ Digital oscilloscope Poulton, ISSCC ‘ Digital oscilloscope Schvan,ISSCC ‘ Digital TV Van der Plas,ISSCC ‘ WPAN 10

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ADC topologies Flash Pipeline Successive Approximation Register (SAR) Sub-ranging Ramp Single slope Dual slope Delta-Sigma 11

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Flash ADC N-bit flash: 2 N -1 comparators V in connected with 2 N -1 comparators in parallel Comparators connected to resistor string 12

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Flash ADC pros and cons Pros Very fast Cons Area and power increase exponentially with resolution Input capacitive loading on V in Noise Offset Jitter sensitivity 13

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Pipelined A/D converters Widely used where high resolution and high throughput is required A pipeline A/D converter is a multi-step amplitude quantizer Cascade of stages of low-resolution analog-to-digital converters Trades latency for speed 14

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Pipeline A/D converters 15 Coarse quantizer

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Pipeline A/D converter Pros and Cons Pros High throughput Easy upgrade to higher resolutions Cons Latency High demands on speed and gain of amplifier(s) High power 16

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SAR ADC Based on binary search Consisted of a comparator, N-bit DAC, binary search logic Compare V D/A with input signal V in Modify V D/A by D 0 D 1 D 2 …D N-1 until closest possible value to V in is reached Sequential converter 17 Successive Approximation Register (SAR) ADC

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3-bit SAR ADC example 18

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SAR ADC pros and cons Pros Small area Low power Cons Low speed: N clock cycle for N-bit SAR ADC. Complex clock generation at high sampling rates: A 6 bit 300MS/s SAR ADC requires 2.1 GHz clock generator with low skew. Clock generator consumes more power than the ADC itself! 19

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Resolution vs sampling rate of ADCs SAR ADCs are the most energy efficient ADC topologies but low speed Can we design ADCs with efficiency of SAR and speed of flash converters. 20

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Asynchronous SAR ADC Problems with SAR The logic delay in the feedback takes up to 75% of clock cycle Complex clock generation Solution: Asynchronous SAR/Binary Search ADC No complex clock gen. No binary search logic /8 0 1 ABS ADC

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Asynchronous SAR ADC Unroll feedback loop N comparators are used. Asynchronous clock is generated from MSB to LSB. Speed is limited by N comparator delays and DAC delays 22

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Asynchronous SAR pros and cons Pros Can operate faster than conventional SAR No need for high speed clock generation Cons Offset between comparators High resolution cannot be achieved like SAR because of the offset. Larger area 23

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Outline 24 Motivation and goals Background New ADC topologies proposed for high speed, low power and medium resolution Asynchronous binary search ADC Pipeline binary search ADC Conclusion

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Proposed ADC topologies Asynchronous topologies 2-bit/stage Asynchronous Binary Search (ABS) ADC Hybrid topologies Pipeline Binary Search (PBS) ADC 25

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Proposed ADC topologies Asynchronous topologies 2-bit/stage Asynchronous Binary Search (ABS) ADC Hybrid topologies Pipeline Binary Search (PBS) ADC 26

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2-bit/stage ABS ADC In a typical asynchronous SAR/binary search ADC speed is limited by N comparator, N DAC delays How to speed up? Resolve two bits in each stage (2-bit flash) Speed limited by N/2 comparator delays and DAC delays. Speed improvement by two times Penalty Power consumption increases by 1.5 times 27

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2-bit/stage ABS ADC Operation Use a 2bit flash quantizer in each stage (3 comparators) Break the reference into 4 intervals. Combines sub-ranging and asynchronous processing ideas. Break the flash ADC operation into multiple steps 28 =9.5/16 Asynch. CLK

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2-bit/stage ABS ADC implementation 29

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2-bit/stage ABS ADC simulation result 30 ParametersValue ProcessRF CMOS, IBM Feature size (nm)90 Resolution (bits)6 Supply (V)1.2 Sampling rate (MS/s)900 SNDR (dB)35.82 Power (mW)3.8 FoM (fJ/conv.step)75

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Proposed ADC topologies Asynchronous topologies 2-bit/stage Asynchronous Binary Search (ABS) ADC Hybrid topologies Pipeline Binary Search (PBS) ADC 31

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Pipelined Binary Search ADC How can we further speed up the binary search operation of Successive Approximation Register ADC? Can we operate the Successive approximation algorithm in pipeline fashion? By combining SAR and Pipeline architectures better performance than proposed ABS ADC were achieved. Two new topologies of PBS are developed. 32

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Pipeline Binary Search (PBS) ADC SAR-ADC loop has to be unrolled. Sampled input signal has to be delayed by an analog delay line. N-comparators and (n-1) digital to analog converters (DACs) have to be used Speed is limited to 1 comparator delay and DAC delays How to delay an analog signal? 33

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How to delay the analog signal? Digital delay can be easily implemented using a D-latch or DFF Analog delay line is implemented by interleaved sampling of the analog signal Example: 2-clock cycle analog delay 34

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6 bit, PBS ADC Circuit Implementation No opamp is used in this pipeline ADC Lower power, higher speed 35

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Layout for the PBS1 ADC 36 DACs Comparator Sample&Holds Clock ditribution R-String

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Simulation result for PBS1 ADC 37 ParametersValue ProcessLLLVT CMOS, UMC Feature size(nm)65 Resolution (bits)6 Supply (V)1.2 Sampling rate (GS/s)1.5 SNDR (dB)35.6 Power (mW)5.8 FoM (fJ/conv.step)78

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Comparison with state of the art ADCs ISSCC’8JSSC’10TCAS I’10CICC ‘10This work:PBS I ADCThis work:ABS ADC Technology (nm) Resolution (bits) # of channels Sampling Rate (GS/s) Peak SNDR (dB) SNDR (dB) Power (mW) Supply (V) FoM (fJ/cs)

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Summary High speed and low power analog to digital converters are essential part of many communication and signal processing applications In this research new ADC topologies that take the advantage of energy efficiency of SAR ADCs while enabling high speed operation compared with conventional SAR ADCs architectures is proposed. A new 2 bit/stage ABS ADC was introduced Twice as fast as conventional ABS ADCs A new ADC concept and implementation (PBS ADC) was introduced Enables binary search operation in a pipelined fashion Application of asynchronous ADCs as quantizers for high resolution ADCs 39

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Thank you for your attention Q&A 40

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