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PDF Folding for Stochastic Flash ADCs

PDF Folding for Stochastic Flash ADCs. December 14, 2010 Skyler Weaver Oregon State University. Presentation Roadmap. Introduction What is a stochastic flash ADC? PDF folding Conclusion. V in. V in. V in. V ref. V ref. V ref. 0. 0. 0. 0. 0. 0. 1. 1. 1. 1. 0. 1. 0. 1.

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PDF Folding for Stochastic Flash ADCs

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  1. PDF Folding for Stochastic Flash ADCs December 14, 2010 Skyler Weaver Oregon State University

  2. Presentation Roadmap • Introduction • What is a stochastic flash ADC? • PDF folding • Conclusion

  3. Vin Vin Vin Vref Vref Vref 0 0 0 0 0 0 1 1 1 1 0 1 0 1 1 1 1 1 1 1 1 Stochastic flash ADC • Comparator offsets create errors

  4. Stochastic flash ADC Comparator offset inversely proportional to size “Digital-size” comparator “Analog-size” comparator

  5. Stochastic flash ADC Offset is random Individual offsets are unknown Offset distribution can be known Input-referred offset

  6. Stochastic flash ADC Apply ramp with random offsets Σ output ramp input reference x1024 6

  7. “Stochastic flash ADC” • Find individual offsets • Choose to create desired function • Computationally expensive MATLAB ramp input reference

  8. Stochastic flash ADC Reference to a group of comparators shifts PDF Σ output ramp input a x1024 8

  9. Two group stochastic flash ADC Shift in reference shifts transfer function Σ –1σ x512 ramp input Σ +1σ x512

  10. Two group stochastic flash ADC Two Gaussian CDFs spaced 2σ sum to a line Σ –1σ x512 Σ ramp input Σ +1σ x512

  11. Two group stochastic flash ADC Two Gaussian CDFs spaced 2σ sum to a line

  12. Two group stochastic flash ADC Two Gaussian CDFs spaced 1.078σ = 9b line

  13. Two group stochastic flash ADC Reference need not be exact when limited by quantization error

  14. Presentation Roadmap • Introduction • PDF folding • Can we increase comparator usage? (yes) • Conclusion

  15. PDF Folding >50% comparators wasted in 2-group stochastic flash

  16. PDF Folding “Wasted” comparator has random offset of wrong polarity Magnitude of offset is OK If polarity can be flipped, comparator becomes useful!

  17. PDF Folding “Wasted” comparator has random offset of wrong polarity Magnitude of offset is OK If polarity can be flipped, comparator becomes useful!

  18. PDF Folding PDF Folding circuit

  19. PDF Folding PDF Folding circuit Polarity toggler Lock bit Toggle bit

  20. PDF Folding PDF Folding circuit “0”

  21. PDF Folding PDF Folding circuit “0”

  22. PDF Folding PDF Folding circuit “1”

  23. PDF Folding PDF Folding circuit “1”

  24. PDF Folding Let’s look at the “left” group 512 comparators

  25. PDF Folding Before and after PDF folding

  26. PDF Folding Before and after PDF folding With comparator noise shape is preserved

  27. PDF Folding Two groups before and after Both groups simulated with noise

  28. PDF Folding Compare transfer functions

  29. Presentation Roadmap • Introduction • PDF folding • Conclusion

  30. Conclusion PDF folding turns wasted comparators into useful ones Improves performance Divide comparator area by 2! Increase # of comparators by 2x! Improve ENOB by 0.5 bits! Runs in background

  31. Special thanks to:Semiconductor Research CorporationIntel Corporation Skyler Weaver Oregon State University

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