Enhanced SAR ADC Energy Efficiency from the Early Reset Merged Capacitor Switching Algorithm

1. Enhanced SAR ADC Energy Efficiency from the Early Reset Merged Capacitor Switching Algorithm Jon Guerber, Hariprasath Venkatram, Taehwan Oh, Un-Ku Moon Oregon State University, Corvallis OR, USA

2. EMCS SAR Outline • MCS SAR Background • EMCS SAR Switching Power • EMCS Linearity • Implementation Techniques • Conclusions

3. SAR Motivation • SAR Contributions • Low Power • Scalable • Low FOM even at small process nodes • Primarily dynamic power • Current SAR Design Issues • DAC takes a large portion of the SAR power budget • Without calibration DAC size (and power) is often based on mismatch concerns

4. Merged Capacitor Switching SAR • Merged Capacitor Switching (MCS) • Sampling reference is initially Vcm • Minimizes switching power by switching only once per phase • Maintains virtual node common mode [1] [Hariprasath 2010] [2] [Zhu 2010]

5. MCS SAR Switching • MCS Switching • Differential • Each phase, current cap charges to VDD or GND • In the end, all caps have either VDD or GND on bottom plate • Switching energy based on code

6. MCS SAR Switching

7. EMCS SAR Switching • EMCS Switching • Differential • Any {10} transition is replaced by {VCM,1} • Alternating code transitions have significantly lower energy • No extra switching events happen since all caps are reset eventually

8. EMCS SAR Switching

9. Switching Energy Comparison • EMCS Energy Savings • 12.5% Lower energy over MCS • 18.4% Lower energy over MCS when MCS is Gaussian distributed • 41.5% Lower energy then set-and-down approach [3] • Even more energy savings is input PDF is concentrated in center

10. EMCS Static Linearity • MCS worst case DNL transition: {1,0,0,0 …} to {0,1,1,1 …} • EMCS worst case DNL transition: {1,VCM, VCM, VCM…} to {VCM,1,1,1 …} and {0,VCM, VCM, VCM …} to {VCM,0,0,0 …} • Variance of virtual ground node charge due to worst case code cap matching is ½ • DNL reduced by factor of 2

11. EMCS Integral Non-Linearity • EMCS INL • INL reduced by factor of 2 • Middle code is when all bits are VCM, hence INL = 0 there • INL Simulation performed with unit cap sigma of 0.02 LSB and 10,000 runs • Reduces size of a matching limited DAC, saves power

12. EMCS Switching Algorithm For (Stage = 1) if Comp = 1 b1 = 1 else b1 = 0 end end For (Stage = 2:End) if CompN = b1 b(n) = b1 else b(n-1) = VCM b(n) = b1 end end • EMCS Switching Algorithm • All final bits end up as either VCM or {b1} • In every phase, b(n) = b1 • In each phase, comparator only dictates whether to reset capacitor b(n-1) or not

13. EMCS Logic Implementation • EMCS Implementation • Efficient AOI gate resets data latches

14. EMCS SAR Summary • Power Reduction • Switching energy reduction without additional driver energy and minimal logic • Accuracy Improvements • Static linearity improvement, relaxed matching • Implementation Method • Low overhead implementation utilizing latch resetting

15. Questions

16. References [1] V. Hariprasath, J. Guerber, S.-H. Lee, U. Moon, “Merged capacitor switching based SAR ADC with highest switching energy-efficiency,” Electron. Lett., vol. 46, pp. 620-621, Apr. 2010. [2] Y. Zhu, C.-H. Chan, U. Cho, S.-W. Sin, S.-P. U, R. Martins, F. Maloberti, “A 10-bit 100-MS/s reference-free SAR ADC in 90nm CMOS,” IEEE J. Solid-State Circuits, vol. 45, no. 6, pp. 1111-1120, Jun. 2010. [3] B. Ginsburg and A. Chandrakasen, “An energy efficient charge recycling approach for a SAR converter with capacitive DAC,” Proc. of IEEE Int. Sym. On Circuits and Systems, ISCAS, pp. 184-187, 2005.