enhanced sar adc energy efficiency from the early reset merged capacitor switching algorithm
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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
EMCS SAR Outline
• MCS SAR Background
• EMCS SAR Switching Power
• EMCS Linearity
• Implementation Techniques
• Conclusions
2
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
SAR
DAC Driver
DAC Driver
Cap DAC
Cap DAC
VIN
DOUT
3
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]
C2^(N-3)C2^(N-2)C
VINP
SAR
DACP
VT
DACN
VINN
C2^(N-3)C2^(N-2)C
VCM
VCMVDD
VDD
4
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
5
VIN+-
VCM VCM VCM
VCM VCM VCM
VDD VCM VCM
GND VCM VCM
GND VCM VCM
VDD VCM VCM
VDD VDD VCM
GND GND VCM
VDD GND VCM
GND VDD VCM
GND VDD VCM
VDD GND VCM
GND GND VCM
VDD VDD VCM
EVDD = (½)CVDD²
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C CEVDD = (1/8)CVDD²
EVDD = (5/8)CVDD²
EVDD = (½)CVDD²
EVDD = (5/8)CVDD²
EVDD = (1/8)CVDD²
Φ1 Φ2
MCS SAR Switching
6
VIN+-
VCM VCM VCM
VCM VCM VCM
VDD VCM VCM
GND VCM VCM
GND VCM VCM
VDD VCM VCM
VDD VDD VCM
GND GND VCM
VDD GND VCM
GND VDD VCM
GND VDD VCM
VDD GND VCM
GND GND VCM
VDD VDD VCM
EVDD = (½)CVDD²
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C CEVDD = (1/8)CVDD²
EVDD = (5/8)CVDD²
EVDD = (½)CVDD²
EVDD = (5/8)CVDD²
EVDD = (1/8)CVDD²
Φ1 Φ2
MSBVDD DD MSB DD
T
2Unit DD
MSB-1VDD DD MSB-1 DD
T
2MSB-1VDD DD
T
2Unit DD
C1E = - V C V
2 2C
C V=
2
C1E = - V C V
2 2C
C+C V
2C
5C V=
8
1
2
For :
For 10 :
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
7
VIN+-
VCM VCM VCM
VCM VCM VCM
VDD VCM VCM
GND VCM VCM
GND VCM VCM
VDD VCM VCM
VDD VDD VCM
GND GND VCM
VCM VDD VCM
VCM GND VCM
VCM GND VCM
VCM VDD VCM
GND GND VCM
VDD VDD VCM
EVDD = (½)CVDD²
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C CEVDD = (1/8)CVDD²
EVDD = (3/8)CVDD²
EVDD = (½)CVDD²
EVDD = (3/8)CVDD²
EVDD = (1/8)CVDD²
Φ1 Φ2
EMCS SAR Switching
8
VIN+-
VCM VCM VCM
VCM VCM VCM
VDD VCM VCM
GND VCM VCM
GND VCM VCM
VDD VCM VCM
VDD VDD VCM
GND GND VCM
VCM VDD VCM
VCM GND VCM
VCM GND VCM
VCM VDD VCM
GND GND VCM
VDD VDD VCM
EVDD = (½)CVDD²
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C C
2C
2C C C
C CEVDD = (1/8)CVDD²
EVDD = (3/8)CVDD²
EVDD = (½)CVDD²
EVDD = (3/8)CVDD²
EVDD = (1/8)CVDD²
Φ1 Φ2
MSBVDD DD MSB DD
T
2Unit DD
MSB-1VDD DD MSB-1 DD
T
2Unit DD
C1E = - V C V
2 2C
C V=
2
C1E = - V C V
2 2C
3C V=
8
1
2
For :
For 10 :
Switching Energy Comparison
9
• 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
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 10
EMCS Integral Non-Linearity
11
• 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
EMCS Switching AlgorithmFor (Stage = 1) if Comp = 1 b1 = 1 else b1 = 0 endend
For (Stage = 2:End) if CompN = b1
b(n) = b1
else b(n-1) = VCM
b(n) = b1
endend 12
• 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
EMCS Logic Implementation
13
• EMCS Implementation– Efficient AOI gate resets data latches
VGP
VGN
A
B
MU
X
LATD Q
Rb
AOI
LATQ D
RbΦ1
LATD Q
Rb
LATQ D
Rb Φ1
B1
B1b
LATD Q
Rb
AOI
LATD Q
Rb
B1
B1b
Φ2
Φ2
b1
b1b
Φ2
RST
b2
b2b
LATD Q
Rb
AOI
LATD Q
Rb
B1
Φ3
Φ3
Φ3
RST
b3
b3bB1b
Φ4
RST
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
14
Questions
15
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.
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