sub-threshold sense amplifier (sa) compensation using auto-zeroing circuitry
DESCRIPTION
Sub-threshold Sense Amplifier (SA) Compensation Using Auto-zeroing Circuitry. Peter Beshay Department of Electrical Engineering University of Virginia, Charlottesville. 01/21/2014. Outline. Motivation Introduction DAZ Circuit 16kB SRAM Chip Measurements Conclusion. Motivation. - PowerPoint PPT PresentationTRANSCRIPT
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RobustLowPowerVLSI
Sub-threshold Sense Amplifier (SA) Compensation Using Auto-zeroing Circuitry
01/21/2014
Peter BeshayDepartment of Electrical EngineeringUniversity of Virginia, Charlottesville
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Outline Motivation Introduction DAZ Circuit 16kB SRAM Chip Measurements Conclusion
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Motivation
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Source: IdeaConnection.com
Source: groups.csail.edu/
Source: Implantable-device.com
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Motivation
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SRAM are used in implantable devices Contribute significantly to the total
System-on-chip (SOC) power consumption
SRAM Power Consumption (1)
(1) N. Verma, Phd thesis
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Motivation
5
Minimum Energy occurs in sub-threshold [1]
Eactive = CVDD2
Etotal/operation minimized in sub-VT
Main LimitationsProcess Variations effect, Slow Speed
VDD (V)
Nor
mal
ized
Ene
rgy
(1) N. Verma, Phd thesisEnergy Consumption vs. VDD (1)
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Motivation
6
Work Focus
Minimizing the energy of the read operation of sub-threshold SRAMs.
Sense Amplifier are utilized during the read operation of the SRAMs.
The intrinsic offset voltage of the SAs causes increased read energy and degraded performance of the SRAM read operation [2].
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Outline Introduction DAZ Circuit 16kB SRAM Chip Measurements Conclusion
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Sense Amplifier
=1 if =0 Otherwise
𝑉 2
𝑨𝒏𝒂𝒍𝒐𝒈𝒚𝑉 1
𝑉 𝑜𝑢𝑡
Enable
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SA Offset Voltage
=1 if =0 Otherwise
𝑉 2
𝐕𝐨𝐟𝐟𝐬𝐞𝐭
𝑨𝒏𝒂𝒍𝒐𝒈𝒚
=1 if =0 Otherwise
𝑉 1
𝑉 2
𝑉 1
𝑉 𝑜𝑢𝑡
𝑉 𝑜𝑢𝑡
Enable
Enable
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=1 if =0 Otherwise
𝑉 2
𝐕𝐨𝐟𝐟𝐬𝐞𝐭
𝑨𝒏𝒂𝒍𝒐𝒈𝒚
=1 if =0 Otherwise
𝑉 1
𝑉 2
𝑉 1
𝐕𝐨𝐟𝐟𝐬𝐞𝐭
𝑉 𝑜𝑢𝑡
𝑉 𝑜𝑢𝑡
Enable
Enable
𝐎𝐜𝐜𝐮𝐫𝐞𝐧𝐜𝐞𝐬
SA Offset Voltage
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...
SAE
Row
Dec
oder
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
......…
6T Bitcell 6T Bitcell 6T Bitcell
6T SRAM Read Operation
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...
SAE
Row
Dec
oder
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
......…
6T Bitcell 6T Bitcell 6T Bitcell
6T SRAM Read Operation
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...
SAE
Row
Dec
oder
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
......…
6T SRAM Read Operation
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...
SAE
Row
Dec
oder
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
......…
6T SRAM Read Operation
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...
SAE
Row
Dec
oder
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
......…
BL= =
6T SRAM Read Operation
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...
SAE
Row
Dec
oder
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
......…
BL= =
WL=
WL
6T SRAM Read Operation
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...
SAE
Row
Dec
oder
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
......…
BL= =
01
WL=
WL
6T SRAM Read Operation
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...
SAE
Row
Dec
oder
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
......…
BL= =
01
WL=
WL
B L ,BL
6T SRAM Read Operation
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...
SAE
Row
Dec
oder
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
......…
BL= =
01
WL=
WL
B L ,BL
∆V >
∆V
6T SRAM Read Operation
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...
SAE
Row
Dec
oder
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
......…
BL= =
01
WL=
WL
B L ,BL
SAE
∆V >
∆V
6T SRAM Read Operation
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...
SAE
Row
Dec
oder
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
......…
BL= =
01
WL=
WL
B L ,BL
SAE
∆V >
∆V
6T SRAM Read Operation
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...
SAE
Row
Dec
oder
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
......…
BL= =
01
WL=
WL
B L ,BL
SAE
Pre-charge
∆V
6T SRAM Read Operation
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...
SAE
Row
Dec
oder
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
......…
BL= =
01
WL=
WL
B L ,BL
SAE
Pre-charge
∆V
6T SRAM Read Operation
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...
SAE
Row
Dec
oder
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
6T Bitcell
......…
BL= =
01
WL=
WL
B L ,BL
SAE
Pre-charge
∆V
𝑬𝒑𝒓𝒆𝒄𝒉𝒂𝒓𝒈𝒆=𝐂𝐁𝐋𝐕𝐃𝐃∆𝐕6T SRAM Read Operation
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PMOS-input Latch SA
𝐄𝐍BL 𝐁𝐋
𝐄𝐍
OUT 𝐎𝐔𝐓
M5 M6
M1 M2
M3 M4
𝐄𝐍Cross coupled inverter to latch the output
Sense the input voltage
Enable the SA
Precharge the output to VDD 25
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𝐄𝐍BL=0.45V 𝐁𝐋=𝟎 .𝟒𝐕
𝐄𝐍
OUT 𝐎𝐔𝐓
M5 M6
M1 M2
M3 M4
𝐄𝐍
EN
OUT,
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PMOS-input Latch SA
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𝐄𝐍BL=0.45V 𝐁𝐋=𝟎 .𝟒𝐕
𝐄𝐍
OUT 𝐎𝐔𝐓
M5 M6
M1 M2
M3 M4
𝐄𝐍
EN
OUT,
V=
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PMOS-input Latch SA
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Offset Voltage
𝐄𝐍BL=0.5 =0.5
𝐄𝐍
OUT 𝐎𝐔𝐓
M5 M6
M1 M2
M3 M4
𝐄𝐍
𝐈𝐃=𝐈𝟎𝐖𝐋 𝐞
𝐕𝐆𝐒−𝐕𝐭𝐡
𝐧𝐕𝐭𝐡𝐞𝐫𝐦𝐚𝐥 (𝟏−𝐞𝐕𝐃𝐒
𝐕 𝐭𝐡𝐞𝐫𝐦𝐚𝐥 )
∆ mismatch causes the currents to Be different, for zero differential input(BL=)
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Digital Auto-zeroing (DAZ)• We propose a digital auto-zeroing (DAZ) scheme
inspired by analog amplifier offset correction.
• The main advantages of the approach are• Near-zero offset after cancellation.• Suitable for sub-threshold operation due to the
repeated offset compensation phase.
• Several attempts have been made before to tackle the problem including:• Redundancy [3]
• Transistor upsizing [4]
• Digitally controlled compensation [5]
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Outline Introduction DAZ Circuit 16kB SRAM Chip Measurements Conclusion
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Auto-zeroing in analog amplifiers• Amplification is done
in two phases
• Φ1: Sample the offset on a capacitor
• Φ2: Subtract the offset from the input signal
(2) K Kang et al, “Dynamic Offset Cancellation Technique” cse.psu.edu/~chip/course/analog/insoo/S04AmpOffset.ppt
Dynamic Offset Cancellation (2)
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DAZ Scheme
• Phase1 (ENR1)A zero differential input is applied to the sense amp.
• Phase2 (ENO)The SA resolves based on its intrinsic offset.
𝑉 𝑜𝑢𝑡
=0
𝑉 𝑜𝑢𝑡
=1
𝑇𝑢𝑛𝑒 h𝑡 𝑒𝑆𝐴
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DAZ Scheme
• Phase3 (ENR2)The differential input is applied to the sense amp.
• Phase4 (ENI)The SA resolves based on the differential input.
𝑉 𝑜𝑢𝑡
=0
𝑉 𝑜𝑢𝑡
=1
BL
BL
BL
BL
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DAZ Circuit
𝐄𝐍
ENR1
𝐁𝐋
𝐄𝐍
OUT 𝐎𝐔𝐓
M5 M6
M1 M2
M3 M4
𝐄𝐍
ENR1
ENR2
ENI
BL
ENR2
ENI
MC2
MC1
• DAZ circuit applied to a latch-based sense amp with PMOS inputs
• DAZ circuit uses a split-phase clock and charge pump (CP) feedback circuit for repetitive compensation.
Charge Pump
𝐂𝐩
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DAZ Circuit
𝐄𝐍
ENR1
𝐁𝐋
𝐄𝐍
OUT 𝐎𝐔𝐓
M5 M6
M1 M2
M3 M4
𝐄𝐍
ENR1
ENR2
ENI
BL
ENR2
ENI
MC2
MC1
Charge Pump
𝐂𝐩
• Transistors MC1 and MC2 control the drive strength of the right side of the SA.
• The CP controls the drive current in both MC1 and MC2 to equalize the strength of the SA right and left sides.
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DAZ Circuit
𝐄𝐍
ENR1
𝐁𝐋
𝐄𝐍
OUT 𝐎𝐔𝐓
M5 M6
M1 M2
M3 M4
𝐄𝐍
ENR1
ENR2
ENI
BL
ENR2
ENI
MC2
MC1
𝐄𝐍𝐈M11
ENO
ENR2
𝐄𝐍𝐑𝟐
M9
M10
M12
M13
CpCharge Pump
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Phase 1
𝐄𝐍
ENR1
𝐁𝐋
𝐄𝐍
OUT 𝐎𝐔𝐓
M5 M6
M1 M2
M3 M4
𝐄𝐍
ENR1
ENR2
ENI
BL
ENR2
ENI
MC2
MC1
𝐄𝐍𝐈M11
ENO
ENR2
𝐄𝐍𝐑𝟐 M12
M13
Cp
M9
M10
ER1: A zero differential input is applied to the sense amp.
Charge Pump
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Phase 2
𝐄𝐍
ENR1
𝐁𝐋
𝐄𝐍
OUT 𝐎𝐔𝐓
M5 M6
M1 M2
M3 M4
𝐄𝐍
ENR1
ENR2
ENI
BL
ENR2
ENI
MC2
MC1
𝐄𝐍𝐈M11
ENO
ENR2
𝐄𝐍𝐑𝟐 M12
M13
Cp
M9
M10
ENO: The SA resolves based on its intrinsic offset.
Charge Pump
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Phase 3
𝐄𝐍
ENR1
𝐁𝐋
𝐄𝐍
OUT 𝐎𝐔𝐓
M5 M6
M1 M2
M3 M4
𝐄𝐍
ENR1
ENR2
ENI
BL
ENR2
ENI
MC2
MC1
𝐄𝐍𝐈M11
ENO
ENR2
𝐄𝐍𝐑𝟐 M12
M13
Cp
M9
M10
ER2: The differential input is applied to the sense amp.
Charge Pump
∆v
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Phase 4
𝐄𝐍
ENR1
𝐁𝐋
𝐄𝐍
OUT 𝐎𝐔𝐓
M5 M6
M1 M2
M3 M4
𝐄𝐍
ENR1
ENR2
ENI
BL
ENR2
ENI
MC2
MC1
𝐄𝐍𝐈M11
ENO
ENR2
𝐄𝐍𝐑𝟐 M12
M13
Cp
M9
M10
ENI: The SA resolves based on the differential input.
Charge Pump
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Precision• The precision of the scheme depends on the accuracy
of setting the voltage on the output capacitor (Cp).
Settling Time= 60us
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Offset Tuning • Accuracy (offset voltage) vs. settling time trade-off
through Cp tuning.
0 2 4 6 8 10 12 14 16 18 205
10
15
20
25
30
35
40
Min Achieved Offset (mV)
Settl
ing
Tim
e (u
s)
Cp=0.74pF
Cp=0.43pF
Cp=0.24pF
Cp=0.14pFCp=0.13pF
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Outline Introduction DAZ Circuit 16kB SRAM Chip Measurements Conclusion
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16kB SRAM Test-case• A 20mV DAZ SA is used in a 16kB SRAM with
1bank, 512 rows and 256 columns using commercial 45nm technology node [6].
• 10% reduction of the read energy• 24% reduction of the read delay
• 45nm technology test chip. • One regular SA array for benchmarking• DAZ SA array with Cp=32fF.
• DAZ circuit limits the absolute value of the maximum offset to 50 mV and provided 80% improvement in σ [6].
Chip Measurements
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Limitation• Area overhead (major concern in SRAM designs)
• 2.5X for 50mV offset compensation• Can be significant for small offsets
• Energy overhead of the continuous calibration (split phases, charge pump)• 3.5X the energy of a regular SA
• Sensitivity to split phase frequency.
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Outline Introduction DAZ Circuit 16kB SRAM Chip Measurements Conclusion
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Conclusion• We proposed a circuit that is capable of improving sense-amp offset to
near zero, which is valuable for sub-threshold operation due to the repeated calibration phase.
• Applying the scheme on a 16 kB SRAM in 45nm technology node showed a reduction in the total energy and delay of 10% and 24% respectively.
• Measurements from a test chip fabricated in 45 nm technology showed the circuit’s ability to limit the absolute maximum value of the offset voltage to 50 mV using a 32fF output capacitance.
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References 1. B. H. Calhoun et al. "Sub-threshold circuit design with shrinking CMOS
devices." ISCAS 2009. 2. J. Ryan et al. “Minimizing Offset for Latching Voltage-Mode Sense Amplifiers
for Sub-threshold Operation” ISQED 2008. 3. N. Verma et al. “A 256 kb 65 nm 8T Sub-threshold SRAM Employing
Sense-Amplifier Redundancy” ISSCC 2008. 4. L. Pileggi et al. “Mismatch Analysis & Statistical Design” CICC 2008. 5. M. Bhargava et al. “Low-Overhead, Digital Offset Compensated, SRAM Sense
Amplifiers” CICC 2009. 6. P. Beshay et al. "A Digital Auto-Zeroing Circuit to Reduce Offset in Sub-
Threshold Sense Amplifiers." JLPEA 2013
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Questions