EE241 Final ProjectHigh Speed Sense Amplifier With Offset

compensation for SRAMs

Yida Duan

Motivation & Proposed Solution

• Time Margin needed for input to grow > offset• Response Time = Time Margin + Sense Time• Time Margin is large for large offset (offset compensation)

Problem (Offset)

Solution (offset Compensation)

Ysel

CBLCBLb

1u 1u

M260n

M160n

M3120n

M4120n

M71u

M5120n

M6120nYsel

resetb

Vdd

resetreset

4fF

4fF

120n

90n

90n

120n

90n

90n

Outn Outp

Iin

inninp

Basis of ComparisonVdd

Vdd

SAEN

Ysel Ysel

CBLCBLb

M8120n

M7120n

M6120n

M3120n

M4120n

1u 1u

M160n

M260n

M5200n

Ysel

120n

60n

120n

60n

OutpOutn

In-In+

Iin

Ysel

CBLCBLb

1u 1u

M260n

M160n

M3120n

M4120n

M71u

M5120n

M6120n

120n60n

Ysel

reset

resetb

resetb

Vdd

120n120n

90n90n

90n 90n

resetb resetb

Outn Outp

M8

M9

In+ In-

Iin

Ysel

CBLCBLb

1u 1u

M260n

M160n

M3120n

M4120n

M71u

M5120n

M6120nYsel

resetb

Vdd

resetreset

4fF

4fF

120n

90n

90n

120n

90n

90n

Outn Outp

Iin

inninp

Vdd

M9120n

M2120n

M460n

M160n

M360n

M6120n

M760n1u

M51u

M860n

CBL

Reset

ResetBResetB Reset

ResetB

ResetB

ResetB

M12120n

M1190n

M1090n

Out

4fF4fF X Y

Iin

(a) Conventional Voltage Sense amp (VSA)

(b) Clamped-bitline current sense amp (CSA)

(c) Offset-compensated current sense amp (OCCSA)

(d) Non-strobed regenerative voltage sense amp (NSR-VSA)

Conventional Voltage Sense AmplifierVdd

Vdd

SAEN

Ysel Ysel

CBLCBLb

M8120n

M7120n

M6120n

M3120n

M4120n

1u 1u

M160n

M260n

M5200n

Ysel

120n

60n

120n

60n

OutpOutn

In-In+

Iin

Vdd

Vdd

SAEN

Ysel Ysel

CBLCBLb

M8120n

M7120n

M6120n

M3120n

M4120n

1u 1u

M160n

M260n

M5200n

Ysel

120n

60n

120n

60n

OutpOutn

In-In+

Iin

Reset Phase

Sense Phase

Good:• No Reset Power

Bad:• Complete bitline discharge• Large Sense Time (Bitline loading)• Large Offset

Clamped-bitline Current Sense AmpReset Phase

Sense Phase

Clamped-bitline Current Sense AmpBetween the 2 phases

CBLRM

tin

M eI

I 727 12

• IM7 > Ios prior to regeneration

• IM7 saturates at Iin/2 for large Time Margin

• M7 in linear region, RM7 is small

IM7

Clamped-bitline Current Sense Amp – Offset Calculation

23

21

2MMos III

Assume ,317 , MMM RRR

• To convert Ios to Vos, Vos=RM7Ios

• Offset is mainly due to M1—M4

333

33

11

11

|)|(

)(

dstpgsM

sattngsM

VVVL

WKI

VVVL

WKI

)()(

)()(

33

33

1

11

tpdsM

tnsatM

VVL

WKI

VVL

WKI

)()( 31 MM II Because 3dssat VV

Clamped-bitline Current Sense Amp

Good:• Bitline clamped to a high voltage after sense phase• Small Sense time (No bitline loading)

Bad:• Reset power• large offset, need Time Margin• Will not work if Iin < Ioffset

Ysel

CBLCBLb

1u 1u

M260n

M160n

M3120n

M4120n

M71u

M5120n

M6120nYsel

resetb

Vdd

resetreset

4fF

4fF

120n

90n

90n

120n

90n

90n

Outn Outp

Iin

inninp

• Same Operation as clamped bit line current sense amplifier• Virtually no offset (any offset is sampled on 4fF caps)

Offset-Compensated Clamped-bitline Current Sense Amplifier

Non-strobed Regenerative Voltage Sense Amplifier

Reset Phase

Sense PhaseGood:• No bitline loading• No Time Margin

Bad:• Reset power• Larger area• Compromise robustness

Simulation Results – Sense Time

Simulation Results - Offset

-0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.080

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Voffset (V)P

roba

bilit

y

Input-referred Offset Probability Obtained From 100-pt Monte Carlo Simulation

NSRVSA

VSACSA

OCCSA

Avg=992.9mVσ=46.2mV

VSA NSR-VSA CSA OCCSA

47mV 8.1mV 15.8mV 1.5mV2OSV

Simulation Results - Yield

-20 0 20 40 60 80 100

30

40

50

60

70

80

90

100

Strobe (SAEN) Signal Delay vs. Yield

Strobe (SAEN) Signal Delay (ps)%

Yie

ld

CSA

OCCSAVSA

0 500 1000 1500 200020

30

40

50

60

70

80

90

100

Strobe (SAEN) Signal Delay vs. Yield

Strobe (SAEN) Signal Delay (ps)

% Y

ield

CSA

OCCSAVSA

Simulation Results – Response Time

Mean (ps)

STD (ps)

Total Response Time (3σ Yield)

NSR-VSA 120.7 4.5 134.2

OCCSA 57.5 1.4 61.7

50 60 70 80 90 100 110 120 1300

5

10

15

20

25

30

35

Response Time (ps)

Num

ber

of S

ampl

es

OCCSA

NSR-VSA

Avg=57.5psσ=1.5ps

Avg=120psσ=4.5ps

Conclusion & Performance Summary

Sense Amp Total Reponse Time(ps)

Reset Power(uW)

Area (um2)

VSA > 2000 0.13 0.052CSA ---- 36.1 0.107

NSR-VSA 134.2 48.9 0.451OCCSA 61.7 36.1 0.473

• OCCSA is 30X faster than VSA, 2X faster than NSR-VSA• OCCSA has comparable reset power with NSR-VSA• OCCSA has comparable area with NSR-VSA

Note: Time Margin Uncertainty is not considered