The Effect of Correlated Level Shifting on Noise ...

The Effect of Correlated Level Shifting on

Noise Performance in Switched

Capacitor Circuits

Benjamin Hershberg, Tawfiq Musah,

Skyler Weaver, Un-Ku Moon

(Presented by Taehwan Oh)

School of Electrical Engineering and Computer Science

Oregon State University

Presentation Outline

• Background

– CLS (Correlated Level-Shifting)

– Split-CLS

• Theoretical Noise Analysis

• Numerical and Simulation Results

• Conclusion

Background

Correlated Level Shifting (CLS)

• Finite opamp gain error becomes 1/A2

• Opamp output tied to different nodes in Φ1 and Φ2

[Gregoire, JSSC 2008]

ФS

ФA

Ф1

Ф2

CCLS

VoФS

ФA

Ф1

Vi

ФA

Ф2

VCMI VCMO

ФS

Ф1

(+/-Vr,0)

CLS Basic Operation

Ф1 :

• opamp charges output

directly

• processes full signal

Opamp Design Requirements

Ф1 Ф2

Output Swing Large Small

Slew Rate Large Small

CCLS

VoФS

ФA

Ф1

Vi

ФA

Ф2

VCMI VCMO

ФS

Ф1

(+/-Vr,0)

CLS Basic Operation

Ф2 :

• opamp is level shifted to

mid-rail

• processes error only


Ф1 Ф2



CCLS

VoФS

ФA

Ф1

Vi

ФA

Ф2

VCMI VCMO

ФS

Ф1

(+/-Vr,0)

Observation: Split-CLS

• Use separate charging devices for Ф1 and Ф2

• Optimized design for each phase

– Increase overall accuracy & efficiency


Ф1 Ф2



Split-CLS (Correlated Level Shifting)

• Split-CLS

– Generalized form of Correlated Level Shifting (CLS)

– Finite opamp gain error approx. 1 / (A1*A2)

CCLS

VOUTФS

ФA

VIN

ФA

VCMO

ФS

(+/-Vr,0)

AΦ1

AΦ2

Vx

VCLS

AMP1

AMP2

VCMO

ФS

[Hershberg, JSSC 2010]


CCLS

VOUTФS

ФA

VIN

ФA

VCMO

ФS

(+/-Vr,0)

AΦ1

AΦ2

Vx

VCLS

AMP1

AMP2

VCMO

ФS

Ф1:• amp charges output directly

• processes full signal

Amplifier Design Requirements

Ф1 Ф2




CCLS

VOUTФS

ФA

VIN

ФA

VCMO

ФS

(+/-Vr,0)

AΦ1

AΦ2

Vx

VCLS

AMP1

AMP2

VCMO

ФS

Ф2:• opamp is level-shifted to

mid-rail

• processes error only

Amplifier Design Requirements

Ф1 Ф2




• Many options for coarse amp / fine amp…– ISSCC 2010: ZCBC (coarse) + Telescopic opamp (fine)

– ISSCC 2012: Ring Amplifier (coarse) + Telescopic opamp (fine)

CCLS

VOUTФS

ФA

VIN

ФA

VCMO

ФS

(+/-Vr,0)

AΦ1

AΦ2

Vx

VCLS

AMP1

AMP2

VCMO

ФS

[Hershberg, ISSCC 2010] ,[Hershberg, ISSCC 2012]

Split-CLS: ISSCC 2010

CCLS

CCLS

Vo+

Vo-

SPC

(ФPC)

VSS

VDDVSS

VDD

VSSVDD

VCM

VCM

+Vref

Vcm

-VrefФS

ФS

ФS

ФS

ФS

ФS

ФA

ФA

SOP

SOP

CDAC

CDACVi+

Vi-

ФA

ФA

Vi+(n+1)

Vi-(n+1)

DC

SDAC

SDAC

SPC

(ФPC)

IN

IP

Vx+

Vx-

• Coarse Amp: Zero-crossing based circuit (ZCBC)

• Fine Amp: Double-cascoded telescopic opamp

[Hershberg, ISSCC 2010]

±VREF

VIN+ VO+

ΦSE

ΦS

ΦA

RAMP

VCMX

CU

CLR

±VREF

VIN+ΦS

CU

VCMO

VOTA_CM

2CU

CLR

±VREF

VIN- VO-

ΦSE

ΦS

ΦA

RAMP

CU

CLR

±VREF

VIN-ΦS x6

(8 total)

CU

VCMO

2CU

CLR

CCLS

ΦSE

ΦSE

ΦCLS

ΦA

ΦA

OTA

x6

(8 total)

CCLS

Split-CLS: ISSCC 2012

• Coarse Amp: Pseudo-differential Ring Amplifier (ringamp)

• Fine Amp: Double-cascoded telescopic opamp

• Best FoM of any high-resolution ADC ever reported– 76.8dB SNDR, 95.4dB SFDR, 5.1mW, 20Msps, 45fJ/c-step FoM

[Hershberg, ISSCC 2012]

Noise Analysis

Noise Analysis: Split-CLS

• The size of CCLS affects:

– Accuracy - feedback factor

– Speed - total load capacitance seem by fine amp

– Opamp requirements - fine amp output swing requirement

• How does level-shifting affect noise performance

– Help? Hurt?


• Noise due to AMP1 is suppressed by gain of AMP2

– Final noise determined by AMP2

• Only fine phase configuration must be considered

CCLS

VOUTФS

ФA

VIN

ФA

VCMO

ФS

(+/-Vr,0)

AΦ1

AΦ2

Vx

VCLS

AMP1

AMP2

VCMO

ФS


C1

C2

CCLS

CLOADCP

ao

β

AOTA(s) HCLS(s) So

Sn

Si

-


C1

C2

CCLS

CLOADCP

ao

β

AOTA(s) HCLS(s) So

Sn

Si

-

Single-stage opamp (as is commonly used in Split-CLS) has one dominant pole:


C1

C2

CCLS

CLOADCP

ao

β

AOTA(s) HCLS(s) So

Sn

Si

-


104

106

108

1010

1012

-80

-60

-40

-20

0

20

40

60

dB

Frequency (Hz)

Forward Path Gain

400fF

200fF

100fF

50fF

25fF

12.5fF

CCLS

=12.5fF

CCLS

=400fF

CCLS

β

AOTA(s) HCLS(s) So

Sn

Si

-

Gain-Bandwidth

Spreading


C1

C2

CCLS

CLOADCP

ao

β

AOTA(s) HCLS(s) So

Sn

Si

-

Normally, factors cancel & gain-bandwidth product is constant


C1

C2

CCLS

CLOADCP

ao

β

AOTA(s) HCLS(s) So

Sn

Si

-

Presence of CP causes gain-bandwidth spreading when CCLS is varied


104

106

108

1010

1012

-80

-60

-40

-20

0

20

dB

Frequency (Hz)

Noise Transfer Function

400fF

200fF

100fF

50fF

25fF

12.5fFCCLS

=12.5fF

CCLS

=400fF

CCLS

β

AOTA(s) HCLS(s) So

Sn

Si

-


C1

C2

CCLS

CLOADCP

ao

β

AOTA(s) HCLS(s) So

Sn

Si

-

Total integrated noise power for noise source Sn(f):


• Telescopic opamp used as noise source

• M1, M2, M3, M4 are the dominant noise contributors

– Can be modeled as single input-referred noise contributor

Vbn

Vbp

Vbt

Vi+ Vi-

Vo+Vo-

VCMFB

VDD

VSS

M1 M2

M3 M4


C1

C2

CCLS

CLOADCP

ao

β

AOTA(s) HCLS(s) So

Sn

Si

-

When 1/f noise is entirely in passband of AOTA(s), noise sources can be

treated as frequency-independent constants:


104

106

108

1010

1012

-240

-220

-200

-180

-160

-140Output Noise

dB

Frequency (Hz)

400fF

200fF

100fF

50fF

25fF

12.5fFC

CLS=12.5fF

CCLS

=400fF

CCLS

β

AOTA(s) HCLS(s) So

Sn

Si

-


0 200 400 600 800 10000

0.5

1

1.5

2

2.5

3

3.5x 10

-8

CCLS

( fF )

Inte

gra

ted N

ois

e P

ow

er

( V

2 )

vno2 vs. C

CLS for various C

P

0fF

50fF

100fF

150fF

200fF

250fF

300fF

350fF

400fF

CP

CCLS and noise are heavily correlated.

CCLS and noise are weakly correlated.

(Mostly likely case in practical designs.)

Comparison with Simulation


• Extracted values

– CP = 45fF

– ao = 58.4dB

– C1 = C2 = 400fF

– CLOAD = 640fF

– v2n(1/f) = 6.31 x 10-10 V2

– Sn(white) = 6.17 x 10-17 V2/Hz

C1

C2

CCLS

CLOADCP

ao

Vbn

Vbp

Vbt

Vi+ Vi-

Vo+Vo-

VCMFB

VDD

VSS

M1 M2

M3 M4


• Simulation and Theoretical model

found to be in good agreement.

200 400 600 800 10000

0.5

1

1.5

2

2.5

x 10-8

CCLS

( fF )

Inte

gra

ted N

ois

e P

ow

er

( V

2 )

Theoretical Model

Simulated Circuit

CP=245fF

CP=145fF

CP=45fF

C1

C2

CCLS

CLOADCP

ao

Vbn

Vbp

Vbt

Vi+ Vi-

Vo+Vo-

VCMFB

VDD

VSS

M1 M2

M3 M4


• Simulation and Theoretical model

found to be in good agreement.

102

103

0

0.5

1

1.5

2

2.5

x 10-8

CCLS

( fF )

Inte

gra

ted N

ois

e P

ow

er

( V

2 )

Theoretical Model

Cadence Simulation

CP=45fF

CP=145fF

CP=245fF

C1

C2

CCLS

CLOADCP

ao

Vbn

Vbp

Vbt

Vi+ Vi-

Vo+Vo-

VCMFB

VDD

VSS

M1 M2

M3 M4

Conclusion

Conclusion

• CLS and Split-CLS reduce finite-opamp gain error by approximately 1/A2

• Addition of CCLS network to MDAC structure introduces bandwidth spreading dependent on CCLS ↔ CP relation.

• For most practical Split-CLS designs, CCLS >> CP

– CCLS should be large enough to minimize swing requirement, maintain sufficient loop gain.

– CP is only intrinsic parasitic capacitances of opamp

• In practical cases, total integrated opamp noise is not significantly affected by Split-CLS.

Thank You For Your Attention

The Effect of Correlated Level Shifting on Noise ...

Documents

Transcript of The Effect of Correlated Level Shifting on Noise ...