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7/24/2019 Lecture22 Offset Cancel 2up http://slidepdf.com/reader/full/lecture22-offset-cancel-2up 1/12 EECS240 – Spring 2012 Lecture 22: Offset Cancellation Elad Alon Dept. of EECS EECS240 Lecture 22 2 Offset Cancellation Overview Two main ideas/approaches Modulate and/or filter offset so that it is outside of signal band CDS (auto-zeroing) Chopping (synch. detection, DEM) Inject a “DC” signal that opposes the offset Feedback-based Trimming - often digitally controlled (especially for comparators)

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EECS240 – Spring 2012

Lecture 22: Offset Cancellation

Elad AlonDept. of EECS

EECS240 Lecture 22 2

Offset Cancellation Overview• Two main ideas/approaches

• Modulate and/or filter offset so that it is outside

of signal band

• CDS (auto-zeroing)

• Chopping (synch. detection, DEM)

• Inject a “DC” signal that opposes the offset• Feedback-based

• Trimming - often digitally controlled (especially

for comparators)

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EECS240 Lecture 22 3

Filtering/Modulating Offset

• General idea:

• Put elements around the amplifier that treat offset

differently than signal

• CDS:

• Configure amplifier so that offset is (approx.)

differentiated

• Chopping:

• Modulate offset to frequencies beyond signal band,then filter it out

EECS240 Lecture 22 4

CDS #1: Output Offset Cancellation

• Relatively insensitive

to switch errors

• Storing amplified

offset

• But, what happens if

gain is large?

in

C osinout 

osC 

 AV 

V V V  AV 

 AV V 

:2Phase

:1Phase

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EECS240 Lecture 22 5

CDS #2: Input Offset Cancellation

EECS240 Lecture 22 6

Multistage Cancellation

• Open switches left to r ight

• Errors from S1 … SN-1 cancelled by final stage

•  Appl ication: continuous t ime comparators

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EECS240 Lecture 22 7

CDS and Flicker Noise

S1

S2 A . S/H  

S2V

iV

o

V1

V2

V1/f 

S1

S2

V1

V2

T = 1/f s

time

[kT]

EECS240 Lecture 22 8

Flicker Noise Analysis

       

s H 

T s

 f nieq

st 

 f  f io

n

nieq

esV sV 

e

T kT V kT V kT V  AkT V 

 

  

 

2/1

errorreferred input

/1/1

signal

1

  by tDelay

TransformLaplace

2

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EECS240 Lecture 22 9

Flicker Noise Frequency Response

2sin

2cos1

1

1

2

2

T  j

e

es H 

T  j

T s

n

  

 

s

 jsn

 jsn

 f 

 f T s H 

T T T 

T T s H 

2

sin2

4

sin2

4sin4

2cos12

2sin

2cos

2cos21

2sin

2cos1

2

1

22

222

   

 

 

   

  

 

 

 

  

 

 

  

 

 

  

 

     

EECS240 Lecture 22 10

Flicker Noise Spectrum

K 1 f s 1

Sf  f ( )K 

f  Hf  f ( ) 2 sin

  f 

2 f s

 

 

 

 

0 1 2 3 4 50

1

2

3

4

5

Hf  f ( )2

Sf  f ( ) Hf  f ( )2

• Flicker noise is

differentiated

• As is thermal

noise

• Noise removed at

low freq.

• But amplif ied at

“ high” freq.

• Noise above f s/2

folds to baseband

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EECS240 Lecture 22 11

Chopping

EECS240 Lecture 22 12

Nested Chopper Amplifier 

• Inner chopper at high freq. to remove 1/f noise

• Outer chopper at low frequency to minimize“ spiking” and remove residual offset from inner

chopper.

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EECS240 Lecture 22 13

 Auxiliary Amplifier Offset Cancellation

EECS240 Lecture 22 14

 Aux. Amplifier Implementation

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EECS240 Lecture 22 15

 Aside: Switched Cap. Resistor 

EECS240 Lecture 22 16

 Aside: Switched Cap Low-Pass Filter 

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EECS240 Lecture 22 17

Offset Trimming

EECS240 Lecture 22 18

Digital Trimming

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EECS240 Lecture 22 19

Comparator Trimming

EECS240 Lecture 22 20

Trim Implementation Issues• Infinite number of ways to introduce digitally

contro lled offset

• People have tried just about all of them

• Key issues:

• Power overhead

• Circuit Imbalance

• Effective resolution

•  Area overhead

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EECS240 Lecture 22 21

Comparator Trim Schemes

EECS240 Lecture 22 22

Pre-Amp Trim

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EECS240 Lecture 22 23

Pre-Amp Trim