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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
V
f f io
n
d
nieq
esV sV
e
T kT V kT V kT V AkT V
2/1
d
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
T
e
es H
T j
T s
n
s
jsn
jsn
f
f T s H
T
T
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
f
• 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