EECS240 – Spring 2010Lecture 22: Offset CancellationElad AlonDept. of EECSEECS240 Lecture 22 2Offset 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• Trimming• Often digitally controlled (especially for comparators)EECS240 Lecture 22 3Filtering/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 outEECS240 Lecture 22 4CDS #1: Output Offset Cancellation• Relatively insensitive to switch errors• Storing amplified offset• But, what happens if gain is large?()inCosinoutosCAVVVVAVAVV=−−=−=:2 Phase:1 PhaseEECS240 Lecture 22 5CDS #2: Input Offset CancellationEECS240 Lecture 22 6Multistage Cancellation• Open switches left to right• Errors from S1…SN-1cancelled by final stage• Application: continuous time comparatorsEECS240 Lecture 22 7Auxiliary Amplifier Offset CancellationEECS240 Lecture 22 8Aux. Amplifier ExampleH. Ohara, H. X. Ngo, M. J. Armstrong, C. F. Rahim, and P. R. Gray, "A CMOS programmable self-calibrating 13-bit eight-channel data acquisition peripheral," IEEE Journal of Solid-State Circuits, vol. 22, pp. 930 - 938, December 1987.EECS240 Lecture 22 9Aux. Amplifier ImplementationEECS240 Lecture 22 10CDS and Flicker NoiseS1S2A . S/H ΣS2ViVoV1V2V1/fS1S2V1V2T = 1/fstime[kT]EECS240 Lecture 22 11Flicker Noise Analysis() () ()() ()()4342144443444421321sHTsfnieqstVffiondnieqesVsVeTkTVkTVkTVAkTV⎭⎬⎫⎩⎨⎧−=→⎪⎪⎭⎪⎪⎬⎫⎪⎪⎩⎪⎪⎨⎧⎟⎠⎞⎜⎝⎛−−+=−−2/1derror referredinput /1/1signal1 by tDelay Transform Laplace2EECS240 Lecture 22 12Flicker Noise Frequency Response()2sin2cos11122TjTeesHTjTsnωωω+−=−=−=−−()()sjsnjsnffTsHTTTTTTTsH2sin24sin24sin42cos122sin2cos2cos212sin2cos12122222πωωωωωωωωωω===⎟⎠⎞⎜⎝⎛−=++−=⎟⎠⎞⎜⎝⎛+⎟⎠⎞⎜⎝⎛−=→→444344421EECS240 Lecture 22 13Flicker Noise SpectrumK1:= fs1:=Sff()Kf:= Hff( ) 2 sinπ f⋅2fs⋅⎛⎜⎝⎞⎠⋅:=012345012345Hff()2Sff()Hff()2⋅f• Flicker noise is differentiated• As is thermal noise• Noise removed at low freq. • But amplified at “high” freq.• Noise above fs/2 folds to basebandEECS240 Lecture 22 14ChoppingEECS240 Lecture 22 15Nested 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. EECS240 Lecture 22 16Offset TrimmingEECS240 Lecture 22 17Digital TrimmingEECS240 Lecture 22 18Comparator TrimmingEECS240 Lecture 22 19Trim Implementation Issues• Infinite number of ways to introduce digitally controlled offset• People have tried just about all of them• Key issues:• Power overhead• Circuit Imbalance• Effective resolution• Area overheadEECS240 Lecture 22 20Comparator Trim SchemesEECS240 Lecture 22 21Pre-Amp TrimEECS240 Lecture 22 22Pre-Amp
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