Chopper Topologies - Slides Version

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    04-Sep-2014

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1 2 OUTLINE A. Auto-zero vs Chopping B. Auto-zero Amplifiers I. Concept: sampling-based II. Limitations: Baseband noise aliasing & residual offset. C. Chopping Amplifiers I. Concept: modulation-based (no sample-data) II. Limitations: bandwidth, ripple and residual offset. Do we still have baseband noise aliasing? III. Summary D. Performance Comparison DOC TECHNIQUES FOR AMPLIFIERS 3 AUTO-ZERO vs CHOPPING 4 A. Concept AUTO-ZERO AMPLIFIERS Overview AUTO-ZERO AMPLIFIERS Concept I u _u Vin*A Vos (Vores = Vos-Vos) 6 B. Limitations I. Base-band noise aliasing AUTO-ZERO AMPLIFIERS Vn+ 7 AUTO-ZERO AMPLIFIERS Limitations: noise aliasing ( ) ( ) ( ) ( ) foldoverfoldbasebandN o AZf S f S f H f S + =2( ) ( )= = ||.|

\| 02 n n sN n foldTnf S f H f S( ) ( ) ( ))`((

+((

=2 22222 cos 122 sin1hhhhofTfTfTfTd f Ht ttt1 tracking AZ s hs hold offset hT T T T dT T T= = = = period AZ full -hfTfThtt 1 > fch 1 3 5 f / fch Modulated signal VOFF + VN 1/f BA CHOPPER AMPLIFIERS Concept I smoothing filter A(f) Modulation process CHOPPER AMPLIFIERS Concept II 13 fk ~ 2KHz Note: scope plots scale: 2sec/div Not only offset is removed ! CHOPPER AMPLIFIERS Concept III 1 14 This is a modulation process WN is then modulated too So baseband noise is again aliased!, but (as opposed to AZ) ( ) ( ) = |.|

\|=odd nnch N2CSnf f Sn1f S22tChoppers output PSD all input signal spectra replicas are weighted such that the sum of all coefficients equals 1 ( )ch A ch o CSf f & /2 f f for S f S >> < ~( ) /2 f f for /f f S f Sch ch k o CS < ~ 85 . 0PSD WN ST foch= / 115 B. Limitations I. Bandwidth vs LF gain & Slew rate II. HF Ripple III. Residual offset CHOPPER AMPLIFIERS CHOPPER AMPLIFIERS Bandwidth vs LF gain / Slew rate Rule of thumb fA > 5 fch or CLOSED LOOP AMPLIFIER (High loop gain can tolerate LF gain loses) Another example > fK=1/f) | In Band WN power is NOT Increased CHOPPER AMPLIFIERS Summary 22 Summarized performance of different offset cancellation techniques DOC Performance Comparison Bandwidth (delay) Offset drift 1/f noise White noise Continuous time Effective gain Trimming N - - N + + Auto-zeroing N + + - - + Chopping -1 or N2 + + N +1 or N2 N / +3 N = Neutral 1CT filtering or feedback ripple reduction techniques 2Tuned filtering techniques 3 Offset compensation / stabilization techniques Closed loop chopped amplifiers 23 CHOPPER AMPLIFIERS FIGHTING LIMITATIONS 24 CHOPPER AMPLIFIERS Fighting Limitations - Overview A. Fighting Bandwidth I. Usual approach II. Currents are faster than voltages 25 CHOPPER AMPLIFIERS Fighting Bandwidth - Overview 2) How can we alleviate DC gain reduction due to limited BW? Closed loop amps DC gain loses are not noticed Slew rate is compromised High speed chopping Slew rate requirement highly reduced Is it required to chop all gain stages ? Currents are faster than voltages Chopped cascodes NEGATIVE FEEDBACK: high loop gain can tolerate LF gain loses without significantly affecting closed loop gain. Aol Aoleff2 Acl CHOPPER AMPLIFIERS Closed loop amplifiers VOFF + VN VSIG u SF uf f fchop fchop fchop Aoleff1 Design Problem !: Very high slew rate requirements dV/dT o VSIG & fchop CHOPPER AMPLIFIERS High speed chopping VOFF + VN VSIG u SF uf f gm1 gm2 So. ? Gm2 is left outside chopper: Its offset contribution is negligible if Gm1*(Ro1//Ri2) = A1 is large (usually the case) since 1122A fVVGmoffref in Gmoff += | Slew rate requirements are reduced Gm2*RL = A2 times !! | Stabilization stage provides a first LPF means ! SF is relaxed CHOPPER AMPLIFIERS High speed chopping II Example of Gm1 VOFF + VN VSIG u SF uf f gm1 gm2 CHOPPER AMPLIFIERS High speed chopping III 2 | Gm1 becomes a wide-band stage (not limited by following stabilization stage bandwidth) fch can be increased ! (until Vresoff hurts) | Cascode devices provides a first filtering means. What happens if demodulation switches are moved to LOW IMPEDANCE nodes? 30 CHOPPER AMPLIFIERS Fighting Limitations Overview (cont) B. Fighting ripple I. CT filtering II. (DT) Tuned filtering: Track and hold demodulation III. (DT) Tuned filtering revisited: low noise approach IV. Offset compensation (feedback) V. Offset stabilization VI. Chopped auto-zero 31 CHOPPER AMPLIFIERS Fighting Ripple - Overview 1) Why do we want to further reduce ripple? Relaxes post-chopper LPF (can be integrated) allows for lower OSR Signal delay can reduced Improves dynamic range 2) How can we further reduce ripple ? CT filtering Low pass (Conventional chopping) High pass Tuned Filtering T&H demodulation Low noise Offset compensation Baseband offset Modulated offset Offset stabilization Auto-zero Chopper Chopped Auto-zero Discrete or Continuous time output CHOPPER AMPLIFIERS Fighting Ripple CT filtering HP filter case 3 32 Modulated signal VOFF + VN 1/f (2) Modulated offset & 1/f VSIG 1 f / fch 3 5 (5) VOFF VSIG (1) VOFF + VN VSIG u SF ufo >fch (This is the ripple we want to kill!) VOFF + VN VSIG u SF (smoothening) u 2Track & hold demodulation CHOPPER AMPLIFIERS Fighting Ripple Tuned filtering T&H demodulation 4 34 VOFF + VN VSIG u SF (smoothening) u 2Track & hold demodulation CHOPPER AMPLIFIERS Fighting Ripple Tuned filtering T&H demodulation (cont) 4 WN aliasing ! + In band Ripple: Virtually eliminated | Independent of circuit parameters | Delay: only limited to smoothing filter | Out of Band 35 VOFF VSIG (1) (2) (3) (4) Sinc filter 2| (5) Sampling is done here Modulated offset & 1/f VSIG 1 f / fch 3 5 1 3 5 2 4 f / fch f / fch 1 3 5 2 4 And averaged out here Conventional chopper-stabilized amplifier SF VOFF + VN VSIG u uCHOPPER AMPLIFIERS Fighting Ripple Tuned filtering Low noise approach 5 36 Sinc filter 2| CHOPPER AMPLIFIERS Fighting Ripple Tuned filtering Low noise approach (cont.) 12 CKSA CKSB CKR TCK TCK TCK This is one possible implementation of a tuned (also known as sinc or notch) filter Input charge from phases A & B is averaged out during CKR phase 37 Sinc filter 2| SF VOFF + VN VSIG u uCHOPPER AMPLIFIERS Fighting Ripple Tuned filtering Low noise approach (cont.) 5 WN PSD is NOT increased | In band Ripple: Virtually eliminated | Independent of circuit parameters | Delay: only limited by AAF | Out of band CHOPPER AMPLIFIERS Fighting Ripple Offset compensation baseband offset 6,7,8 38 Conventional chopper-stabilized amplifier Modulated offset & 1/f VSIG 1 f / fch 3 5 1 3 5 VOFF + VN 1/f VOFF + VN Modulated signal VOFF + VN 1/f uVo fo