L18 lecture - Offset Cancellation Overview Techniques...

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EECS 240 Lecture 18: Offset Cancellation V. Petkov 1 Offset Cancellation Overview Techniques: Autozero (sampled) Chopper stabilization (continuous-time) Objective: Reject offset and 1/f noise Reference Ch. Enz and G. C. Temes, “Circuit Techniques for Reducing the Effects of Op-Amp Imperfections,” Proc. IEEE, Nov. 1996, pp. 1584-1614. EECS 240 Lecture 18: Offset Cancellation V. Petkov 2 Outline Autozero technique: Offset cancellation Output Input Using auxiliary amplifier Flicker noise rejection Design example: capacitive sensor interface Chopper stabilization Principle of operation Residual offset and nested chopper stabilization Comparison and summary
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EECS 240 Lecture 18: Offset Cancellation V. Petkov 3 Output Offset Cancellation Output still corrupted by switch charge injection Æ bottom plate sampling Requires ( ) in C os in out os C AV V V V A V AV V = = = : 2 Phase : 1 Phase DD os V AV < EECS 240 Lecture 18: Offset Cancellation V. Petkov 4 Multistage Implementation R. Poujois and J. Borel, "A low drift fully integrated MOSFET operational amplifier," IEEE Journal of Solid-State Circuits, vol. 13, pp. 499 - 503, August 1978.
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EECS 240 Lecture 18: Offset Cancellation V. Petkov 5 Input Offset Cancellation ( ) ( ) 1 A V V 1 A V V A V V V V A V : (b) Phase 1 A A V V V V V A V : (a) Phase os res , os os in o c os in o os o c o os o + = + = + = + = = + = EECS 240 Lecture 18: Offset Cancellation V. Petkov 6 Multistage Cancellation Open switches left to right Residual offset from S 1 … S N-1 cancelled by final stage Capacitive coupling reduces gain Application: comparators
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EECS 240 Lecture 18: Offset Cancellation V. Petkov 7 Example: Offset Compensated SC Gain Stage S1 S2 C1 S1 C2 S2 S1 S2 V i V o V os V C1 V C2 EECS 240 Lecture 18: Offset Cancellation V. Petkov 8 SC Gain Stage: Φ 1 Assuming infinite open-loop gain. S1 C1 S1 C2 S1 S2 V i V o V os V C1 V C2 ( ) os i C C tot os C os i C V C C V C V C V C Q V V V V V 2 1 1 2 2 1 1 2 1 1 + = + = = = Φ
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EECS 240 Lecture 18: Offset Cancellation V. Petkov 9 SC Gain Stage: Φ 2 ( ) ( ) i o tot tot os i tot os o tot os o C os C V C C V Q Q V C C V C Q V C C V C Q V V V V V 2 1 2 1 1 2 1 2 2 1 1 2 1 2 = = + = + = = = Φ Φ Φ Φ S2 C1 C2 S2 S2 V o V os V C1 V C2 Charge on C1, C2 is redistributed Total charge stays same! At end of phase 2: EECS 240 Lecture 18: Offset Cancellation V. Petkov 10 SC Gain Stage Implementation S1 S2 C1 S1 C2 S2 S1 S2 V i V o V os V C1 V C2 C x Amplifier must be unity-gain stable Output pulled back to V os in each cycle
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