EE 435 Lect 24 Spring 2010

EE 435 Lect 24 - EE 435 Lecture 24 Data Converters Basic Operation of CMFB Block OXX FB O1 O2 01 02 FB V V V = A s 2 ⎛ ⎞ ⎜ ⎟ ⎝ ⎠ V OXX

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Unformatted text preview: EE 435 Lecture 24 Data Converters Basic Operation of CMFB Block OXX FB O1 O2 ( ) 01 02 FB V +V V = A s 2 ⎛ ⎞ ⎜ ⎟ ⎝ ⎠ V OXX is the desired quiescent voltage at the stabilization node (irrespective of where V FB goes) .• • • • • Review from last lecture .• • • • • Common-mode offset voltage Definition: The common-mode offset voltage is the voltage that must be applied to the biasing node at the CMFB point to obtain the desired operating point at the stabilization node .• • • • • Review from last lecture .• • • • • Common-mode gains 02 01 03 COM C1 05 V g +g /2 A- V s C + g = ¡ 02 m5 COM2 C2 05 V g A- V s C + g = ¡ 02 m3 COM3 C3 05 V g / 2 A- V s C + g = ¡ 1 / 2 2 01 03 COM0 05 g +g /2 A- g T T I I λ λ = − = − ¡ 5 5 2 / 4 / 2 m5 COM20 05 g A- g T E B T E B I V I V λ λ = − = − ¡ T m3 EB3 COM30 05 T EB3 2I /2 g /2 V 2 A =- g I / 2 V λ λ − = − ¡ Although the common-mode gain A COM0 is very small, A C0M20 is very large! Shift in V 02Q from V OXX is the product of the common-mode offset voltage and A COM20 .• • • • • Review from last lecture .• • • • • How much gain is needed in the CMFB amplifier? Do a small-signal analysis, only input is V COFF Want to guarantee 02Q 0XX OUT-ACCEPTABLE V- V < Δ V The CMFB Loop ( ) 02 02 COFF COM2 V = V A+V A M 2 M’ 3 M 4 V O2 V C1 V DD V C3 C 2 V C1 V COFF V OXX V FB A V AVG COM2 02 COFF COM2 A V =V 1-AA COM2 0UT-ACCEPTABLE COFF COM2 A V = V 1-AA Δ .• • • • • Review from last lecture .• • • • • How much gain is needed in the CMFB amplifier? The CMFB Loop M 2 M’ 3 M 4 V O2 V C1 V DD V C3 C 2 V C1 V COFF V OXX V FB A V AVG COM2 0UT-ACCEPTABLE COFF COM2 A V = V 1-AA Δ • This does not require a particularly large gain • This is the loop that must be compensated since A and A COMP2 will be frequency dependent • Miller compensation capacitor for compensation of differential loop will often appear in shunt with C 2 • Can create this loop without CM inputs on fully differential structure for simulations • Results extend readily to two-stage structures with no big surprises .• • • • • Review from last lecture .• • • • • CMFB Circuits 01 V 02 V Several (but not too many) CMFB circuits exist Can be classified as either continuous-time or discrete-time- o V + o V .• • • • • Review from last lecture .• • • • • Data Converters...
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This note was uploaded on 01/31/2012 for the course EE 345 taught by Professor Geiger during the Fall '11 term at Iowa State.

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EE 435 Lect 24 - EE 435 Lecture 24 Data Converters Basic Operation of CMFB Block OXX FB O1 O2 01 02 FB V V V = A s 2 ⎛ ⎞ ⎜ ⎟ ⎝ ⎠ V OXX

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