EE 435 Lect 37 Spring 2010

EE 435 Lect 37 Spring 2010 - EE 435 Lecture 37 DAC Design...

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EE 435 Lecture 37 DAC Design
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Problem Solution Mismatch of resistors due to gradient effects Use anti-parallel layout (be careful of too many interconnects) Mismatch of resistors due to local random variations Make coarse resistors physically large and make its impedance small Switch impedance is code dependent Add an upper string DAC (to make all Vgs same) Settling time is code dependent Add series impedance in all lines to make R same or add a differential pair on the bottom to compensate Too many routing is required to get the switch control signal into R string Use logic cells physically located in terminal to the resistor array β is code dependent Add a dummy resistor in series with a switch on the bottom. How to determine the impedance of the resistor and switch It depends on the matching characteristics of resistors relative to switches’. Use NMOS switch on the bottom Save area, improve speed (larger Vgs for NMOS) Thanks to Ziyue for providing this material Steer Current rather than switch current to minimize Cp effects Keep current from V DD constant Dump current can often be used to generate differential output with minimal overhead Area in MSB portion of circuit (e.g. R-2R) more important Segmentation offers favorable tradeoffs between performance and logic complexity NMOS switch on bottom also reduces signal dependent delay when turned on
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Current Steering DACs Segmented Resistor Arrays R F 1 2 V OUT V REF Thermometer Coded Array Binary Coded Array X MSB n 1 Binary to Thermometer X LSB n 2 .• •• Review from last lecture .•
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EE 435 Lect 37 Spring 2010 - EE 435 Lecture 37 DAC Design...

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