EE C245 ME C218 Introduction to MEMS Design Fall 2011 Prof. Clark T.-C. Nguyen Dept. of Electrical Engineering & Computer Sciences University of California at Berkeley Berkeley, CA 94720 Lecture EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 1 Lecture Outline Reading: Senturia, Chpt. 14 Lecture Topics: Detection Circuits Velocity Sensing Position Sensing EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 2 1
Velocity-to-Voltage Conversion To convert velocity to a voltage, use a resistive load EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 3 Velocity-to-Voltage Conversion To convert velocity to a voltage, use a resistive load EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 4 2
Velocity-to-Voltage Conversion To convert velocity to a voltage, use a resistive load EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 5 Position-to-Voltage Conversion To sense position (i.e., displacement), use a capacitive load C D EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 6 3
Position-to-Voltage Conversion To sense position (i.e., displacement), use a capacitive load C D EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 7 Velocity Sensing Circuits EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 8 4
Velocity-to-Voltage Conversion To convert velocity to a voltage, use a resistive load EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 9 Problems With Purely Resistive Sensing x b F d1 k Electrode 1 d 1 d 2 m Electrode 2 i o R D v o i 1 C 1 C 2 v 1 V P Includes C o, line C, bond pad C, and next stage C EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 10 5
F d1 Electrode 1 Problems With Purely Resistive Sensing x d 1 d 2 m b k Electrode 2 In general, the sensor output must be connected to the inputs of further signal conditioning circuits input R i of these circuits can load R D i o R D v o R i i 1 C 1 C 2 These change w/ hook-up not good. v 1 V P Problem: need a sensing circuit that is immune to parasitics or loading. Soln: use op amps. EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 11 F d1 The TransR Amplifier Advantage x b k The virtual ground provided by the ideal op amp eliminates the parasitic capacitance and R i R 2 Electrode 1 d 1 d 2 m i 1 C 1 C 2 v 1 Electrode 2 i 2 - + i 2 R o = 0Ω The zero output resistance of the (ideal) op amp can drive virtually anything V P EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 12 6
Position Sensing Circuits EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 13 Problems With Pure-C Position Sensing To sense position (i.e., displacement), use a capacitive load EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 14 7
F d1 Electrode 1 The Op Amp Integrator Advantage x d 1 d 2 m b k Electrode 2 The virtual ground provided by the ideal op amp eliminates the parasitic capacitance i o R 2 - + C 2 1 R 2 >> sc2 (for biasing) i 1 C 1 C 2 v 1 V P EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 15 Differential Position Sensing EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 16 8
Differential Position Sensing Example: ADXL-50 Tethers with fixed ends Proof Mass Sense Finger C 1 C 2 Fixed Electrodes V P Suspension Beam in Tension C 1 V o C 2 -V P EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 17 Buffer-Bootstrapped Position Sensing +V P Includes capacitance from interconnects, bond pads, and C gs of the op amp C gd + - Unity Gain Buffer -V P C gd = gate-to-drain capacitance of the input MOS transistor Bootstrap the ground lines around the interconnect and bond pads No voltage across It s effectively not there! Interconnect Ground Plane 1 EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 18 9
Effect of Finite Op Amp Gain +V P Total ADXL-50 Sense C ~ 100fF C gd + - Unity Gain Buffer -V P EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 19 Integrator-Based Diff. Position Sensing +V P R 2 i o C F - 1 R 2 >> sc2 (for biasing) C 0 p + v -V P EE C245: Introduction to MEMS Design LecM 14 C. Nguyen 11/18/08 20 10