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Transcript
EEL5225: Principles of MEMS Transducers (Fall 2003) Instructor: Dr. Hui-Kai Xie Accelerometers z Capacitive Position Sensing Circuits for Capacitive Sensing ADI Capacitive Accelerometers Other MEMS Accelerometers Reading: Senturia, Chapter 19, p.497-530 z z z 11/24/2003 Note: Most of figures in this lecture are copied from Senturia, Microsystem Design, Chapter 19. EEL5225: Principles of MEMS Transducers (Fall 2003) Lecture 33 by 1 H.K. Xie 11/24/2003 Capacitive Position Sensing z Capacitive Position Sensing MEMS Capacitive Sensors: • High impedance • Small sensing capacitance • Very small signal • Parasitic capacitance • Noise 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) 2 Differential Capacitive Sensing V0 = −Vs + = z C1 ( 2Vs ) C1 + C2 C1 − C2 Vs C1 + C2 Differential Capacitive Sensing z First order cancellation of many effects z z 11/24/2003 Temperature variations Common mode rejection EEL5225: Principles of MEMS Transducers (Fall 2003) 3 Circuits for Capacitive Sensing z Interface circuits z z z z z Demodulation Methods z z z Peak detectors Synchronous demodulators Offset cancellation circuits z z 11/24/2003 Transimpedance amplifier Transimpedance amplifier with feedback capacitor Switched-capacitor circuits Voltage follower Chopper-Stabilized Amplifiers Correlated Double Sampling EEL5225: Principles of MEMS Transducers (Fall 2003) 4 Transimpedance amplifier Q = C ( x ) Vs iC = C ( x ) dVs ∂C dx + Vs dt ∂x dt Vo = − RF iC • • • • Parasitic capacitance is negligible Output voltage depends on both the position x and velocity dx/dt DC Vs: Output voltage is directly proportional to the velocity. AC Vs: High frequency of Vs is desired. • Large DC offset • Sensitivity is proportional to RF. But large resistors are difficult to implement on-chip for integrated sensors. • Vs also generates electrostatic force which disturbs the position of the rotor. Æ Small Vs or Short pulses 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) 5 Transimpedance Amplifier z Transimpedance amplifier with a feedback capacitor • Assume a high-frequency AC source. Then velocitydependent term of iC can be ignored. • Assume ωRFCF >> 1. C (x) iC Vo ≈ − ≈− Vs sC F CF • RF provides DC feedback to clamp the DC value at the inverting input node to zero voltage. • This circuit suppresses the effect of parasitic capacitance because the inverting input is set at virtual ground. • Large RF is normally required, which may be difficult to implement on-chip. 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) 6 Switched-Capacitor Circuit Fig.14.34 • Two non-overlapping clock pulses • High switching frequency for the clocks • DC source for Vs 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) 7 Switched-Capacitor Circuit • φ1 turns on T1 and T3 ÆUnity-gain buffer ÆCharge C(x)Vs on capacitor C(x) • φ1 is low and turns off T1 and T3 ÆIsolating C(x) and turning the op-amp into an integrator • φ2 turns on T2 ÆGrounding left-terminal of C(x) ÆShifting the charge C(x)Vs of the right-terminal of C(x) to the leftterminal of C2 ÆThe circuit settles at Vo = C ( x) Vs ( ∵ C2Vo = C ( x)Vs ) C2 • Repeat the clock cycles. Vo alternates between zero and [C(x)/C2]Vs. A followed low-pass filter will give the average output. This circuit suppresses the parasitic capacitance effect because of the virtual ground of the inverting input. 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) 8 Voltage Follower z Voltage follower for differential capacitor • Symmetric positive and negative sinusoidal or pulse signals (+/-Vs) Vx = C1 − C2 Vs C1 + C2 + CP • Parasitic capacitance reduces the signal Æ Solution: a guard electrode driven by Vo - Increased fabrication complexity - Difficult to cancel all parasitics Guard electrode Substrate electrode 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) 9 Differential Capacitive Sensing z Transimpedance amplifier for differential capacitor C1 − C2 V0 = − Vs CF 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) 10 Demodulation: Peak Detector z Demodulation of a capacitive signal using a peak detector 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) 11 Synchronous Demodulators z Analog multiplier S (t ) cos (ωc t ) ⋅ Vr cos (ωc t + θ ) = S ( t ) Vr 2 After low-pass filtering, the output is which is phase-sensitive. cos θ + cos ( 2ωc t + θ ) Vr cos θ S (t ) 2 Analog Devices MLT04 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) 12 Synchronous Demodulators z Track-and-hold circuit • T4 and T2 are synchronized through φ2, • CT always holds previous C(x)Vs/C2 for one period and updates C(x)Vs/C2 every clock cycle. • R3C3 forms a low-pass filter that smoothes out the sampling steps. 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) 13 A Capacitive Measurement System z 11/24/2003 System block diagram EEL5225: Principles of MEMS Transducers (Fall 2003) 14 Offset Cancellation z Chopper-stabilized amplifiers Vos1, Vos2: input offsets of op-amp V0 = A( R1 + R2 ) ( v+ − Vos 2 ) AR1 + R1 + R2 During the φ1 phase, v+ = Vos1 During the φ2 phase, v+ = Vs + Vos1 ⇒ V2 = Vos 2 − Vos1 ⇒ V2 = Vs + Vos1 − Vos 2 After LPF, only Vs remains • This circuit can also cancel out low-frequency amplifier noise, 1/f noise in particular • Still affected by parasitic capacitance at the input node 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) 15 Offset Cancellation z Correlated Double Sampling Vos1, Vos2: input offsets of op-amp φ1 phase: V0,1 = − A1 AA Vos1 − 1 2 Vos 2 1 + A1 A2 1 + A1 A2 φ2 phase: V0,2 = − A1 ( C1 − C2 ) C1 + C2 + A1CF wher B = B→ 11/24/2003 Vs − B ⋅ V0,1 C1 + C2 + CF C1 + C2 + (1 + A1 ) CF 1 for large A1 A1 • This circuit can also cancel out low-frequency amplifier noise, 1/f noise in particular • Vos1 is attenuated by a factor of A1A2, while Vos2 is attenuated by a factor of A1 • NOT affected by parasitic capacitance at the input node EEL5225: Principles of MEMS Transducers (Fall 2003) 16 Capacitive Accelerometer z Accelerometer model a Anchor Spring Proof mass ma x = kx a m x = a x = x2 k ωr Brownian noise: an ,rms = 4k B Tb∆f = 11/24/2003 4k B T ωr ∆f mQ Displacement is proportional to acceleration, and can be picked up 9 piezoresistively 9 Piezoelectrically 9 Capacitively 9 Optically 9Thermally EEL5225: Principles of MEMS Transducers (Fall 2003) 17 Analog Devices (ADI) Accelerometers Form transistors on bare wafers first Then deposit and anneal MEMS structural materials NPN No CMP needed Only one interconnect metal layer Wet etch to release MEMS structures Need a dedicated production line NMOS Sensor Area Sensor Poly Met Passivations BPSG Thox Nwell 11/24/2003 Emitter Base NSD EEL5225: Principles of MEMS Transducers (Fall 2003) Courtesy of Mr. John Geen of Analog Devices, Inc. 18 Analog Devices (ADI) Accelerometers z z Accelerometer structure Accelerometer system block diagram 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) 19 Analog Devices (ADI) Accelerometers z Sensing mechanism anchor spring shuttle Vout 11/24/2003 Vs = ± α + β Vs a 2 EEL5225: Principles of MEMS Transducers (Fall 2003) 20 Analog Devices (ADI) Accelerometers 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) 21 Other MEMS accelerometers z Tunneling Accelerometer (T. Kenny, et al) dt ( I t ∝ VB exp −α I Φ dt ) VB: Bias voltage • Small dt is typically obtained by moving the tip closer to the counter electrode through an actuation force after the microstructure is released. • Force feedback to maintain constant distance. • High resolution: sub-µg/Hz1/2. 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) 22 Other MEMS accelerometers z DRIE CMOS-MEMS z-axis accelerometer (Xie, et al) z-spring anchor proof mass self-test actuator y x z 11/24/2003 Self-test actuator Top view sense comb fingers ¾ Size: 0.5mm x 0.6mm ¾ Resonance: 3.9 kHz ¾ Sensitivity: 2.6 mV/g (calculated 4.0 mv/g) ¾ Range: > 10 g ¾ Linearity: 0.5% (F.S.) ¾ Noise floor: 1 mg/Hz1/2 (Brownian 2.5 µg/Hz1/2) EEL5225: Principles of MEMS Transducers (Fall 2003) 23 Other MEMS accelerometers z Thermal MEMS accelerometer (MEMSIC, Inc.) • Consists of thermal resistor, thermocouples and air as the inertial mass. • Thermal heating creates a warm air bubble over the heating element. • Any change in the sensor’s motion and/or orientation causes the cooler air to force the heated bubble toward the end of the package cavity in the direction of acceleration. • This movement creates a temperature differential in the vicinity of the two thermocouples. Amplifying this difference produces an output signal that characterizes both the nature (e.g., shock or tilt) and the direction of the applied force. 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) www.memsic.com 24