Instructor: Dr. Hui-Kai Xie. Accelerometers

Transcription

1 Instructor: Dr. Hui-Kai Xie Accelerometers Capacitive Position Sensing Circuits for Capacitive Sensing ADI Capacitive Accelerometers Other MEMS Accelerometers Reading: Senturia, Chapter 19, p Note: Most of figures in this lecture are copied from Senturia, Microsystem Design, Chapter /24/ Lecture 33 by H.K. Xie 11/24/2003

2 Capacitive Position Sensing Capacitive Position Sensing MEMS Capacitive Sensors: High impedance Small sensing capacitance Very small signal Parasitic capacitance Noise 11/24/2003 2

3 Differential Capacitive Sensing ( 2 ) 1 V0 = Vs + Vs C1 + C2 C = C C C V + C s Differential Capacitive Sensing First order cancellation of many effects Temperature variations Common mode rejection 11/24/2003 3

4 Circuits for Capacitive Sensing Interface circuits Transimpedance amplifier Transimpedance amplifier with feedback capacitor Switched-capacitor circuits Voltage follower Demodulation Methods Peak detectors Synchronous demodulators Offset cancellation circuits Chopper-Stabilized Amplifiers Correlated Double Sampling 11/24/2003 4

5 Transimpedance amplifier ( ) Q = C x V ( ) s ic = C x + Vs s dv dt Cdx x dt V o = R i F C Parasitic capacitance is negligible Output voltage depends on both the position x and velocity dx/dt DC V s : Output voltage is directly proportional to the velocity. AC V s : High frequency of V s is desired. Large DC offset Sensitivity is proportional to R F. But large resistors are difficult to implement on-chip for integrated sensors. Vs also generates electrostatic force which disturbs the position of the rotor. Small Vs or Short pulses 11/24/2003 5

6 Transimpedance Amplifier Transimpedance amplifier with a feedback capacitor Assume a high-frequency AC source. Then velocitydependent term of i C can be ignored. Assume ωr F C F >> 1. V o i C sc F C ( x) C F V s R F 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 R F is normally required, which may be difficult to implement on-chip. 11/24/2003 6

7 Switched-Capacitor Circuit Fig Two non-overlapping clock pulses High switching frequency for the clocks DC source for V s 11/24/2003 7

8 Switched-Capacitor Circuit φ 1 turns on T 1 and T 3 Unity-gain buffer Charge C(x)V s on capacitor C(x) φ 1 is low and turns off T 1 and T 3 Isolating C(x) and turning the op-amp into an integrator φ 2 turns on T 2 Grounding left-terminal of C(x) Shifting the charge C(x)V s of the right-terminal of C(x) to the leftterminal of C 2 The circuit settles at ( ) V = Cx V ( C V = C( x) V ) o s 2 o s C2 Repeat the clock cycles. V o alternates between zero and [C(x)/C 2 ]V s. 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 8

9 Voltage Follower Voltage follower for differential capacitor Symmetric positive and negative sinusoidal or pulse signals (+/-V s ) V x = C C C + C + C P V s Parasitic capacitance reduces the signal Solution: a guard electrode driven by V o - Increased fabrication complexity - Difficult to cancel all parasitics Guard electrode Substrate electrode 11/24/2003 9

10 Differential Capacitive Sensing Transimpedance amplifier for differential capacitor V C = C V s CF 11/24/

11 Demodulation: Peak Detector Demodulation of a capacitive signal using a peak detector 11/24/

12 Analog multiplier Synchronous Demodulators S t Vr St ( )cos( ωct) Vr cos( ωct+ θ) = cosθ + cos( 2ωct+ θ) 2 V r cosθ After low-pass filtering, the output is S() t which is phase-sensitive. 2 ( ) Analog Devices MLT04 11/24/

13 Track-and-hold circuit Synchronous Demodulators T 4 and T 2 are synchronized through φ 2, C T always holds previous C(x)V s /C 2 for one period and updates C(x)V s /C 2 every clock cycle. R 3 C 3 forms a low-pass filter that smoothes out the sampling steps. 11/24/

14 A Capacitive Measurement System System block diagram 11/24/

15 Offset Cancellation Chopper-stabilized amplifiers V os1, V os2 : input offsets of op-amp AR ( + R) V = v V ( ) os2 AR1 + R1 + R2 During the φ phase, v = V V = V V 1 + os1 2 os2 os1 During the φ phase, v = V + V V = V + V V 2 + s os1 2 s os1 os2 After LPF, only V s 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/

16 Correlated Double Sampling Offset Cancellation V os1, V os2 : input offsets of op-amp φ 1 phase: φ 2 phase: A1 A1A2 V0,1 = Vos 1 Vos2 1+ AA AA 1 2 This circuit can also cancel out low-frequency amplifier noise, A1( C1 C2) V 1/f noise in particular 0,2 = Vs B V0,1 C1 + C2 + AC 1 F V os1 is attenuated by a factor of C A 1 A 2, while V os2 is attenuated 1 + C2 + CF wher B = C ( ) by a factor of A 1 + C A 1 C F 1 NOT affected by parasitic 1 B for large A capacitance at the input node A /24/

17 Accelerometer model Capacitive Accelerometer Anchor a ma x = x = m k Spring kx a x nrms, 4 = Brownian noise: a B a ω x 2 r = k Tb f = 4kT B ωr f mq Proof mass Displacement is proportional to acceleration, and can be picked up piezoresistively Piezoelectrically Capacitively Optically Thermally 11/24/

18 Analog Devices (ADI) Accelerometers Form transistors on bare wafers first Then deposit and anneal MEMS structural materials No CMP needed Only one interconnect metal layer Wet etch to release MEMS structures Need a dedicated production line NPN NMOS Sensor Area Sensor Poly Passivations BPSG Met Thox Nwell Emitter Base NSD Courtesy of Mr. John Geen of Analog Devices, Inc. 11/24/

19 Analog Devices (ADI) Accelerometers Accelerometer structure Accelerometer system block diagram 11/24/

20 Sensing mechanism Analog Devices (ADI) Accelerometers spring shuttle anchor V out Vs = ± α + 2 βv a s 11/24/

21 Analog Devices (ADI) Accelerometers 11/24/

22 Other MEMS accelerometers Tunneling Accelerometer (T. Kenny, et al) d t It VB exp( α I Φdt) V B : Bias voltage Small d t 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/hz 1/2. 11/24/

23 Other MEMS accelerometers DRIE CMOS-MEMS z-axis accelerometer (Xie, et al) z-spring Top view anchor Self-test actuator self-test actuator proof mass z y x sense comb fingers Size: 0.5mm x 0.6mm Resonance: 3.9 khz Sensitivity: 2.6 mv/g (calculated 4.0 mv/g) Range: > 10 g Linearity: 0.5% (F.S.) Noise floor: 1 mg/hz 1/2 (Brownian 2.5 µg/hz 1/2 ) 11/24/

24 Other MEMS accelerometers 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/