SensorsResistive sensorsCapacitive sensorsStrain SensorsSimple piezoresistive pressure sensorSimple piezoresistive accelerometerSimple capacitive accelerometerSimple capacitive pressure sensorADXL50 AccelerometerADXL50 Sensing MechanismAnalog Devices Polysilicon MEMSADXL50 – block diagramMEMS Gyroscope ChipSlide 14Thermal NoiseEquipartitionNyquistSensor and interface electronicsSummaryksjp, 7/01MEMS Design & FabSensors•Resistive, Capacitive•Strain gauges, piezoresistivity•Simple XL, pressure sensor•ADXL50•Noiseksjp, 7/01MEMS Design & FabResistive sensors•R(x) = R0(1+x)•E.g. TCR, gauge factor•Generate thermal noise•Wheatstone bridge minimizes sensitivity to•Nominal resistance value•Power supply variation•Other inputs•R(x) = R0(1+x) (1+y)R0R(x)R0R0VxV+V-ksjp, 7/01MEMS Design & FabCapacitive sensors•Typically used to measure displacement•C ~= 0 A/d•Can be used in Wheatstone bridge (with AC excitation)•Sensitive to environmental coupling•Typically want amplifier very close•Typically need to shield other varying conductors•Definitely don’t want charge-trapping dielectrics nearby•No intrinsic noiseSeparation (d)Area (A)ksjp, 7/01MEMS Design & FabStrain Sensors•Shape changeL/L = a/a = - (Poisson’s ratio)•R(a,b,L) = L/A•R() = R0(1+(1+2) )•R() = R0(1+G )•Piezoresistive() = 0(1+ GP )•R() = R0(1+(GP+G) )•GP ~ -20, 30 (poly), ~100 (SCS)•Piezoelectric•Strain generates charge, charge generates strainFFksjp, 7/01MEMS Design & FabSimple piezoresistive pressure sensorksjp, 7/01MEMS Design & FabSimple piezoresistive accelerometerksjp, 7/01MEMS Design & FabSimple capacitive accelerometer•Cap wafer may be micromachined silicon, pyrex, …•Serves as over-range protection, and damping•Typically would have a bottom cap as well.C(x)=C(x(a))Cap waferksjp, 7/01MEMS Design & FabSimple capacitive pressure sensorC(x)=C(x(P))ksjp, 7/01MEMS Design & FabADXL50 Accelerometer•+-50g•Polysilicon MEMS & BiCMOS •3x3mm dieksjp, 7/01MEMS Design & FabADXL50 Sensing Mechanism•Balanced differential capacitor output•Under acceleration, capacitor plates move changing capacitance and hence output voltage•On-chip feedback circuit drives on-chip force-feedback to re-center capacitor plates.ksjp, 7/01MEMS Design & FabAnalog Devices Polysilicon MEMSksjp, 7/01MEMS Design & FabADXL50 – block diagramksjp, 7/01MEMS Design & FabSense CircuitElectrostatic Drive CircuitProof MassDigital OutputMEMS Gyroscope ChipRotation induces Coriolis accelerationJ. Seeger, X. Jiang, and B. Boserksjp, 7/01MEMS Design & FabMEMS Gyroscope Chip1m Drive0.01Å SenseJ. Seeger, X. Jiang, and B. Boserksjp, 7/01MEMS Design & FabThermal Noise•Fundamental limitation to sensor performance due to thermal noise•“White” noise, Johnson noise, Brownian motion all the same•Not the same as flicker, popcorn, 1/f noise•Equipartition theorem (energy perspective)•every energy storage mode will have ½ kBT of energy•Nyquist (power perspective):•Every dissipator will contribute PN = 4 kBT B•B = bandwidth of interest in Hzksjp, 7/01MEMS Design & FabEquipartition•½ kBT = 4x10-21 J @ room temperature (300K)•½ C V2 = ½ kBT •C=1pF Vn = 60uV (RMS value)•½ k x2 = ½ kBT•K = 1N/m xn = 0.06nm•½ m v2 = ½ kBT•m = 10-9 kg (~100um cube) vn = 2x10-6 m/sksjp, 7/01MEMS Design & FabNyquist•PN = 4 kBT B•In a resistor•PN = VN2/R = 4 kBT B•VN = sqrt(4 kBT R B)• = sqrt (4 kBT R) sqrt(B)•If R = 1kZ then•VN = 4nV/sqrt(Hz) sqrt(B)ksjp, 7/01MEMS Design & FabSensor and interface electronicsmVmeasurandtransducerLNANADCLow noiseamplifierFilterAnalog to DigitalConverterksjp, 7/01MEMS Design & FabSummary•Resistive and capacitive sensors most common•Sensing, amplification, filtering, feedback on the same chip ~$2•Minimum detectable signal limited by thermal
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