HO19_315aSP09_cap_sensing

HO19_315aSP09_cap_sensing - Capacitive Sensing Boris...

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Unformatted text preview: Capacitive Sensing Boris Murmann Stanford University murmann@stanford.edu Copyright © 2009 by Boris Murmann EE315A ― HO #19 B. Murmann 1 Applications • Touchpad sensors p • Fingerprint sensors • Flow rate measurement • Biosensors • Inertial sensors – Accelerometers – Gyroscopes • … B. Murmann EE315A ― HO #19 2 Fingerprint Sensor (1) [Tartagni, JSSC 1/1998] B. Murmann EE315A ― HO #19 3 Fingerprint Sensors (2) [Tartagni, JSSC 1/1998] B. Murmann EE315A ― HO #19 4 Flow Rate Measurement [Chiang, JSSC 12/2006] EE315A ― HO #19 B. Murmann 5 Capacitive DNA Detection (1) [Stagni, JSSC 12/2006] B. Murmann EE315A ― HO #19 6 Capacitive DNA Detection (2) [Stagni, JSSC 12/2006] EE315A ― HO #19 B. Murmann 7 Automotive Inertial Sensor Applications Accelerometers and Gyroscopes for Navigation Accelerometers for Airbags http://www.semiconductors.bosch.de/pdf/SMB120_170_Product_Info.pdf Gyroscope for Rollover Detection B. Murmann EE315A ― HO #19 8 Electronic Stability Program (ESP) B. Murmann EE315A ― HO #19 9 Consumer-Grade Accelerometer B. Murmann EE315A ― HO #19 10 A Closer Look at Accelerometers a F = m ⋅a F k x m = a k x= mass spring constant http://www.semiconductors.bosch.de/pdf/SMB120_170_Product_Info.pdf EE315A ― HO #19 B. Murmann Displacement 11 Capacitance Change Structure at Rest With Applied Acceleration [ADXL50 datasheet] B. Murmann EE315A ― HO #19 12 Model of a Typical Sensor Element [Petkov] EE315A ― HO #19 B. Murmann 13 High-Impedance Readout Circuit [Boser & Howe, JSSC 3/1996] • Use chopper stabilization to mitigate offset and 1/f noise issues • CP is often comparable to sense capacitance – I t d Introduces undesired d i d attenuation Need several MΩ B. Murmann EE315A ― HO #19 Bootstrapping of parasitic capacitance (shield) ( hi ld) 14 Sense Voltage http://www.ee.ucla.edu/~wu/ee250b/Case%20study-Capacitive%20Accelerometer.pdf EE315A ― HO #19 B. Murmann 15 How Much Signal Can We Get? x m 1 = = 2 a k ωr • At low frequencies • Want low ωr – But this limits the usable bandwidth • Unless special “tricks” (like feedback) are used sed [Boser & Howe, JSSC 3/1996] B. Murmann • Let’s assume ωr=2π·5kHz and a=1mg – Displacement will be only ~10pm p y p (~x0/100,000 for x0=1μm) – Assuming C=1pF and x0=1μm, capacitance change is only 10aF p g y – Assuming Vs=2.5V, and 2x attenuation output voltage is 2.5V/100,000/2 = 12.5μV μ EE315A ― HO #19 16 Noise (1) • Damping “b” introduces noise – Due to Brownian motion of air molecules • For ωr=2π·5kHz, m=0.25μg and Q 0.5, Q=0.5, we have a2 μg ≅ 200 Δf Hz 2 vx μg μV μV ≅ 200 ⋅ 12.5 = 2.5 mg Δf Hz Hz [Boser & Howe, JSSC 3/1996] EE315A ― HO #19 B. Murmann 17 Noise (2) • For vacuum packaged devices much higher Q can be achieved • Suppose ωr=2π·5kHz m=0 25μg and Q=50 000; we then have =2π·5kHz, m=0.25μg Q=50,000; a2 μg ≅1 Δf Hz 2 vx μg μV nV ≅1 ⋅ 12.5 = 12.5 Δf mg Hz Hz • MOS thermal noise • Assuming γ=1, equal noise from sensor and amplifier and no other noise source th i 4kT gm = ≅ 100 μS 2 2 vn 1 = 4kT γ gm Δf nV ⎞ ⎛ ⎜ 12.5 ⎟ Hz ⎠ ⎝ B. Murmann EE315A ― HO #19 18 Readout Circuit Variants (1) • Correlated double sampling • Eliminates large bias resistor and demodulator • Noise folding penalty due to output sampling g [Boser & Howe, JSSC 3/1996] EE315A ― HO #19 B. Murmann 19 Readout Circuit Variants (2) • Correlated double sampling at output of first amplifier • Differential signal Diff ti l i l path comes with usual benefits (PSRR, ) (PSRR …) • kT/C noise on Cs is cancelled along with offset and flicker ff t d fli k noise of the amplifier [Petkov] B. Murmann EE315A ― HO #19 20 C/V Amplifier Implementation • Small i S ll signals, 5V l supply plenty of headroom for cascodes [Petkov] B. Murmann EE315A ― HO #19 21 Continuous Time Force Feedback B. Murmann EE315A ― HO #19 22 Benefits of Feedback • Sensing fingers are kept near zero displacement g g p p – Improves linearity – Prevents structure from pull-in/stiction when excited near resonance (important for high Q vacuum) Q, • Usable bandwidth increased by loop gain – Can reduce sensor resonance frequency to improve sensitivity • Reduces drift – Gain no longer set by sensor parameters EE315A ― HO #19 B. Murmann 23 Digital Force Feedback (1) • Provides inherent A/D conversion • Output is a pulse density modulated d it d l t d bit stream – Sigma-delta modulation, d l ti see EE315B B. Murmann [Petkov] EE315A ― HO #19 24 Digital Force Feedback (2) [Petkov] B. Murmann EE315A ― HO #19 25 Architecture Comparison (1) B. Murmann EE315A ― HO #19 26 Architecture Comparison (2) EE315A ― HO #19 B. Murmann 27 Gyroscope Principle (1) Frame S Sense Proof Mass Drive [Chinwuba D. Ezekwe] B. Murmann EE315A ― HO #19 28 Gyroscope Principle (2) Proof mass oscillates ~10μm • Rotation causes sub pico-meter pico meter displacement in sense direction • Displacement is measured through capacitive readout, similar to accelerometer Se nse • D ri ve Clockwise rotation [Chinwuba D. Ezekwe] B. Murmann EE315A ― HO #19 29 Example: ADXRS150/300 • Resolves a capacitance change of 10-20 F in a BW of 1 Hz • Corresponding displacement is 10-14 m – Classical radius of an electron (!) http://www.analog.com/library/analogDialogue/archives/37-03/gyro.html http://www analog com/library/analogDialogue/archives/37 03/gyro html B. Murmann EE315A ― HO #19 30 References • Bernhard Boser’s MEMS notes, http://www.eecs.berkeley.edu/ boser/pdf/index.htm http://www eecs berkeley edu/~boser/pdf/index htm • V. P. Petkov and B. E. Boser, "Capacitive interfaces for MEMS," in Enabling technology for MEMS and Nanodevices, Baltes, Brand, Fedder, Hi ld Korvink, Tabata, d Wiley-VCH, B d F dd Hierold, K i k T b t eds., Wil VCH 2004, pp. 49-92. • N. Yazdi, F. Ayazi, and K. Najafi, "Micromachined inertial sensors," Proceedings of the IEEE, vol. 86, pp. 1640-1659, 1998. • V.P. Petkov and B.E. Boser, "A fourth order Σ∆ interface for A fourth-order micromachined inertial sensors," IEEE J. Solid-State Circuits, vol.40, no.8, pp. 1602-1609, Aug. 2005. B. Murmann EE315A ― HO #19 31 ...
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This note was uploaded on 08/13/2009 for the course EE 315 taught by Professor Borismurmann during the Spring '09 term at Stanford.

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