Lectures 1. Accelerometer conditioning circuits Micro-Nanoscience and Nanotechnology Lectures 1. Accelerometer conditioning circuits © 2013 Universitat Politècnica de Catalunya (EPSEVG) José Antonio Soria Pérez - Associate Professor - ETSEIAT (UPC). Office hours: EET - D126 (TR2, 2nd floor): TU 10-14h and 17-19h THU: 10-14h E-mail: jose.antonio.soria@eel.upc.edu, jasoria@eel.upc.edu

No room for spring coil, mass and dashpot in microscale accelerometers Accelerometer basics Measurement of dynamic forces associated to moving objects Model: Spring, “proof mass” and dashpot. Acceleration, Speed, Distance and even Force (Vibration). The accelerometer is attached to the vibrating solid body Spring No room for spring coil, mass and dashpot in microscale accelerometers ks Mass ? F(t).- Force (N) m (g) a (t).- Acceleration (m/s2 or ‘g’) Dashpot (with damping) Vibrating body Distance: (m.- metter) Speed: (m/s) Acceleration: (m/s2) 1m/s2 = 9.8-1G

MEM Accelerometer (at rest) Accelerometer basics Measurement of dynamic forces associated to moving objects Model: Spring, “proof mass” and dashpot. Acceleration, Speed, Distance and even Force (Vibration). Two micro-accelerometer types: 1.- The cantilever beam accelerometer 2.- Balance force micro-accelerometer (most commonly used) Silicon Cantilever Beam Piezoresistor Vibrating body Casing Constraint Base MEM Accelerometer (at rest) Stationary Electrodes Moving Electrode PLATE BEAM End Tethers Acceleration Plate beam = proof mass Two end tethers = coil spring Surrounding air = dashpot Cantilever = coil spring Piezoresistor = proof mass Surrounding fluif = dashpot Movement carried out by attached piezoresistor Beam Movement C2 C1 Movement measurement as capacitance change between electrodes

Acceleration transduction Single vs. Diferential configuration Capacitance (C or ΔC) as displacement change (x) Non-linear behavior in single configuration Diferential configuration most commonly used C0 Cx Single Configuration d Diferential Configuration d - x d + x C1 C2 d Diferential Configuration d - x d + x C2 = C0- ΔC x C1 = C0-+ΔC C0 Cx Single Configuration d x Neglecting ΔCx2 in small dispacements x x ε0.- permitivity (air) = 8.85pF/m εr.- permitivity (dielectric material) A.- Area of conductive plates d.- In between distance of plates x.- Displacement Linear expression

Acceleration transduction MEM’s layout Paral·lel capacitors increase sensibility AC voltage source necessary base (substrate) spring ks proof mass: movable microsturcture Fixed outer plates C01 C02 C0N Paral·lel Association motion x Diferential Configuration d - x d + x C2 = C0- ΔC x C1 = C0+ΔC 0º 180º C1 C2 vo + Sinusoid or Square Waveform vo is a transient source Sensibility Sensor Output (Vx) N.- Number of built-in capacitor pairs Sensibility = [Vs2/m] Units Need of a conditioning circuit for magnitude and sign detection!! V0.- Input peak value

Another capacitor in series matching nominal value Reactance circuits Capacitive transducers are a class of reactance circuits Capacitor’s impedance as main sensing variable when driving with AC sources Example: Analysis of the Voltage Divider conditioning circuit C0 Cx d x ≡ Z0(1+x) x ≡ Displacement of mass proof Assume: Transfer Function Voltage divider v0 + Z Z0 (1+x) Another capacitor in series matching nominal value = Z0 i + _ vx

2) Differential capacitor Reactance circuits Capacitive transducers are a class of reactance circuits Common conditioning circuits for capacitive sensors Voltage dividers: Output refferred to ground Bridge configurations: Differential Output 1) Single Capacitor v0 + Z0 Z0(1+x) _ vx 2) Differential capacitor Z0(1-x) Non-linear with offsett Linear 4) Adjacent connection Z0(1-x) Z0(1+x) v0 + _ vx 3) Fixed branch Z0 Double sensibility without offset Fourfold sensibility

Positive acceleration Negative acceleration Reactance circuits Capacitive transducers are a class of reactance circuits Common conditioning circuits for capacitive sensor Acceleration codification Shifted-phase square differential AC source Amplifier for coding acceleration at peak positions t vy(t) KF×a(t) -KF×a(t) Positive acceleration Negative acceleration v0(t) t vx(t) V0 -Vx Vs2/m v0 + C1 C2 K -V0 Vx + _ vx Signal Amplification + _ vy t vy(t) KF×a(t) -KF×a(t) MEM accelerometer with differential capacitor configuration Acceleration coded at peak positions Need of sign detection !!

Sign detection (Full accelerometer circuit) Sign detection by phase demodulation Sign changed at each positive input cycle by the analog comparator Analog Comparator + _ SW Phase demodulator 1 v0 + C1 C2 vx K vy vout = a(t) Negative Acceleration Positive Acceleration Increase of Acc. magnitude v0 t vy vout SW=0 v0 t vy vout v0 t vy vout SW=1

The ADXL345 accelerometer The conditioning circuit just presented is the sense electronics block of the ADXL345 accelerometer (one per axis) These consist of a relaxation oscillator, the MEM sensor, an amplifier and a phase demodulator Other necessary elements to handle accelreation data digitally are: The Analog-to-Digital converter for transforming the data into a “discrete-time” domain A digital low-pass filter for preventing from high-frequency switching noise The communication peripheral blocks: SPI (Serial Peripheral Interface) or I2C (Inter-Integrated Circuit) Relaxation Oscillator MEM Sensor Amplifer Phase demodulator v0 + C1 C2 K _ SW 1 To the ADC Converter The ADXL345 Evaluation Board

Reading datasheets Datsheets are instruction manuals for electronic components Usual sections: 1st. Page – General description of the device (A revision history is introduced here if necessary) 1srt. Section - Electrical Specifications and Absolute Ratings (Recommendded operation conditions) 2nd – Pinout configuration and function description 3rd – Plots showing device performance. 4rth – Timming diagrams and operating instructions (Programming devices only. It should explain exactly how it works) ... – Some manufactures introduce relevant application information in additional sections (Example schematics, operation principle, design examples…) Final – Package information (Outline Layout design and soldering instructions)

ADXL345 features 3-axis (Output range ± 2g, ± 4g, ± 8g, ± 16g; each axis) Supply voltage range: 2.0 – 3.6V (ultralow power device: 23μA) DAQ resolution: 13 bits (± 2g = 10, ± 4g = 11; ± 8g = 12; ± 16g = 13) Sensitivity (10 bit res.): ± 2g;± 4g = 256LSB/g; ± 4g = 128LSB/g; ± 8g = 64LSB/g; ± 16g = 32LSB/g Motion resolution: 3.9mg/LSB Communications: SPI (3 and 4 wire - syncronous) or I2C interfaces I/O volatage range: 1.7V Output rate (DAQ Bandwidth): 3200Hz ≡ 3200 samp./sec. Operating temperature range: from -70ºC to +105ºC (Test conditions: from -45ºC to +85ºC) Shock survival: 10000g.