1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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 !!

9. 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

10. 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

11. 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)

12. 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.