Download Lecture33

EEL5225: Principles of MEMS Transducers (Fall 2003)
Instructor: Dr. Hui-Kai Xie
Accelerometers
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Capacitive Position Sensing
Circuits for Capacitive Sensing
ADI Capacitive Accelerometers
Other MEMS Accelerometers
Reading: Senturia, Chapter 19, p.497-530
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Note: Most of figures in this lecture are copied from
Senturia, Microsystem Design, Chapter 19.
EEL5225: Principles of MEMS Transducers (Fall 2003)
Lecture 33 by
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H.K. Xie 11/24/2003
Capacitive Position Sensing
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Capacitive Position Sensing
MEMS Capacitive Sensors:
• High impedance
• Small sensing capacitance
• Very small signal
• Parasitic capacitance
• Noise
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Differential Capacitive Sensing
V0 = −Vs +
=
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C1
( 2Vs )
C1 + C2
C1 − C2
Vs
C1 + C2
Differential Capacitive Sensing
z First order cancellation of many effects
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Temperature variations
Common mode rejection
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Circuits for Capacitive Sensing
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Interface circuits
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Demodulation Methods
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Peak detectors
Synchronous demodulators
Offset cancellation circuits
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Transimpedance amplifier
Transimpedance amplifier with feedback capacitor
Switched-capacitor circuits
Voltage follower
Chopper-Stabilized Amplifiers
Correlated Double Sampling
EEL5225: Principles of MEMS Transducers (Fall 2003)
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Transimpedance amplifier
Q = C ( x ) Vs
iC = C ( x )
dVs
∂C dx
+ Vs
dt
∂x dt
Vo = − RF iC
•
•
•
•
Parasitic capacitance is negligible
Output voltage depends on both the position x and velocity dx/dt
DC Vs: Output voltage is directly proportional to the velocity.
AC Vs: High frequency of Vs is desired.
• Large DC offset
• Sensitivity is proportional to RF. But large resistors are difficult to
implement on-chip for integrated sensors.
• Vs also generates electrostatic force which disturbs the position
of the rotor. Æ Small Vs or Short pulses
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EEL5225: Principles of MEMS Transducers (Fall 2003)
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Transimpedance Amplifier
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Transimpedance amplifier with a feedback capacitor
• Assume a high-frequency
AC source. Then velocitydependent term of iC can
be ignored.
• Assume ωRFCF >> 1.
C (x)
iC
Vo ≈ −
≈−
Vs
sC F
CF
• RF 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 RF is normally required, which may be difficult to implement
on-chip.
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EEL5225: Principles of MEMS Transducers (Fall 2003)
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Switched-Capacitor Circuit
Fig.14.34
• Two non-overlapping clock pulses
• High switching frequency for the clocks
• DC source for Vs
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EEL5225: Principles of MEMS Transducers (Fall 2003)
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Switched-Capacitor Circuit
• φ1 turns on T1 and T3
ÆUnity-gain buffer
ÆCharge C(x)Vs on capacitor C(x)
• φ1 is low and turns off T1 and T3
ÆIsolating C(x) and turning the op-amp into an integrator
• φ2 turns on T2
ÆGrounding left-terminal of C(x)
ÆShifting the charge C(x)Vs of the right-terminal of C(x) to the leftterminal of C2
ÆThe circuit settles at Vo = C ( x) Vs ( ∵ C2Vo = C ( x)Vs )
C2
• Repeat the clock cycles. Vo alternates between zero and [C(x)/C2]Vs. 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.
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EEL5225: Principles of MEMS Transducers (Fall 2003)
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Voltage Follower
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Voltage follower for differential capacitor
• Symmetric positive and negative
sinusoidal or pulse signals (+/-Vs)
Vx =
C1 − C2
Vs
C1 + C2 + CP
• Parasitic capacitance reduces the signal
Æ Solution: a guard electrode driven by Vo
- Increased fabrication complexity
- Difficult to cancel all parasitics
Guard electrode
Substrate electrode
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EEL5225: Principles of MEMS Transducers (Fall 2003)
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Differential Capacitive Sensing
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Transimpedance amplifier for differential
capacitor
C1 − C2
V0 = −
Vs
CF
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EEL5225: Principles of MEMS Transducers (Fall 2003)
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Demodulation: Peak Detector
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Demodulation of a capacitive signal using a
peak detector
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Synchronous Demodulators
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Analog multiplier
 S (t ) cos (ωc t )  ⋅ Vr cos (ωc t + θ )  =
S ( t ) Vr
2
After low-pass filtering, the output is
which is phase-sensitive.
 cos θ + cos ( 2ωc t + θ ) 
Vr cos θ
S (t )
2
Analog Devices MLT04
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Synchronous Demodulators
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Track-and-hold circuit
• T4 and T2 are synchronized through φ2,
• CT always holds previous C(x)Vs/C2 for one period and updates
C(x)Vs/C2 every clock cycle.
• R3C3 forms a low-pass filter that smoothes out the sampling
steps.
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EEL5225: Principles of MEMS Transducers (Fall 2003)
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A Capacitive Measurement System
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System block diagram
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Offset Cancellation
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Chopper-stabilized amplifiers
Vos1, Vos2: input
offsets of op-amp
V0 =
A( R1 + R2 )
( v+ − Vos 2 )
AR1 + R1 + R2
During the φ1 phase, v+ = Vos1
During the φ2 phase, v+ = Vs + Vos1
⇒ V2 = Vos 2 − Vos1
⇒ V2 = Vs + Vos1 − Vos 2
After LPF, only
Vs 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
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Offset Cancellation
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Correlated Double Sampling
Vos1, Vos2: input
offsets of op-amp
φ1 phase: V0,1 = −
A1
AA
Vos1 − 1 2 Vos 2
1 + A1 A2
1 + A1 A2
φ2 phase: V0,2 = −
A1 ( C1 − C2 )
C1 + C2 + A1CF
wher B =
B→
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Vs − B ⋅ V0,1
C1 + C2 + CF
C1 + C2 + (1 + A1 ) CF
1
for large A1
A1
• This circuit can also cancel out
low-frequency amplifier noise,
1/f noise in particular
• Vos1 is attenuated by a factor of
A1A2, while Vos2 is attenuated
by a factor of A1
• NOT affected by parasitic
capacitance at the input node
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Capacitive Accelerometer
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Accelerometer model
a
Anchor
Spring
Proof mass
ma x = kx
a
m
x = a x = x2
k
ωr
Brownian noise:
an ,rms = 4k B Tb∆f
=
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4k B T ωr ∆f
mQ
Displacement is proportional to
acceleration, and can be picked up
9 piezoresistively
9 Piezoelectrically
9 Capacitively
9 Optically
9Thermally
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Analog Devices (ADI) Accelerometers
ƒ Form transistors on bare wafers first
ƒ Then deposit and anneal MEMS
structural materials
ƒ
ƒ
ƒ
ƒ
NPN
No CMP needed
Only one interconnect metal layer
Wet etch to release MEMS structures
Need a dedicated production line
NMOS
Sensor Area
Sensor Poly
Met
Passivations
BPSG
Thox
Nwell
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Emitter
Base
NSD
EEL5225: Principles of MEMS Transducers (Fall 2003)
Courtesy of Mr. John Geen
of Analog Devices, Inc.
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Analog Devices (ADI) Accelerometers
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Accelerometer structure
Accelerometer system block diagram
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EEL5225: Principles of MEMS Transducers (Fall 2003)
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Analog Devices (ADI) Accelerometers
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Sensing mechanism
anchor
spring
shuttle
Vout
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Vs
=
± α + β Vs a
2
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Analog Devices (ADI) Accelerometers
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Other MEMS accelerometers
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Tunneling Accelerometer (T. Kenny, et al)
dt
(
I t ∝ VB exp −α I Φ dt
)
VB: Bias voltage
• Small dt 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/Hz1/2.
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EEL5225: Principles of MEMS Transducers (Fall 2003)
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Other MEMS accelerometers
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DRIE CMOS-MEMS z-axis accelerometer (Xie, et al)
z-spring
anchor
proof
mass
self-test
actuator
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x
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‰ Self-test actuator
‰ Top view
sense comb
fingers
¾ Size: 0.5mm x 0.6mm
¾ Resonance: 3.9 kHz
¾ Sensitivity: 2.6 mV/g
(calculated 4.0 mv/g)
¾ Range: > 10 g
¾ Linearity: 0.5% (F.S.)
¾ Noise floor: 1 mg/Hz1/2
(Brownian 2.5 µg/Hz1/2)
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Other MEMS accelerometers
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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.
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EEL5225: Principles of MEMS Transducers (Fall 2003)
www.memsic.com
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