EEL5225: Principles of MEMS Transducers (Fall 2003) Accelerometers Capacitive Position Sensing Circuits for Capacitive Sensing ADI Capacitive Accelerometers Other MEMS Accelerometers Reading: Senturia, Chapter 19, p.497-530 Lecture 33 by H.K. Xie 11/24/2003 EEL5225: Principles of MEMS Transducers (Fall 2003) Instructor: Dr. Hui-Kai Xie Note: Most of figures in this lecture are copied from Senturia, Microsystem Design, Chapter 19.
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11/24/2003 1EEL5225: Principles of MEMS Transducers (Fall 2003)
Accelerometers
Capacitive Position SensingCircuits for Capacitive SensingADI Capacitive AccelerometersOther MEMS Accelerometers
Reading: Senturia, Chapter 19, p.497-530
Lecture 33 by H.K. Xie 11/24/2003
EEL5225: Principles of MEMS Transducers (Fall 2003)Instructor: Dr. Hui-Kai Xie
Note: Most of figures in this lecture are copied from Senturia, Microsystem Design, Chapter 19.
11/24/2003 2EEL5225: Principles of MEMS Transducers (Fall 2003)
Capacitive Position Sensing
Capacitive Position Sensing
MEMS Capacitive Sensors:• High impedance• Small sensing capacitance• Very small signal• Parasitic capacitance• Noise
11/24/2003 3EEL5225: Principles of MEMS Transducers (Fall 2003)
( )10
1 2
1 2
1 2
2s s
s
CV V V
C CC C
VC C
= − ++
−=
+
Differential Capacitive Sensing
Differential Capacitive SensingFirst order cancellation of many effects
Temperature variationsCommon mode rejection
11/24/2003 4EEL5225: Principles of MEMS Transducers (Fall 2003)
Interface circuitsTransimpedance amplifierTransimpedance amplifier with feedback capacitorSwitched-capacitor circuitsVoltage follower
11/24/2003 5EEL5225: Principles of MEMS Transducers (Fall 2003)
( )
( )
s
sC s
Q C x VdV C dxi C x Vdt x dt
=
∂= +
∂
Transimpedance amplifier
o F CV R i= −
• 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. Small Vs or Short pulses
11/24/2003 6EEL5225: Principles of MEMS Transducers (Fall 2003)
Transimpedance amplifier with a feedback capacitor
Transimpedance Amplifier
( )Co s
F F
C xiV V
sC C≈ − ≈ −
• Assume a high-frequency AC source. Then velocity-dependent term of iC can be ignored.
• Assume ωRFCF >> 1.
• 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.
11/24/2003 7EEL5225: Principles of MEMS Transducers (Fall 2003)
• Two non-overlapping clock pulses• High switching frequency for the clocks• DC source for Vs
Fig.14.34
Switched-Capacitor Circuit
11/24/2003 8EEL5225: Principles of MEMS Transducers (Fall 2003)
22
( ) ( ( ) ) o s o sC xV V C V C x VC
= =∵
• φ1 turns on T1 and T3Unity-gain bufferCharge C(x)Vs on capacitor C(x)
• φ1 is low and turns off T1 and T3Isolating C(x) and turning the op-amp into an integrator
• φ2 turns on T2Grounding left-terminal of C(x)Shifting the charge C(x)Vs of the right-terminal of C(x) to the left-terminal of C2The circuit settles at
• Repeat the clock cycles. Vo alternates between zero and [C(x)/C2]Vs. A followed low-pass filter will give the average output.
Switched-Capacitor Circuit
This circuit suppresses the parasitic capacitance effect because of the virtual ground of the inverting input.
11/24/2003 9EEL5225: Principles of MEMS Transducers (Fall 2003)
Voltage follower for differential capacitor
1 2
1 2x s
P
C CV V
C C C−
=+ +
Voltage Follower
• Parasitic capacitance reduces the signalSolution: a guard electrode driven by Vo
- Increased fabrication complexity- Difficult to cancel all parasitics
• Symmetric positive and negative sinusoidal or pulse signals (+/-Vs)
Guard electrode
Substrate electrode
11/24/2003 10EEL5225: Principles of MEMS Transducers (Fall 2003)
Transimpedance amplifier for differential capacitor
1 20 s
F
C CV V
C−
= −
Differential Capacitive Sensing
11/24/2003 11EEL5225: Principles of MEMS Transducers (Fall 2003)
Demodulation of a capacitive signal using a peak detector
Demodulation: Peak Detector
11/24/2003 12EEL5225: Principles of MEMS Transducers (Fall 2003)
Analog multiplier
Synchronous Demodulators
( ) ( ) ( ) ( )( ) cos cos cos cos 22
rc r c c
S t VS t t V t tω ω θ θ ω θ⋅ + = + +
( )cos2
rV S tθAfter low-pass filtering, the output is
which is phase-sensitive.
Analog Devices MLT04
11/24/2003 13EEL5225: Principles of MEMS Transducers (Fall 2003)
Track-and-hold circuit
Synchronous Demodulators
• 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.
11/24/2003 14EEL5225: Principles of MEMS Transducers (Fall 2003)
System block diagram
A Capacitive Measurement System
11/24/2003 15EEL5225: Principles of MEMS Transducers (Fall 2003)
Chopper-stabilized amplifiers
Offset Cancellation
( )1 20 2
1 1 2
( )os
A R RV v V
AR R R +
+= −
+ +
Vos1, Vos2: input offsets of op-amp
1 1 2 2 1
2 1 2 1 2
During the phase, During the phase,
os os os
s os s os os
v V V V Vv V V V V V V
φφ
+
+
= ⇒ = −
= + ⇒ = + −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
11/24/2003 16EEL5225: Principles of MEMS Transducers (Fall 2003)
Correlated Double Sampling
Offset Cancellation
Vos1, Vos2: input offsets of op-amp
1 1 20,1 1 2
1 2 1 21 1os osA A A
V V VA A A A
= − −+ + • 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
( )
( )
1 1 20,2 0,1
1 2 1
1 2
1 2 1
11
wher 1
1 for large A
sF
F
F
A C CV V B V
C C ACC C C
BC C A C
BA
−= − − ⋅
+ ++ +
=+ + +
→
φ1 phase:
φ2 phase:
11/24/2003 17EEL5225: Principles of MEMS Transducers (Fall 2003)
Accelerometer model
Capacitive Accelerometer
2r
xx
x
aa
kmx
kxma
ω==
=
Proof massSpring
Anchor a
Displacement is proportional to acceleration, and can be picked up
11/24/2003 18EEL5225: Principles of MEMS Transducers (Fall 2003)
NPN NMOS Sensor Area
Thox
Nwell EmitterBase NSD
BPSG
Sensor Poly
MetPassivations
Courtesy of Mr. John Geen of Analog Devices, Inc.
Analog Devices (ADI) Accelerometers
Form transistors on bare wafers firstThen deposit and anneal MEMS structural materialsNo CMP neededOnly one interconnect metal layerWet etch to release MEMS structuresNeed a dedicated production line
11/24/2003 19EEL5225: Principles of MEMS Transducers (Fall 2003)
Accelerometer structure
Analog Devices (ADI) Accelerometers
Accelerometer system block diagram
11/24/2003 20EEL5225: Principles of MEMS Transducers (Fall 2003)
Sensing mechanism
2s
out sV
V V aα β= ± +
Analog Devices (ADI) Accelerometers
spring
anchor
shuttle
11/24/2003 21EEL5225: Principles of MEMS Transducers (Fall 2003)
Analog Devices (ADI) Accelerometers
11/24/2003 22EEL5225: Principles of MEMS Transducers (Fall 2003)
Tunneling Accelerometer (T. Kenny, et al)
Other MEMS accelerometers
dt
( )expt B I tI V dα∝ − Φ
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.
11/24/2003 23EEL5225: Principles of MEMS Transducers (Fall 2003)
11/24/2003 24EEL5225: Principles of MEMS Transducers (Fall 2003)
Thermal MEMS accelerometer (MEMSIC, Inc.)
Other MEMS accelerometers
www.memsic.com
• 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.