3-axis accelerometer and strain sensor readout MEMS-based capacitive

sensors

Juan Santana

Richard van den Hoven

Imec-Holst Centre

The Netherlands

Imec- Holst Centre1st MEMSCON Event - 07 October 2010, Bucharest

1st MEMSCON Event - 07 October 2010, Bucharest

Overview

Summary

• MEMS based charge sensing

• Accelerometers & Strain sensors

Technical Specifications• Programmable gain by timing control, sensitivity and

accuracy read-out

• Low-power

• TSMC CMOS 0.25m with operating voltage (3.0V)

• ±2 g input range, 1mg resolution

• High linearity (1%)

• Parasitics insensitive

Applications• Accelerometers, strain sensors for structural

assessment, automotive and BAN, industrial and healthcare

Accelerometer

Sensor area

Strain sensor

Imec- Holst Centre

1st MEMSCON Event - 07 October 2010, Bucharest

Architecture

Cin

SW5

VREF2VREF1

OA

Cs

Cr

26SW2

SW3SW1

SW4

GND

V1

V2

CONTROL

CLK,V1,V2\

Reset

Yout Zout Xout

Integrate

Sample

250

160

310

Xvalid34a

610

Architecture

Timing sequence

Novel Technique

Integration of multiple pulses

Variable gain set by number of pulses

Integrated noise proportional to √N

Timing allows duty-cycle control

Signal-to-noise ratio control

Decouples sensor from amplifier

Allows readout of wide range of sensors

Accurate sin(x)/x suppresses artifacts

Multiplexed output

Imec- Holst Centre

1st MEMSCON Event - 07 October 2010, Bucharest

ASIC

2mm

Y-channel

X-channel

Z-channel

4mm

Bias stage

MUX+buffer+digital

ESDSystem Level characteristics

3-channel architecture

1 Analog multiplexed buffered output

1 Analog non-multiplexed (X channel)

Synchronization signals available

Embedded filtering (sin(x)/x response)

1 Bias voltage (VDD) and 1 clock

Minimum capacitance ESD structures

Imec- Holst Centre

1st MEMSCON Event - 07 October 2010, Bucharest

Results

0

500

1000

1500

2000

2500

-30000 -20000 -10000 0 10000 20000 30000

Vout

[mV

]

Strain [µstrain]

0

5

10

15

20

25

0 100 200 300 400

Sens

itivi

ty (V

/g)

Number of pulses

Accelerometer

Strain sensor

Variable sensitivity ~N

y = 0.0003x0.4206

0.000

0.001

0.002

0.003

0.004

0.005

0 100 200 300 400

Tota

l inte

grat

ed n

oise

(V)

Number of pulses

Integrated Noise ~√N

Imec- Holst Centre

1st MEMSCON Event - 07 October 2010, Bucharest

Benchmarking

Parameter Denison Paavola

ISSCC2008

IMEC-NL

Acceleration

strain range

~±1g ±2g ±2.5g±20,000µeVariable gain

Supply voltage 1.7-2.2V 1.0V 3.0VPower 1.5µW 1.5µW 15µWNoise Floor

(accel.)

1mg/√Hz 704µg/√Hz 70µg/√Hz

Non-linearity <1% --- <1% (accelerometer)<0.6% (strain sensor)

Bandwidth 10Hz 1Hz 100HzFOM

F√(W/Hz)

µW·µg/Hz

8.1×10-22

1400 4.41×10-20

881

Technology 0.8µm CMOS 0.25µm CMOS 0.25µm CMOS

Area ---- 2.25mm² c 2 mm² (active)8 mm2

Imec- Holst Centre

State-of-the-art readout architectures

1st MEMSCON Event - 07 October 2010, Bucharest

Conclusions

Company

Versatile architecture for MEMS-based capacitive sensors

Cost effective design requires less external components

Low Power

Sampled output relaxes demands of following stages

Embedded filtering

Novel technique to control gain and SNR of the readout

Can be extended to many other types of capacitive based sensors