1. Basics of Sensor Compensation for Capacitive Sensors
January 22, 2013 Randy Frank

2. Agenda Capacitive sensing
Techniques and available options that simplify users design-in of capacitive sensors
Touch sensors
Capacitive proximity sensor
Capacitive pressure sensor
Capacitive sensor interfaces

3. Capacitive Sensing
Global Capacitive Sensors Market worth $17.89 billion by 2018
Growing at a CAGR of 15% from 2013 to 2018
Motion
Proximity
Pressure
Touch

4. Capacitive Sensors C = K* Ε0 * A/D Where Q =C*V Where:
C= Capacitance (farads)
K= The Dielectric Constant of the material between the plates. (Air=1.0)
Ε0 = Permittivity of free space (a constant)
A= Area of the plates (Meters2)
D= Distance between the plates (Meters)
Q =C*V
Where:
Q = Current (amps)
C = Capacitance (farads)
V = Voltage

5. Touch Sensing The body provides one electrode

6. Types of Capacitive Touch Sensing
A capacitive touch sensor is based on one of the following capacitance types:
1. Surface capacitance: uniformly coated glass
2. Projected Capacitance: These are subdivided again into two types:
Self capacitance
Mutual capacitance

7. Mutual Capacitance Dual layer ITO (indium tin oxide)
Every intersection has its own mutual capacitance

8. Sigma-Delta Circuit Integrating to reduce measurement time and noise

9. Touch Sensing Comparison
Cypress Semiconductor

10. Capacitive and More
Freescale capacitive and resistive touch screen sensing
CRTOUCH
Ready to use

11. Touch Eval Kit
Microchip Technology: enhanced mTouch Capacitive Touch Evaluation Kit
Example touch designs
MCU hardware
Software

12. Proximity Sensor

13. Proximity Sensing

14. Capacitive Sensing Vs. Inductive sensors – metallic objects
Photoelectric sensors – needs light source
Magnetic sensors – detect magnetic materials

15. Tubular Capacitive Sensors
Eaton E53 for sensing liquid level and powdered or granulated materials. The self-contained devices detect both metallic and nonmetallic targets.

16. Interface Circuitry in Proximity Sensor
PEPPERL+FUCHS
As a target approaches a capacitive sensor, the oscillations increase until they reach a threshold level and activate a switching device.

17. Capacitive Pressure Sensor IC
Infineon KP236 - Absolute Pressure Sensor
Full compensation
Broken wire detection
High sensitivity and linearity
Customized transfer functions
Optional output clamping

18. Capacitive Sensor Interface
ZMDI’s ZSSC3123
CMOS IC for capacitance-to-digital conversion
For sensors with capacitances up to 260pF and a sensitivity of 125aF/LSB to 1pF/LSB (depending on resolution, speed, and range settings)
Accuracy as high as ±0.25% -40 to 125°C
Output: I2C, SPI, PDM or alarms
Avoids external calibration trimming

19. Capacitive IC Block Diagram
Humidity sensing
Weight scale
Tension control
ZSSC3123
Pulse Density Modulation (PDM) output

20. Programmable Controller
Analog Devices AD7142/43 >1 fF resolution 14 capacitance sensor input channels

21. Summary Capacitive circuits are not nearly as common as piezoresistive
Touch sensors are the exception
Many suppliers offer touch sensing
Capacitive Interface ICs simplify this task

22. Topics Later This Week Wednesday
A case study of smart sensor development
Thursday
Transitioning from analog sensing to the digital control world
Friday
Today’s smart sensors