MC33941

Sensors

6

Freescale Semiconductor

PRINCIPLE OF OPERATION

The 33941 generates a low radio frequency sine wave with

nominal 5.0 V peak-to-peak amplitude. The frequency is set

by an external resistor and is optimized for 120 kHz. An

internal multiplexer routes the signal to one of the 7 terminals

under control of the ABC input terminals. A receiver

multiplexer simultaneously connected to the selected

electrode and routes its signal to a detector, which converts

the sine wave to a DC level. The DC level is filtered by an

external capacitor, is multiplied and offset to increase

sensitivity. All electrode outputs are grounded internally by

the device when not selected.

The amplitude and phase of the sinusoidal wave at the

electrode are affected by objects in proximity. A “capacitor” is

formed between the driving electrode and the object, each

forming a “plate” that holds the electric charge. The voltage

measured is an inverse function of the capacitance between

the electrode being measured, the surrounding electrodes,

and other objects in the electric field surrounding the

electrode. Increasing capacitance results in decreasing

voltage. The value of the series resistor (22kohm) was

chosen to provide a near linear relationship at 120 kHz over

a range of 10pF to 70pF.

While exploring applications using the E-Field chip, it is

always useful to approach the problem using the capacitor

model.

Figure 2. Conceptual Block Diagram

CAPACITOR MODEL

The capacitance measured by the E-Field IC is:

•

Proportional to the area of the electrode

•

Proportional to the dielectric constant of the material

between the electrodes

•

Inversely proportional to the distance between the objects

Figure 3. Capacitor Model

Detector

Low Pass Filter

Voltage Level Proportional to 1/C (voltage divider)

Drive level ~ 5 v p-p

Load Resistor

(22 K ohms)

Sine Generator

(120 KHz)

Detected Signal

Level Decreases

with Increasing

Capacitance

Electrodes

Capacitance

increases as

electrodes move

closer together

Capacitor Model

Virtual Ground

Stray Variable

Capacitance

Object

Detector

Low Pass Filter

Voltage Level Proportional to 1/C (voltage divider)

Drive level ~ 5 v p-p

Load Resistor

(22 K ohms)

Sine Generator

(120 KHz)

Detected Signal

Level Decreases

with Increasing

Capacitance

Electrodes

Capacitance

increases as

electrodes move

closer together

Capacitor Model

Virtual Ground

Stray Variable

Capacitance

Object

d

A

k

C

0

ε

=

C

k

d

C=The Capacitance in Farads (F)

A=The area of the plates in square meters (m2)

d=The distance between the plates in meters (m)

k=The dielectric constant of the material separating the plates

0=Is the permittivity of free space (8.85 x 10-12 F/m)

Table 4 Dielectric Constants of Various Materials

Dielectric Material

Thickness (mil)

k

Acrylic

84.5

2.4-4.5

Glass

74.5

7.5

Nylon Plastic

68

3.0-5.0

Polyester Film

10

3.2

Flexible Vinyl Film

9

2.8-4.5

Air

-

1

Water

-

80

Ice

-

3.2

Automotive Oil

-

2.1