MPR03X

Sensors

16

Freescale Semiconductor

7

Theory of Operation

7.1

Introduction

The MPR03X utilizes the principle that a capacitor holds a fixed amount of charge at a specific electric potential. Both the

implementation and the configuration will be described in this section.

7.2

Capacitance Measurement

The basic measurement technique used by the MPR03X is to charge up the capacitor C on one electrode input with a DC current

I for a time T (the charge time). Before measurement, the electrode input is grounded, so the electrode voltage starts from 0 V

and charges up with a slope, Equation 1, where C is the pad capacitance on the electrode (Figure 21). All of the other electrodes

are grounded during this measurement. At the end of time T, the electrode voltage is measured with a 10 bit ADC. The voltage

is inversely proportional to capacitance according to Equation 2

rate it was charged.

Equation 1

Equation 2

Figure 21. MPR03X Electrode Measurement Charging Pad Capacitance

When measuring capacitance there are some inherent restrictions due to the methodology used. On the MPR03X the voltage

after charging must be in the range that is shown in Figure 22.

Figure 22.

C

I

dt

dV

=

C

T

I

V

×

=

Electrode Charge Time

T

V

Electrode

Charging

Electrode

Discharging

Electrode voltage measured here

2T

Electrode Discharge Time

Valid ADC Values vs. VDD

0

100

200

300

400

500

600

700

800

900

1.71

1.91

2.11

2.31

2.51

2.71

VDD (V)

ADChigh

ADCmid

ADClow