DEMOMPR031 Freescale, DEMOMPR031 Datasheet - Page 16

• Manufacturer Part Number: DEMOMPR031
• Manufacturer: Freescale

Specifications of DEMOMPR031

• Lead Free Status / RoHS Status: Compliant

MPR03X
16
7
7.1
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
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,
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
rate it was charged.
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
Theory of Operation
Introduction
Capacitance Measurement
V
900
800
700
600
500
400
300
200
100
0
1.71
Figure 21. MPR03X Electrode Measurement Charging Pad Capacitance
Electrode Charge Time
Equation
1, where C is the pad capacitance on the electrode
1.91
Electrode
Charging
V
Figure
dV
Equation 2
dt
Valid ADC Values vs. V DD
=
2.11
I
=
Figure 22.
×
C
22.
V DD (V)
C
ADChigh
ADCmid
ADClow
I
T
T
.
The electrode is then discharged back to ground at the same
Electrode voltage measured here
Discharging
Electrode
2.31
Electrode Discharge Time
2.51
(Figure
21). All of the other electrodes
Freescale Semiconductor
2.71
2T
Equation 1
Equation 2
Sensors