MPR121 datasheet(12/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

Figure 4. MPR121 Electrode Measurement Charging Pad Capacitance

When measuring capacitance there are some inherent restrictions due to the methodology used. On the MPR121 the voltage

after charging must be in the range that is shown in Figure 5.

Figure 5.

The valid operating range of the electrode charging source is 0.7 V to (VDD - 0.7 V). This means that for a given VDD the valid

ADC (voltage visible to the digital interface) range is given by

Equation 3

and

Equation 4

These equations are represented in the graph. In the nominal case of VDD = 1.8 V the ADC range is shown below in Table 7.

Table 7.

VDD

ADChigh

ADClow

ADCmid

1.8

625.7778

398.2222

512

Electrode Charge Time

Electrode

Charging

Electrode

Discharging

Electrode voltage measured here

Electrode Discharge Time

Valid ADC Value s vs. VDD

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

ADC

low

0.7

------------ 1024

()

ADC

high

0.7

–

-------------------------- 1024

()

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