MPR121_10 Datasheet, PDF (12/57 Pages) Freescale Semiconductor, Inc – Proximity Capactive touch Senosr controller

Electrode voltage measured here
V
Electrode
Charging
Electrode
Discharging
Electrode Charge Time _T
Electrode Discharge Time 2T
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.
Valid ADC Values vs. VDD
900
800
ADChigh
700
600
ADCmid
500
400
ADClow
300
200
100
0
1.71
1.91
2.11
VDD (V)
2.31
Figure 5.
2.51
2.71
The valid operating range of the electrode charging source is 0.7 V to (V_DD - 0.7 V). This means that for a given V_DD the valid
ADC (voltage visible to the digital interface) range is given by
ADClow = _V---⁰-D--^.--⁷-D---(1024),
and
ADChigh
=
V----D-----D------–_{----0---.--7- (1024)}
VDD
.
Equation 3
Equation 4
These equations are represented in the graph. In the nominal case of V_DD = 1.8 V the ADC range is shown below in Table 7.
Table 7.
V_DD
ADC_high
ADC_low
ADC_mid
1.8
625.7778 398.2222
512
MPR121
12
Sensors
Freescale Semiconductor