MPR031(2009) データシートの表示（PDF） - Freescale Semiconductor
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MPR031 Datasheet PDF : 32 Pages
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The valid operating range of the electrode charging source is 0.7V to (VDD-.7)V. This means that for a given VDD the valid ADC
(voltage visible to the digital interface) range is given by
ADClow
=
.7
V DD
(1024)
,
and
ADChigh
=
(VDD − .7) (1024)
VDD
.
Equation 3
Equation 4
These equations are represented in the graph. In the nominal case of VDD = 1.8V the ADC range is shown below in Table 10.
Table 10.
VDD
1.8
ADChigh ADClow ADCmid
625.7778 398.2222
512
Any ADC counts outside of the range shown are invalid and settings must be adjusted to be within this range. If capacitance
variation is of importance for an application after the current output, charge time and supply voltage are determined then the
following equations can be used. The valid range for capacitance is calculated by using the minimum and maximum ADC values
in the capacitance equation. Substituting the low and high ADC equations into the capacitance equation yields the equations for
the minimum and maximum capacitance values which are
7.3 Sensitivity
Clow
= I ×T
VDD − .7
and
Chigh
=
I
×T
.7
.
Equation 5
The sensitivity of the MPR03X is relative to the capacitance range being measured. Given the ADC value, current and time
settings capacitance can be calculated,
C = I × T ×1024 .
VDD × ADC
Equation 6
For a given capacitance the sensitivity can be measured by taking the derivative of this equation. The result of this is the
following equation, representing the change in capacitance per one ADC count, where the ADC in the equation represents the
current value.
dC = − I × T ×1024
dADC VDD × ADC 2
Equation 7
This relationship is shown in the following graph by taking the midpoints off all possible ranges by varying the current and time
settings. The midpoint is assumed to be 512 for ADC and the nominal supply voltage of 1.8V is used.
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