MPR031EPR2 Freescale, MPR031EPR2 Datasheet - Page 17

• Manufacturer Part Number: MPR031EPR2
• Manufacturer: Freescale

Specifications of MPR031EPR2

• Lead Free Status / RoHS Status: Compliant

Sensors
Freescale Semiconductor
The valid operating range of the electrode charging source is 0.7V to (V
(voltage visible to the digital interface) range is given by
These equations are represented in the graph. In the nominal case of V
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
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,
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.
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.
Sensitivity
C
Table 10.
low
VDD
=
dADC
1.8
dC
V
DD
C
ADC
I
×
=
ADC
−
T
=
ADChigh
7 .
V
625.7778
I
high
−
×
DD
V
and
T
low
I
=
×
DD
×
×
(
=
ADC
T
1024
V
×
and
C
V
DD
×
V
ADC
7 .
high
DD
1024
DD
398.2222
ADClow
−
(
7 .
=
1024
.
2
) (
I
DD
DD
1024
7 .
×
-.7)V. This means that for a given V
)
= 1.8V the ADC range is shown below in
T
ADCmid
)
.
512
.
,
DD
the valid ADC
Equation 3
Equation 4
Equation 5
Equation 6
Equation 7
Table
MPR03X
10.
17