MPR121_10 FREESCALE [Freescale Semiconductor, Inc], MPR121_10 Datasheet - Page 13

• Manufacturer Part Number: MPR121_10
• Description: Proximity Capactive touch Senosr controller
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

Sensors
Freescale Semiconductor
variation is of importance for an application after the current output, charge time and supply voltage are determined then the fol-
lowing 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
SENSITIVITY
and settings capacitance can be calculated,
following equation, representing the change in capacitance per one ADC count, where the ADC in the equation represents the
current value.
settings. The midpoint is assumed to be 512 for ADC and the nominal supply voltage of 1.8 V is used.
Table
range the equation is nonlinear, thus the sensitivity is best with the lowest capacitance. This graph shows the sensitivity derivative
reading across the valid range of capacitances for a set I, T, and V
21 pF) and a nominal 1.8 V supply.
Any ADC counts outside of the range shown are invalid and settings must be adjusted to be within this range. If capacitance
The sensitivity of the MPR121 is relative to the capacitance range being measured. Given the ADC value, current and time
For a given capacitance the sensitivity can be measured by taking the derivative of this equation. The result of this is the
This relationship is shown in the following graph by taking the midpoints off all possible ranges by varying the current and time
Smaller amounts of change indicate increased sensitivity for the capacitance sensor. Some sample values are shown in
In the previous cases, the capacitance is assumed to be in the middle of the range for specific settings. Within the capacitance
8.
-0.5
-1.5
-2.5
-3.5
-4.5
-1
-2
-3
-4
-5
0
0
C low
Figure 7
Table 8.
500
=
Sensitivity vs. Midpoint Capacitance for V DD = 1.8 V
100
pF
10
--------------- -
dADC
is representative of this effect.
------------------------- -
V
dC
DD
C
I
×
=
–
T
dC/dADC @cmid (pF/1 ADC Count)
=
-------------------------------
V
0.7
I
×
DD
Midpoint Capacitance (pF)
---------------------------------- -
V DD
1000
T
I
Sensitivity (pF/ADC count)
and
×
×
Figure 6.
×
T
1024
ADC
×
×
C high
ADC
1024
DD
.
-0.01953
-0.19531
. For simple small electrodes (that are approximately
2
1500
=
I
---------- -
0.7
×
T
.
2000
2500
Equation 5
Equation 6
Equation 7
MPR121
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