Freescale Semiconductor Evaluation Board MPR031/2 for 2- or 3-pad touch sensor controllers DEMOMPR031 DEMOMPR031 データシート

Sensors
Freescale Semiconductor

17

The valid operating range of the electrode charging source is 0.7V to (V

DD

-.7)V. This means that for a given V

DD 

the valid ADC 

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

 , 

and

 . 

 

These equations are represented in the graph. In the nominal case of V

DD

 = 1.8V the ADC range is shown below in 

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

  

 and 

.

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.

1.8

625.7778

398.2222

512





1024

7

.





 

1024

7

.





7

.







7

.













1024

2

1024









