MPR03X

Sensors

Freescale Semiconductor

17

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

,

Equation 3

and

.

Equation 4

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

.

Equation 5

7.3

Sensitivity

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,

.

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.

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.

Table 10.

VDD

ADChigh

ADClow

ADCmid

1.8

625.7778

398.2222

512

()

1024

7

.

DD

low

V

ADC

=

()()

1024

7

.

DD

DD

high

V

V

ADC

−

=

7

.

−

×

=

DD

low

V

T

I

C

7

.

T

I

C

high

×

=

ADC

V

T

I

C

×

×

=

1024

2

1024

ADC

V

T

I

dADC

dC

×

×

−

=