MPR03X

Sensors

18

Freescale Semiconductor

Figure 23.

Smaller amounts of change indicate increased sensitivity for the capacitance sensor. Some sample values are shown in Table 11.

In the above cases, the capacitance is assumed to be in the middle of the range for specific settings. Within the capacitance

range the equation is nonlinear, thus the sensitivity is best with the lowest capacitance. This graph shows the sensitivity derivative

21 pF) and a nominal 1.8V supply the following graph is representative of this effect.

Figure 24.

Table 11.

pF

Sensitivity (pF/ADC count)

10

-0.01953

100

-0.19531

Sensitivity vs. Midpoint Capacitance for VDD =1.8 V

-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

500

1000

1500

2000

2500

Midpoint Capacitance (pF)

dC/dADC @cmid (pF/1 ADC Count)

0

Sensitivity vs. Capacitance for VDD =1.8 Vand I=36 μA and T=.5 μS

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

10

12

14

16

18

20

22

24

26

28

30

Maximum

Minimum

C/ADC

Capacitance