Data Sheet

MPR121 Sensor
4 Freescale Semiconductor
Capacitance Measurement and Touch Sensing
1. Capacitance Measurement
The complete capacitance measurement system is composed by sensing electrode pads connected to the MPR121 sensing
inputs, and the MPR121 communicating with the host processor via the I
2
C bus and Interrupt output. The total measureable
sensing channels is 13 channels, including 12 physical electrode inputs and one multiplexed 13
th
channel for proximity
detection. A multiplexer is included internally at the front end, so the 13 channels are measured sequentially in time. After the
capacitance is measured, it then get noise filtered and finally touch /release status is determined.
Apart from touch sensing, MPR121 can also be used for general purpose capacitive detection in various industry applications.
The 10bit output data (or even the 8bit baseline value providing an even higher level of noise rejection for slowly changing
mediums) can be used as the capacitance measurement output relating to the measured parameters such as the water level,
displacement, or medium content change.
Figure 3. MPR121 Block Diagram and Capacitance Measurement
The capacitance measured on each sensing channel, is the total capacitance to ground which can be the combination of
background parasitic capacitance to ground (Cb) and finger touch induced capacitance to ground (Cx). This ground refers to
the common ground when the device is not electrically floated, and refers to the MPR121 device ground when the device is
floated for example powered by batteries. For a typical application, for example the Cb can be about 10pF, and the Cx can be
below 1pF. The two values can be much smaller in battery powered portable application, which requires more attention on
touch sensitivity design.
The MPR121 uses a constant DC charge current scheme for capacitance measurement. Each channel is charged and then
discharged completely to ground periodically to measure the capacitance (see Figure 4). All the channels are measured
sequentially, when one channel is in the charge/discharge and measurement period the other channels are shorted to ground.
Figure 4. MPR121 Capacitance Measurement
Cap Cx: ~0.01pf
induced by finger
Common Ground
10bit ADC
CC/ Charge current &
time control
Multiplex
LED,
GPIO control
To
host
auto configure,
baseline tracking,
touch/release detection
I2C
logic
Clock Gen
Power control
Cap Cb: ~10pf
background
Stray on input
12 channels
Filter
C2 :10~100pf,Stray Cap
to GND, or short to GND
directly
C1 : ~100pf,Stray
Cap to GND