Datasheet

AD7142
Rev. A | Page 30 of 72
OUTPUTS
EXCITATION SOURCE
The excitation source onboard the AD7142 is a square wave
source with a frequency of 250 kHz. This excitation source
forms the electric field between the transmitter and receiver in
the external capacitance sensor PCB. The source is output from
the AD7142 on two pins, the SRC pin and the
SRC
pin (outputs
an inverted version of the source square wave). The
SRC
signal
offsets large external sensor capacitances.
SRC
is not used in the
majority of applications.
The source output can be disabled from both output pins
separately by writing to the control register bits (Address
0x000[13:12]). Setting Bit 12 in this register to 1 disables the
source output on the SRC pin. Setting Bit 13 in this register to 1
disables the inverted source output on the
SRC
pin.
C
SHIELD
OUTPUT
To prevent leakage from the external capacitance sensors, the
sensor traces can be shielded. The AD7142 has a voltage output
that can be used as the potential for any shield traces, C
SHIELD
.
The C
SHIELD
voltage is equal to AV
DD
/2.
The C
SHIELD
potential is derived from the output of the AD7142
internal amplifier, and is of equal potential to the CIN input
lines. Because the shield is at the same potential as the sensor
traces, no leakage to ground occurs. To eliminate any ringing on
the C
SHIELD
output, connect a 10 nF capacitor between the
C
SHIELD
pin and ground. This capacitor is required, whether
C
SHIELD
is used in the application or not
.
For most applications, C
SHIELD
is not used, and a ground plane is
used instead around the sensors.
GPIO
The AD7142 has one GPIO pin located at Pin 26. It can be
configured as an input or an output. The GPIO_SETUP
Bits[13:12] in the interrupt enable register determine how the
GPIO pin is configured.
Table 15. GPIO_SETUP Bits
GPIO_SETUP GPIO Configuration
00 GPIO disabled
01 Input
10 Output low
11 Output high
When the GPIO is configured as an output, the voltage level on
the pin is set to either a low level or a high level, as defined by
the GPIO_SETUP bits shown in
Table 1 5.
When the GPIO is configured as an input, the
GPIO_INPUT_CONFIG bits in the interrupt enable register
determine the response of the AD7142 to a signal on the GPIO
pin. The GPIO can be configured as either active high or active
low, as well as either edge-triggered or level-triggered, as listed
in
Tabl e 16.
Table 16. GPIO_INPUT_CONFIG Bits
GPIO_INPUT_CONFIG GPIO Configuration
00 Triggered on negative level (active low)
01 Triggered on positive edge (active high)
10 Triggered on negative edge (active low)
11 Triggered on positive level (active high)
When GPIO is configured as an input, it triggers the interrupt
output on the AD7142.
Table 14 lists the interrupt output
behavior for each of the GPIO configuration setups.
USING THE GPIO TO TURN ON/OFF AN LED
The GPIO on the AD7142 can be used to turn on and off LEDs
by setting the GPIO as either output high or low. Setting the
GPIO output high turns on the LED; setting the GPIO output
low turns off the LED. The GPIO pin connects to a transistor
that provides the drive current for the LED. Suitable transistors
include the KTC3875.
AD7142
GPIO
V
CC
KTC3875
OR SIMILAR
05702-061
Figure 45. Controlling LEDs Using the GPIO