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AD7142 Datasheet

  • AD7142

  • Programmable Capacitance-to-Digital Converter with Environme...

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AD7142
OUTPUTS
EXCITATION SOURCE
The excitation source on board the AD7142 is a square wave
source with a frequency of 240 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.
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
00
01
10
11
GPIO Configuration
GPIO disabled
Input
Output low
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 15.
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 Table 16.
Table 16. GPIO_INPUT_CONFIG Bits
GPIO_INPUT_CONFIG
00
01
10
11
GPIO Configuration
Triggered on negative level
(active low)
Triggered on positive edge
(active high)
Triggered on negative edge
(active low)
Triggered on positive level
(active high)
C
SHIELD
OUTPUT
To prevent leakage from the external capacitance sensors, the
sensor traces are 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.
For 4-layer sensors, C
SHIELD
is connected to a copper plane
around the sensors on layers 2, 3, and 4. A ground copper plane
is used on Layer 1 around the source electrodes. On a 2-layer
sensor, C
SHIELD
is connected to a copper plane around the
sensors on the bottom layer. A ground copper plane is used on
the top layer around the source electrodes.
Figure 42 shows how the sensor traces are shielded by running
traces connected to the shield potential around the sensor traces.
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.
V
CC
KTC3875
OR SIMILAR
AD7142
SENSOR PCB
C
SHIELD
10nF
05702-032
AD7142
GPIO
Figure 42. Shielding the Sensor Traces
05702-061
Figure 43. Controlling LEDs Using the GPIO
Rev. 0 | Page 29 of 68

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