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AD7142

Rev. 0 | Page 29 of 68

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.

CSHIELD 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, CSHIELD.

The CSHIELD voltage is equal to AVDD/2.

The CSHIELD 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 CSHIELD output, connect a 10 nF capacitor between the

CSHIELD pin and ground.

For 4-layer sensors, CSHIELD 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, CSHIELD 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.

10nF

AD7142

Figure 42. Shielding the Sensor Traces

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

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

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)

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

VCC

KTC3875

OR SIMILAR

Figure 43. Controlling LEDs Using the GPIO

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