AD7142 datasheet(11/68 Pages) AD

11 / 68 page

AD7142

Rev. 0 | Page 11 of 68

THEORY OF OPERATION

The AD7142 and AD7142-1 are capacitance-to-digital

converters (CDCs) with on-chip environmental compensation,

intended for use in portable systems requiring high resolution

user input. The internal circuitry consists of a 16-bit, ∑-Δ con-

verter that converts a capacitive input signal into a digital value.

There are 14 input pins on the AD7142 and AD7142-1, CIN0 to

CIN13. A switch matrix routes the input signals to the CDC.

The result of each capacitance-to-digital conversion is stored in

on-chip registers. The host subsequently reads the results over

the serial interface. The AD7142 contains an SPI interface and

the AD7142-1 has an I2C interface ensuring that the parts are

compatible with a wide range of host processors. Because the

AD7142 and AD7142-1 are identical parts, with the exception of

the serial interface, AD7142 refers to both the AD7142 and

AD7142-1 throughout this data sheet.

The AD7142 interfaces with up to 14 external capacitance

sensors. These sensors can be arranged as buttons, scroll bars,

wheels, or as a combination of sensor types. The external

sensors consist of electrodes on a 2- or 4-layer PCB that

interfaces directly to the AD7142.

The AD7142 can be set up to implement any set of input

sensors by programming the on-chip registers. The registers can

also be programmed to control features such as averaging,

offsets, and gains for each of the external sensors. There is a

sequencer on-chip to control how each of the capacitance

inputs is polled.

The AD7142 has on-chip digital logic and 528 words of RAM

that are used for environmental compensation. The effects of

humidity, temperature, and other environmental factors can

effect the operation of capacitance sensors. Transparent to the

user, the AD7142 performs continuous calibration to compen-

sate for these effects, allowing the AD7142 to give error-free

results at all times.

The AD7142 requires some minor companion software that

runs on the host or other microcontroller to implement high

resolution sensor functions such as a scroll bar or wheel.

However, no companion software is required to implement

buttons, including 8-way button functionality. Button sensors

are implemented completely in digital logic on-chip.

The AD7142 can be programmed to operate in either full power

mode, or in low power automatic wake-up mode. The

automatic wake-up mode is particularly suited for portable

devices that require low power operation giving the user

significant power savings coupled with full functionality.

The AD7142 has an interrupt output, INT, to indicate when

new data has been placed into the registers. INT is used to

interrupt the host on sensor activation. The AD7142 operates

from a 2.6 V to 3.6 V supply, and is available in a 32-lead, 5 mm ×

5 mm LFCSP_VQ.

CAPACITANCE SENSING THEORY

The AD7142 uses a method of sensing capacitance known as

the shunt method. Using this method, an excitation source is

connected to a transmitter generating an electric field to a

receiver. The field lines measured at the receiver are translated

into the digital domain by a ∑-Δ converter. When a finger, or

other grounded object, interferes with the electric field, some of

the field lines are shunted to ground and do not reach the

receiver (see Figure 16). Therefore, the total capacitance

measured at the receiver decreases when an object comes close

to the induced field.

EXCITATION

SIGNAL

240kHz

Σ-Δ

ADC

16-BIT

DATA

PLASTIC COVER

AD7142

PCB LAYER 1

PCB LAYER 2

Figure 16. Sensing Capacitance Method

In practice, the excitation source and ∑-Δ ADC are implemented

on the AD7142, while the transmitter and receiver are constructed

on a PCB that makes up the external sensor.

Registering a Sensor Activation

When a sensor is approached, the total capacitance associated

with that sensor, measured by the AD7142, changes. When the

capacitance changes to such an extent that a set threshold is

exceeded, the AD7142 registers this as a sensor touch.

Preprogrammed threshold levels are used to determine if a

change in capacitance is due to a button being activated. If the

capacitance exceeds one of the threshold limits, the AD7142

registers this as a true button activation. The same thresholds

principle is used to determine if other types of sensors, such as

sliders or scroll wheels, are activated.

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