AD7143 datasheet(12/57 Pages) AD

12 / 57 page

AD7143

Rev. 0 | Page 11 of 56

THEORY OF OPERATION

The AD7143 is a capacitance-to-digital converter (CDC) 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 converter that

converts a capacitive input signal into a digital value. There are

eight input pins, CIN0 to CIN7, on the AD7143. 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 AD7143 has an I2C interface, ensuring that the parts are

compatible with a wide range of host processors.

The AD7143 interfaces with up to eight 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 single or multiple layer PCB

that interface directly to the AD7143.

The AD7143 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 AD7143 has on-chip digital logic and 528 words of RAM

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

AD7143 performs continuous calibration to compensate for

these effects, allowing the AD7143 to give error-free results at

all times.

The AD7143 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 host software is required to implement buttons,

including 8-way button functionality. Button sensors are

implemented completely in digital logic on-chip with the status

of each button reported in interrupt status registers.

The AD7143 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 AD7143 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 AD7143 operates from a 2.6 V to 3.6 V supply, and is

available in a 16-lead, 4 mm × 4 mm LFCSP_VQ.

CAPACITANCE SENSING THEORY

The AD7143 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 17). Therefore, the total capacitance

measured at the receiver decreases when an object comes close

to the induced field.

EXCITATION

SIGNAL

250kHz

Σ-Δ

ADC

16-BIT

DATA

PLASTIC COVER

AD7143

PCB LAYER

Figure 17. Single Layer Sensing Capacitance Method

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

on the AD7143, while the transmitter and receiver are constructed

on a PCB that comprises the external sensor.

Registering a Sensor Activation

When a sensor is approached, the total capacitance associated

with that sensor, measured by the AD7143, changes. When the

capacitance changes to such an extent that a set threshold is

exceeded, the AD7143 registers this as a sensor touch and then

automatically updates the internal interrupt status registers.

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 AD7143

registers this as a true button activation. The same threshold

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

sliders or scroll wheels, are activated.

AD7143 Datasheet(PDF) 12 Page - Analog Devices

AD7143 Datasheet(HTML) 12 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

Datasheet Download

Link URL