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AD7147
(Rev.:Rev0)

CapTouch™ Programmable Controller for Single-Electrode Capacitance Sensors

Analog Devices 

THEORY OF OPERATION

The AD7147 and AD7147-1 are CDCs with on-chip

environmental compensation. They are intended for use in

portable systems requiring high resolution user input. The

internal circuitry consists of a 16-bit, ∑-Δ converter that can

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

13 input pins, CIN0 to CIN12, on the AD7147 or AD7147-1. 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 AD7147 has an SPI interface, and the AD7147-1

has an I2C interface, ensuring that the parts are compatible with

a wide range of host processors. AD7147 refers to both the

AD7147 and AD7147-1, unless otherwise noted, from this

point forward in this data sheet.

The AD7147 interfaces with up to 13 external capacitance

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

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

sensors consist of an electrode on a single- or multiple-layer

PCB that interfaces directly to the AD7147.

The AD7147 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 an

on-chip sequencer that controls how each of the capacitance

inputs is polled.

The AD7147 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

affect the operation of capacitance sensors. Transparent to the

user, the AD7147 performs continuous calibration to compen-

sate for these effects, allowing the AD7147 to consistently

provide error-free results.

The AD7147 requires a companion algorithm that runs on the

host or another microcontroller to implement high resolution

sensor functions, such as scroll bars or wheels. However, no

companion algorithm is required to implement buttons. Button

sensors are implemented on chip, entirely in digital logic.

The AD7147 can be programmed to operate in either full power

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

wake-up mode is particularly suited for portable devices that

AD7147

require low power operation to provide the user with significant

power savings and full functionality.

The AD7147 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 AD7147 operates

from a 2.6 V to 3.6 V supply and is available in a 24-lead, 4 mm ×

4 mm LFCSP.

CAPACITANCE-SENSING THEORY

The AD7147 measures capacitance changes from single-electrode

sensors. The sensor electrode on the PCB comprises one plate

of a virtual capacitor. The other plate of the capacitor is the user’s

finger, which is grounded with respect to the sensor input.

The AD7147 first outputs an excitation signal to charge the

plate of the capacitor. When the user comes close to the sensor,

the virtual capacitor is formed, with the user acting as the second

capacitor plate.

PLASTIC COVER

SENSOR PCB

Σ-Δ

ADC

16-BIT

DATA

AD7147

EXCITATION

SIGNAL

250kHz

Figure 19. Capacitance-Sensing Method

A square wave excitation signal is applied to CINx during

the conversion, and the modulator continuously samples the

charge going through CINx. The output of the modulator is

processed via a digital filter, and the resulting digital data is

stored in the CDC_RESULT_Sx registers for each conversion

stage, at Address 0x00B to Address 0x016.

Rev. 0 | Page 11 of 68

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