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AD7148

Rev. 0 | Page 10 of 56

THEORY OF OPERATION

The AD7148 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 on

the AD7148: CIN0 to CIN7. 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 AD7148 has an

I2C interface, ensuring that the part is compatible with a wide

range of host processors.

The AD7148 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 an electrode on a single or multiple layer PCB that

interfaces directly to the AD7148.

The AD7148 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 to control how each of the capacitance inputs

is polled.

The AD7148 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 AD7148 performs continuous calibration to compen-

sate for these effects, allowing the AD7148 to give error-free

results at all times.

The AD7148 requires minimal companion software that runs

on the host or other microcontroller to implement high resolution

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

companion software is required to implement buttons. Button

sensors are implemented on chip, entirely in digital logic.

The AD7148 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

require low power operation, providing the user with significant

power savings and full functionality.

The AD7148 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 AD7148 operates from a 2.6 V to

3.6 V supply and is available in a 16-lead, 4 mm × 4 mm LFCSP.

CAPACITANCE SENSING THEORY

The AD7148 measures capacitance changes from sensors where

one plate is connected to ground. The sensor electrode on the

PCB makes up 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 AD7148 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.

Σ-Δ

ADC

PLASTIC COVER

AD7148

SENSOR PCB

EXCITATION

SIGNAL

250kHz

16-BIT

DATA

Figure 16. Capacitance Sensing Method

A square wave excitation signal is applied to the CINx input during

the conversion, and the modulator continuously samples the

charge going through the CINx pin. 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, located at Address 0x00B to Address 0x012.

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