AD7148 Analog Devices, AD7148 Datasheet - Page 10

• Manufacturer Part Number: AD7148
• Manufacturer: Analog Devices

Specifications of AD7148

• Resolution (bits): 16bit
• # Chan: 8
• Sample Rate: 250kSPS
• Interface: I²C/Ser 2-Wire,Ser
• Analog Input Type: SE-Uni
• Ain Range: ± 8 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD7148
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
I
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.
2
C interface, ensuring that the part is compatible with a wide
Rev. A | Page 10 of 56
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.
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.
Figure 16. Capacitance Sensing Method
AD7148
SENSOR PCB
ADC
Σ-Δ
EXCITATION
SIGNAL
250kHz
PLASTIC COVER
16-BIT
DATA