AD7147A Analog Devices, AD7147A Datasheet - Page 11

• Manufacturer Part Number: AD7147A
• Manufacturer: Analog Devices

Specifications of AD7147A

• Resolution (bits): 16bit
• # Chan: 13
• Sample Rate: 111SPS
• Interface: I²C/Ser 2-Wire,Ser,SPI
• Analog Input Type: Capacitive
• Ain Range: ± 8 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

THEORY OF OPERATION
The AD7147A and AD7147A-1 are CDCs with on-chip environ-
mental compensation. They are intended for use in portable
systems requiring high resolution user input. The internal
circuitry consists of a 16-bit, Σ-Δ converter that can change a
capacitive input signal into a digital value. There are 13 input
pins, CIN0 to CIN12. 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 AD7147A has an SPI interface,
and the AD7147A-1 has an I
are compatible with a wide range of host processors. AD7147A
refers to both the AD7147A and AD7147A-1, unless otherwise
noted, from this point forward in this data sheet.
The AD7147A 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 AD7147A.
The AD7147A 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 AD7147A 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 AD7147A performs continuous calibration to compen-
sate for these effects, allowing the AD7147A to consistently
provide error-free results.
The AD7147A 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.
2
C interface, ensuring that the parts
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The AD7147A 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 to provide the user
with significant power savings and full functionality.
The AD7147A 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 AD7147A operates from a 2.6 V to
3.6 V supply and is available in a 2.3 mm × 2.1 mm WLCSP.
CAPACITANCE SENSING THEORY
The AD7147A 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 AD7147A 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 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.
AD7147A
Figure 19. Capacitance-Sensing Method
SENSOR PCB
ADC
Σ-Δ
PLASTIC COVER
EXCITATION
SIGNAL
250kHz
16-BIT
DATA
AD7147A