sx8643 Semtech Corporation, sx8643 Datasheet - Page 15

• Manufacturer Part Number: sx8643
• Description: Ultra Low Power, Capacitive Button Touch Controller 12 Sensors With Enhanced Led Drivers
• Manufacturer: Semtech Corporation

SX8643
Ultra Low Power, Capacitive Button Touch Controller
(12 sensors) with Enhanced LED Drivers
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
Analog Sensor Interface
Figure 11
To get the ticks representing the charge on a specific sensor the ASI will execute several steps.
The charge on a sensor cap (e.g. CAP0) will be accumulated multiple times on the external integration capacitor,
Cint.
This results in an increasing voltage on Cint proportional to the capacitance on CAP0.
At this stage the offset compensation DAC is enabled. The compensation DAC generates a voltage proportional
to an estimation of the external capacitance. The estimation is obtained by the offset compensation procedure
executed e.g. at power-up.
The difference between the DAC output and the charge on Cint is the desired signal. In the ideal case the
difference of charge will be converted to zero ticks if no finger is present and the number of ticks becomes high in
case a finger is present.
The difference of charge on Cint and the DAC output will be transferred to the ADC (Sigma Delta Integrator).
After the charge transfer to the ADC the steps above will be repeated.
The larger the number the cycles are repeated the larger the signal out of the ADC with improved SNR. The
sensitivity is therefore directly related to the number of cycles.
The SX8643 allows setting the sensitivity for each sensor individually in applications which have a variety of
sensors sizes or different overlays or for fine-tuning performances. The optimal sensitivity is depending heavily on
the final application. If the sensitivity is too low the ticks will not pass the thresholds and it is not possible to detect
fingers. In case the sensitivity is set too large a finger hovering above the sensors will already be detected before
the finger really touches the overlay resulting in false detections.
Once the ASI has finished the first sensor, the ticks are stored and the ASI will start measuring the next sensor
until all (enabled) sensors pins have been treated.
In case some sensors are disabled then these result in lower power consumption simply because the ASI is active
for a shorter period and the following processing period will be shorter.
The ticks from the ASI will then be handled by the digital processing.
3.8 Offset Compensation
The capacitance at the CAP pins is determined by an intrinsic capacitance of the integrated circuit, the PCB
traces, ground coupling and the sensor planes. This capacitance is relatively large and might become easily some
tens of pF. This parasitic capacitance will vary only slowly over time due to environmental changes.
Revision 7_5, February 10
© 2010 Semtech Corp.
www.semtech.com
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