sx8639 Semtech Corporation, sx8639 Datasheet - Page 18

• Manufacturer Part Number: sx8639
• Description: Low Power, Capacitive Button And Wheel Touch Controller 8 Sensors With Enhanced Led Drivers And Proximity Sensing
• Manufacturer: Semtech Corporation

SX8639
Low Power, Capacitive Button and Wheel Touch Controller
(8 sensors) with Enhanced LED Drivers and Proximity Sensing
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
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.
A finger touch is in the order of one pF. If the finger approaches the sensor this occurs typically fast.
The ASI has the difficult task to detect and distinguish a small, fast changing capacitance, from a large, slow
varying capacitance. This would require a very precise, high resolution ADC and complicated, power consuming,
digital processing.
The SX8639 features a 16 bit DAC which compensates for the large, slow varying capacitance already in front of
the ADC. In other words the ADC converts only the desired small signal. In the ideal world the ADC will put out
zero ticks even if the external capacitance is as high as 100pF.
At each power-up of the SX8639 the Digital Compensation Values (DCV) are estimated by the digital processing
algorithms. The algorithm will adjust the compensation values such that zero ticks will be generated by the ADC.
Once the correct compensation values are found these will be stored and used to compensate each CAP pin.
If the SX8639 is shut down the compensation values will be lost. At a next power-up the procedure starts all over
again. This assures that the SX8639 will operate under any condition. Powering up at e.g. different temperatures
will not change the performance of the SX8639 and the host does not have to do anything special.
The DCVs do not need to be updated if the external conditions remain stable.
However if e.g. temperature changes this will influence the external capacitance. The ADC ticks will drift then
slowly around zero values basically because of the mismatch of the compensation circuitry and the external
capacitance.
In case the average value of the ticks become higher than the positive noise threshold (configurable by user) or
lower than the negative threshold (configurable by user) then the SX8639 will initiate a compensation procedure
and find a new set of DCVs.
Compensation procedures can as well be initiated by the SX8639 on periodic intervals. Even if the ticks remain
within the positive and negative noise thresholds the compensation procedure will then estimate new sets of
DCVs.
Finally the host can initiate a compensation procedure by using the I2C interface (in Active or Doze mode). This is
e.g. required after the host changed the sensitivity of sensors.
Revision 7_8, September 10
© 2010 Semtech Corp.
www.semtech.com
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