SX8663 datasheet(17/64 Pages) SEMTECH

17 / 64 page

ADVANCED COMMUNICATIONS & SENSING

Rev5 4

th August 2011

www.semtech.com

SX8663

Capacitive Button Matrix (up to 36) and Proximity Controller

with Individual LED Drivers and Buzzer Output

3.7

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 SX8663 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 SX8663 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 SX8663 is shut down the compensation values will be lost. At a next power-up the procedure starts all over

again. This assures that the SX8663 will operate under any condition. Powering up at e.g. different temperatures

will not change the performance of the SX8663 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 SX8663 will initiate a compensation procedure

and find a new set of DCVs.

Compensation procedures can as well be initiated by the SX8663 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. This is e.g. required after the

host changed the sensitivity of sensors.

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