SX8661 datasheet(22/88 Pages) SEMTECH

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ADVANCED COMMUNICATIONS & SENSING

DATASHEET

Revision v3.2, January 2011

www.semtech.com

SX8661

Low Power, Capacitive Button Touch and Proximity Controller

(8 sensors) with LED Drivers and Analog Output

Figure 18

Analog Sensor Interface

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 SX8661 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 touch/proximity detection

will not be possible. In case the sensitivity is set too large, some power will be wasted and false touch/proximity

information may be output (i.e. for touch buttons => finger not touching yet, for proximity sensors => finger/hand

not close enough).

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.

The ASI will shut down and wait until new sensing period will start.

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