QST108 datasheet(8/51 Pages) STMICROELECTRONICS

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QST touch sensing technology

QST108

8/51

QST touch sensing technology

3.1

Functional description

QST devices employ bursts of charge-transfer cycles to acquire signals. Burst mode permits

low power operation, dramatically reduces RF emissions, lowers susceptibility to RF fields,

and yet permits excellent speed. Signals are processed using algorithms pioneered by

Quantum which are specifically designed to provide reliable, trouble-free operation over the

life of the product.

The QST switches and charge measurement hardware functions are all internal to the

device. An external CS capacitor accumulates the charge from sense-plate CX, which is

then measured. Larger values of CX cause the charge transferred into CS to rise more

rapidly, reducing available resolution. As a minimum resolution is required for proper

operation, this can result in dramatically reduced gain. Larger values of CS reduce the rise

of differential voltage across it, increasing available resolution by permitting longer QST

bursts. The value of CS can thus be increased to allow larger values of CX to be tolerated.

The device is responsive to both CX and CS, and changes in either can result in substantial

changes in sensor gain.

Figure 2.

QTouch™ measuring circuitry

3.2

Spread-spectrum operation

The bursts operate over a spread of frequencies, so that external fields will have minimal

effect on key operation and emissions are very weak. Spread-spectrum operation works

with the Detection Integrator mechanism (DI) to dramatically reduce the probability of false

detection due to noise.

Ai12569

SNS_SCKn

SNSK_SCKn

Sense capacitor

CS (a few nF)

Cx (~20 pF)

CT (~5 pF)

Earth

Serial resistor

RS (10 kΩ)

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