AN4312 datasheet(31/36 Pages) STMICROELECTRONICS

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AN4312

Surface sensor design

Figure 27. Track routing recommendation

Electrode banks

A set of electrodes that are driven simultaneously during the acquisition is called an

acquisition bank.

This set of electrodes and tracks interact less with each other and can be routed closer.

Typically, a spacing of twice the track width is sufficient.

For electrodes not belonging to the same bank, coupling must be avoided, a spacing of at

least 2 mm is required and 4 to 5 mm is recommended (see Figure 27).

Electrode spacing

To avoid cross detection on adjacent electrodes, it is recommended to keep a gap of at least

twice the panel thickness between electrodes (see Figure 27).

Interaction with other tracks

To avoid creating coupling with lines driving high frequency signals, it is recommended to

cross the sensor tracks perpendicularly with the other tracks. This is especially true for

communication lines, where it is forbidden to route them in parallel with the sensor tracks.

To avoid such a configuration, the pins of the microcontroller must be selected and grouped

by function. When it is possible, all the sensor pins are consecutively distributed on one or

several sides of the microcontroller package (the pins are then used as GPIOs like the LED

drivers and communication lines).

It is strongly recommended to dedicate the pins to be used as sensors and not to share

them with other features. Sharing tracks produces parasitic capacitance due to re-routing of

the sensor tracks, and impacts the sensitivity.

MS18980V2

Ground plane or

ground track

Touchkey bank 1

Touchkey bank 2

Any application track

(LED, power, Com.)

At least twice the

panel thickness

As thin as

PCB

technology

allows

At least

twice the

track width

At least 2 mm (4 - 5 mm is recommended)

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