AN2869 datasheet(31/43 Pages) STMICROELECTRONICS

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AN2869

Surface electrode design

Doc ID 15298 Rev 6

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3.5.5

Component placement

To reduce the sensor track lengths, it is recommended to place the microcontroller very

close to the sensor electrodes. It is also recommended to center the microcontroller among

the sensors to balance the parasitic capacitance and to put a ground layer above it.

The ESD protection resistors must also be placed as close as possible to the microcontroller

to reduce the track length which could drive ESD disturbance directly to the microcontroller

without protection. These ESD resistors must be selected according to the acquisition

method recommendations.

3.5.6

Ground considerations

Ground plane

It is recommended to route the sensors and the ground on the same layer while the

components and other tracks are routed on the other(s) layer(s).

When a multilayer PCB is used, both sides of the PCB are commonly grounded to improve

the immunity to noise. Nevertheless, the ground has an effect on the sensitivity of the

sensor. The ground effect is to increase CX, which reduces the sensitivity as the ratio CT/CX

decreases. So, to balance between noise immunity and sensitivity, it is recommended to use

partial grounding on both sides of the PCB through a 15% mesh on the sensor layer and a

10% copper mesh for the opposite side with the electrodes and tracks.

Ground around sensor

When the ground plane is on the same layer as the sensor, it surrounds the sensors. To

avoid increasing CX, it is recommended to keep a gap between the sensor and the ground.

This gap size must be at least 2 mm (4-5 mm recommended) and must also be respected

with any noisy application track or power supply voltage.

There are two different cases:

●

distance to GND and power supply voltage, shorter distance is possible, but impacts

significantly the key sensitivity

●

distance to noisy signal, the detection may completely stop working in case the

distance is not respected

Special care must be taken to balance the ground around the sensors. This is particularly

true for a rotary or linear sensor (see Section 3.5.2: Driven shield).

Caution:

Floating planes must never be placed close to the sensors.

Ground plane example

A full ground plane is mandatory below the MCU up to serial resistors (see Figure 25).

●

It must cover the tracks between the MCU and the serial resistors

●

In RC, it must also cover the load resistors

●

In CT, it must cover the sampling capacitors

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