AN2869 datasheet(38/43 Pages) STMICROELECTRONICS

38 / 43 page

Projected sensor design

AN2869

38/43

Doc ID 15298 Rev 6

Figure 33.

Rotary sensor made of 5 parcels

To design the rotary sensor:

Define the diameter (D) of the rotary sensor (12 mm to 30 mm) and the number of keys

(N minimum = 5)

Verify the outer arc (W) of each parcel in the rotary sensor; it should be from 6 to

10 mm

If necessary, adjust the parcel number by using the following formula: N = 3.14 * D / W

(with W chosen between 6 and 10 mm to obtain N as an integer)

4.4

Specific recommendations

4.4.1

PCB and layout

Ground considerations

One of the advantages of a projected sensor compared to a surface sensor is that its Tx and

Rx signals are less sensitive to the external environment than the ones used with the

surface sensor because they are coupled together. Rx is impacted by the ground in any

case (but less than the surface sensor); on the other hand, Tx is shielded by the ground. So

the sensor can be flooded as shown in Figure 34.

)

Global clearance

Rx to Tx: 1mm

Global Rx width

0.2 to 0.5mm

0.5 m m

(bottom

Rx1

Rx2

Rx3

Rx5

Rx4

Global

width

0.2 to 0.5mm

0.5mm

W/2

0.5mm

0.5 m m

MS18989V2

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