SX9331 datasheet(13/48 Pages) SEMTECH

13 / 48 page

SX9331

Final Datasheet

April 21, 2023

www.semtech.com

13 of 48

Rev. 2

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Semtech

When the conductive object to be detected (finger/palm/face, etc) is not present, the sensor only sees an inherent

capacitance value CEnv created by its electrical field’s interaction with the environment, in particular with ground areas.

When the conductive object (finger/palm/face, etc) approaches, the electrical field around the sensor will be modified

and the total capacitance seen by the sensor increased by the user capacitance CUser. This phenomenon is illustrated

in the figure below.

Overlay

PCB

Overlay

PCB

CSensor = CEnv

CSensor = CEnv + CUser

Figure 7: Proximity Effect on Electrical Field and Sensor Capacitance

The challenge of capacitive sensing is to detect this relatively small variation of CSensor (CUser usually contributes for a

few percent only) and differentiate it from environmental noise (CEnv also slowly varies together with the environment

characteristics like temperature, etc). For this purpose, the chip integrates an auto offset compensation mechanism

which dynamically monitors and removes the CEnv component to extract and process CUser only.

In first order, CUser can be estimated by the formula below:

A is the common area between the two electrodes hence the common area between the user’s finger/palm/face and

the sensor.

d is the distance between the two electrodes hence the proximity distance between the user and the system.

ε is the free space permittivity and is equal to 8.85 10e-12 F/m (constant)

ε is the dielectric relative permittivity.

Typical permittivity of some common materials is given in the table below.

Material

Typical rε

Glass

FR4

Acrylic Glass

Wood

Air

Table 8: Typical Permittivity of Some Common Materials

From the discussions above we can conclude that the most robust and efficient design will be the one that minimizes

CEnv value and variations while improving CUser.

User

⋅

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