DocID024663 Rev 2

5/20

AN4299

Conducted noise immunity

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1

Conducted noise immunity

1.1

Signal to Noise ratio (SNR)

Signal to noise ratio (SNR) is an important characteristic in the evaluation of a touch sensing

system.

SNR measurement results are only valid for a specified board and for the noise environment

at the moment of the measure. This is why the noise immunity is better evaluated by

referring to a standard like IEC61000-4-6 where the noise level and the test conditions are

specified.

1.2

IEC61000-4-6 standard

The IEC61000-4-6 standard specifies the test procedure to evaluate the noise immunity of

an EUT (equipment under test).

1.2.1

Test setup

The test consists in using a noise generator to inject modulated noise signals into the EUT

power supply lines as shown in Figure 1.

Figure 1. Standard IEC61000-4-6 test setup

Touch sensing systems are based on capacitor variation measurements. The system must

be able to detect capacitive variations as low as few picofarads on the sensor electrodes.

Therefore such systems may be sensitive to conducted noise.

In a real touch sensing system, the main source of perturbation is introduced by the finger of

the user. This is because the user is in the electric path between the system and the earth.

In the test setup shown in Figure 1, the injected signal simulates the noise perturbations to

which a system may be exposed. By varying the frequency and the level of the injected

signal, the test setup allows to characterize the situations where the touch system becomes

unreliable.