DocID024663 Rev 2

7/20

AN4299

Conducted noise immunity

19

1.2.3

Noise immunity evaluation

The EUT noise immunity is evaluated by testing the ability of the EUT to behave according

the definition of a given Class when it is submitted to a given noise level. The Table 2 below

summarizes the different noise levels and Classes.

1.2.4

IEC61000-4-6 standard limitation

As mentioned above, the minimum frequency step recommended by the standard to sweep

the injected signal from 150 kHz to 80 MHz is 1 % of the preceding frequency value. At

500 kHz this represents a 5 kHz step. On most touch sensing systems, these steps are too

large to be able to isolate the worst case situations. Some applications with narrow critical

wave band can pass 3Vrms if the critical frequency falls just in the middle between tested

frequencies, whereas the same application does not pass 1 Vrms if you set the test exactly

on the critical frequency.

This is why it is important to set smaller steps around the critical frequencies. Sometime the

standard bench is not able to do the appropriate step (for example 100Hz). Section 3

proposes a method to detect the worst case.

Table 2. Test levels

Standard class \ noise level

Level 1

1 Vrms

Level 2

3Vrms

Level 3

10 Vrms

Class A: system work normally

Pass/fail

Pass/fail

Pass/fail

Class B

occur (false touch detection or touch masking),

but the product recovers once the stress is

removed without any operator intervention

Pass/fail

Pass/fail

Pass/fail

Class C: same as class B but need external

action (such as reset or power off/on) to return

to normal state

Pass/fail

Pass/fail

Pass/fail

Class D: system that losses function or

degradation of performance which is not

recoverable

Pass/fail

Pass/fail

Pass/fail