LDC2114: Inductive Touch Buttons EMI issue

test_user

Part Number: LDC2114

Hello,

we were measuring our system (pcb with four inductive touch buttons) in an EMC laboratory.

We didn't have any problems with emission, but failed at radiated immunity.

We tested two setups: The first one with 0.3mm SSL surface, and the second one with 0.5mm glass fascia and a very thin copper foil as target (less than 50 um)

The setup with 0.3mm SSL surface had no problems with radiation, but the glass-version failed. (I've attached a diagram of the Output Datas)

The output datas started to act up at 500MHz over a wide frequency range.

Why do these two systems behave so different? Its the same hardware, only the mechanical setup is different.

Kind regards,

Clemens

over 7 years ago

0 Chris Oberhauser66 over 7 years ago

TI__Genius 14270 points

Hi Clemens,

Are the PCBs the same? Is the SSL grounded or AC coupled to the sensor PCB? Is the sensor-target distance shorter with the SSL?

My first thought is that you may have improved coupling between the sensor and the SSL. Try to increase that coupling - see if a 1nF cap between the inside copper and the sensor ground fixes the issue. Also, check to see if the COM capacitor is within the recommended range.

Regards,

ChrisO

0 Clemens Zintgraf over 7 years ago in reply to Chris Oberhauser66

Prodigy 40 points

Hi Chris,

the PCBs are the same and sensor distance also.

SSL is not grounded, it's floating. Placing that cap won't be too easy but i will try.

Thanks for your answer.

Regards,

Clemens

0 Clemens Zintgraf over 7 years ago in reply to Chris Oberhauser66

Prodigy 40 points

Hi Chris,
could you explain me what exactly is the function of the COM capacitor, and how does it effect emi issues?
Regards,
Clemens

0 Chris Oberhauser66 over 7 years ago in reply to Clemens Zintgraf

TI__Genius 14270 points

Hi Clemens,
The COM capacitor provides a offset ground for the sensor driver. It should be set to provide appear as a low impedance to ground, but not too much of a load on the sensor driver.

The AC coupling of the plate with a fixed 1nF is only for debugging to see if that is the cause.

Is your layout based on the recommended layout in the datasheet?

Regards.
ChrisO

0 Clemens Zintgraf over 7 years ago in reply to Chris Oberhauser66

Prodigy 40 points

Hi Chris,

the layout is based on the recommened layout, and IN-lines are routed in middle layers between com planes.

The only difference between the two systems is the target.

Do you think the thickness of the target could be a problem for emi? It's only 0,75 skin depths.

Regards,

Clemens

0 Chris Oberhauser66 over 7 years ago in reply to Clemens Zintgraf

TI__Genius 14270 points

Hello Clemens,

Can you provide some more information on the sensor parameters and device settings?

Regards,

ChrisO

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LDC2114: Inductive Touch Buttons EMI issue