FDC2214: Connecting a capacior to ground, signal unexpected in oscilloscope

Eduardo Gomez Suarez

Prodigy 60 points

Part Number: FDC2214

Configuration:

- Single-ended configuration

- FDC2214 is grounded

- 0Resistors replace with a common mode chocke

- 100pf between INA and GND

I am facing the following problem when measuring the signals in the oscilloscope:

When I connect 100pF between INA and GND I do not get an expected signal in the pin INA:

Meanwhile in the pin INB, I am able to measure the normal signal, something like that:

I would like to know which problem I am facing, I think it could be because of:

- Common mode choke

- Something related with the ground

- Need of another system characterization.

over 8 years ago

0 Rachel Liao over 8 years ago

TI__Expert 6480 points

Hi Eduardo,

If you take the 100pF off of INA, do the waveforms then become symmetrical?

Thanks,
Rachel

0 Eduardo Gomez Suarez over 8 years ago in reply to Rachel Liao

Prodigy 60 points

Yes then the waveforms become completely simmetrical, and I can measure the expected behaviour.

Thank you for your fast answer.

0 Rachel Liao over 8 years ago in reply to Eduardo Gomez Suarez

TI__Expert 6480 points

Hi Eduardo,

Apologies for the delay - does this behavior occur when the common-mode chokes are removed but the 100pF is still connected to INA? If not, I'm thinking it could be caused by a second resonance affect from the extra inductance of the choke, especially on the side with the extra capacitance.

Thanks,
Rachel

0 Eduardo Gomez Suarez over 8 years ago in reply to Rachel Liao

Prodigy 60 points

No, it does not happen without the common mode choke. I totally agree with your explanation. What should I do to fix this problem. My application should work in Single-ended configuration and I have a big problem with this second oscillation.

Thank you in advance.

Eduardo

0 Rachel Liao over 8 years ago in reply to Eduardo Gomez Suarez

TI__Expert 6480 points

Hi Eduardo,

Is the 100pF required? What is it being used for?
Could you send me a schematic and/or layout so I can take a look at where the capacitor, inductor, and common mode chokes are placed in relation to the chip?

Thanks,
Rachel

0 Eduardo Gomez Suarez over 8 years ago in reply to Rachel Liao

Prodigy 60 points

Hi Rachel,

As I am working for a company (automotive application), I can not share any schematics in internet. May I have your email?

Thanks,
Eduardo

0 Rachel Liao over 8 years ago in reply to Eduardo Gomez Suarez

TI__Expert 6480 points

Hi Eduardo,

Please send me your email so I can send you more information.

Thanks,
Rachel

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FDC2214: Connecting a capacior to ground, signal unexpected in oscilloscope