VDDA parallel LC

Mark Peffley

Hello, I am looking at the schematics for the F28035 control card. Can anyone explain the purpose of the parallel L/C on the VSSA / ADCGnd pin?

www.ti.com/.../tmdscncd28035

over 9 years ago

0 Adam Dunhoft over 9 years ago

TI__Expert 7885 points

Hi Mark,

This is a low pass LC filter. I believe the intent wis to filter the analog GND node from high frequency switching noise which could be seen on the digital GND node.

Best Regards,
Adam Dunhoft

0 Mark Peffley over 9 years ago in reply to Adam Dunhoft

Prodigy 110 points

I'm not sure I follow that. Seems like the capacitor would provide a high frequency path between the analog and digital grounds. Is it maybe some kind of resonant trap to block noise from a particular clock or switching converter on that board?

Could it just be an error? Like maybe the capacitor was supposed to bypass VDDA to VSSA?

0 Adam Dunhoft over 9 years ago in reply to Mark Peffley

TI__Expert 7885 points

Mark,

The person who designed this will be back in office tomorrow. I will check with him and get back to you.

Best Regards,
Adam Dunhoft

0 Brett Larimore over 9 years ago in reply to Adam Dunhoft

TI__Guru 57255 points

Hi Mark,

This is a situation that started off as an experiment, but stayed in the design.

The bead shouldn't do much in the VSSA path - as you've mentioned high-frequency noise can come in through the capacitor and any low-frequency parts will flow in through the bead.

I would recommend not placing this particular ferrite bead in your design (largely just because of the unnecessary extra BOM cost).

Thank you,
Brett

0 Mark Peffley over 9 years ago in reply to Brett Larimore

Prodigy 110 points

Got it. Thanks!

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VDDA parallel LC