TIDM-NFC-RW PCB IMPEDANCE CONTROL

Michael Dollard

I'm looking at the example PCB in tidrjd7 and the stack-up description in tirjd6. The impedance calculation for the antenna feed stripline from L2 doesn't yield 50ohm given the 30mil traces. The trace impedance calculates to about 95ohms. I must be missing something. Can anyone give me a hint what that might be?

over 7 years ago

0 Ralph Jacobi over 7 years ago

TI__Guru*** 128185 points

Hello Michael,

I am not clear on what impedance calculation you are trying to work through. Can you give me some more details to help me understand where you are in the calculation process so I can help you out?

Can you point on the schematic located on page 8 of this document where you are at on your calculations in regards to passives?: www.ti.com/.../slou372a.pdf

The 50 ohm point is highlighted in that document as TP8 for reference.

0 Michael Dollard over 7 years ago in reply to Ralph Jacobi

Prodigy 120 points

Hi Ralph, thanks for the reply. That's the schematic and that's the right place on it. I'm not yet calculating the passives for antenna matching I'm calculating the trace-width needed between L2 and C23. I'm also looking at the PCB stack-up on page 9 of "tidrjd6.pdf" www.ti.com/.../tidrjd6. Given that stack-up and the 15mil clearance the 30mil microstrip trace is calculating to 96 ohms.

0 Josh Wyatt over 7 years ago in reply to Michael Dollard

TI__Guru* 91086 points

@13.56MHz? you do realize that calculating the trace impedance at this frequency typically is a waste of time unless its very long or very skinny.

0 Michael Dollard over 7 years ago in reply to Josh Wyatt

Prodigy 120 points

Yes, I did arrive at that conclusion. Thanks!

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TIDM-NFC-RW PCB IMPEDANCE CONTROL