LMK00334: Clock input and output impedance

Lisa Xu

Intellectual 305 points

Part Number: LMK00334

Dear TI Team:

We have LMK00334RTVR design in our project,

Now we have trace impedance 85ohmm for CLKin0/n,

trace impedance 100ohm for CLKin1/n

trace impedance 100ohm for CLKoutA0/n

trace impedance 85ohm for CLKoutA1/n

1. Is it ok for our trace impedance as above?

2. what is your suggestion?

Best Regards

Lisa

2 months ago

0 William Pfennigwerth 2 months ago

TI__Expert 6971 points

Lisa,

85 ohm traces will cause reflections due to the impedance mismatch. It is recommended to use 100 ohm differential (50 ohm single-ended) trace impedance.

Regards,

Will

0 Lisa Xu 2 months ago in reply to William Pfennigwerth

Intellectual 305 points

Hi Will:

It means LMK00334 needs 100ohm differential impedance which will cause mismatch with 85ohm trace impedance.

Or input 85ohm and output 100ohm trace impedance difference will cause mismatch?

which one is the main reason?

Best Regards

Lisa

+1 William Pfennigwerth 2 months ago in reply to Lisa Xu

TI__Expert 6971 points

Lisa,

Sorry for the confusion. Impedance matching mainly concerns the source impedance getting matched to the trace impedance. If you have a 85 ohm source matched to 85 ohm traces you could use this to drive the LMK00334. But because the LMK00334 is made for 50 ohm impedance if you were to use 85 ohm traces on the output you could see reflections.

Regards,

Will

0 Lisa Xu 2 months ago in reply to William Pfennigwerth

Intellectual 305 points

Hi Will:

Thanks for your explanation.

It means 85ohmm trace impedance for CLKin0/n is ok because our CPU is 85ohn drive,

but trace impedance 85ohm for CLKoutA1/n is not ok because LMK00334 is 100ohm differential drive.

Best Regards

Lisa

+1 William Pfennigwerth 2 months ago in reply to Lisa Xu

TI__Expert 6971 points

Lisa,

Yes. This is correct.

If you want a clock buffer with 85 ohm outputs, I would recommend our latest PCIe clock buffer the LMKDB1104. The LMKDB1104Z85 is the 85 ohm variant.

Regards,

Will

0 Lisa Xu 2 months ago in reply to William Pfennigwerth

Intellectual 305 points

Got it,Thanks Will

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LMK00334: Clock input and output impedance