LMK00301: Common Mode/DC Offset Voltage requirement

Mike Rusovick

Part Number: LMK00301

The LMK00301 Datasheet Sec 7.5 (Electrical Characteristics) states that the Differential Input Common Mode Voltage (Vcmd) is 0.25V MIN.

If AC coupling is used, like you say it can be, is an external bias network required to create this DC offset/Common Mode bias?

On your LMK00301 Eval Board you have series caps, but no external bias network. There seems to be a conflict here between your datasheet requirement and the Eval Board.

-By the way, we are driving the LMK00301 clock0 input with an AC coupled LVDS diff clock, and no external bias network (0V bias).

7 months ago

0 Andrea Vallenilla 7 months ago

TI__Expert 8315 points

Hello Mike,

The input pins can be AC-coupled as shown in the data sheet on p. 30. The LMK00301 has an internal bias that applies the common mode to the pin. Note that this is specific to LVDS input (as you mentioned you were using). For other signal types, continue on to p. 31.

Please let me know if you have any other questions.

Good luck!

Andrea

0 Mike Rusovick 7 months ago in reply to Andrea Vallenilla

Prodigy 10 points

Andrea,

Thanks for the response, the datasheet does not say that there is internal biasing provided. The 0.25V Common Mode Voltage requirement in Sec 7.5 (under Clock Inputs) gives the impression that it is required externally. Maybe this can be clarified in your next datasheet update?

Thanks, Mike

0 Andrea Vallenilla 7 months ago in reply to Mike Rusovick

TI__Expert 8315 points

Hello Mike,

You are correct, the LMK00301 does not have internal bias and it needs to be included externally like the EVAL board. I will take a note of this and include this feedback on our next data sheet update.

More info on this thread: https://e2e.ti.com/support/clock-timing-group/clock-timing-internal/f/clock-timing---internal-forum/1196538/lmk00301-lmk00301-termination-bias-and-coupling?tisearch=e2e-sitesearch&keymatch=LMK003%2A%20%26%20bias#

Good luck,

Andrea

0 Mike Rusovick 6 months ago in reply to Andrea Vallenilla

Prodigy 10 points

Andrea, Thanks for the update. Did you change your answer because you consulted with someone?

I have some follow-up questions on the LMK00301 Eval Board schematic:

On Sheet 2, C7/C12 and C14/C17 show a value of "0". It appears the capacitors were replaced with 0 ohm resistors, correct?

Also on Sheet 2, it seems like the bias networks should be on the other side of the caps, because if you want to AC couple the clock inputs, you would not be able to provide a bias because the caps would be blocking it. Do you agree?

Thanks, Mike

0 Andrea Vallenilla 6 months ago in reply to Mike Rusovick

TI__Expert 8315 points

Hello Mike,

I realized I was looking at the outputs and not the inputs which was your question, that was my mistake. My correction came from reading that thread I tagged from one of my coworkers also explaining the biasing of the part.

Mike Rusovick said:
On Sheet 2, C7/C12 and C14/C17 show a value of "0". It appears the capacitors were replaced with 0 ohm resistors, correct?

Correct, I just double checked this in the BOM.

Mike Rusovick said:
Also on Sheet 2, it seems like the bias networks should be on the other side of the caps, because if you want to AC couple the clock inputs, you would not be able to provide a bias because the caps would be blocking it. Do you agree?

I also agree with this. Thank you for catching all of this, I will take a note about this questions.

Best,

Andrea

0 Mike Rusovick 6 months ago in reply to Andrea Vallenilla

Prodigy 10 points

Thanks for the response.

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LMK00301: Common Mode/DC Offset Voltage requirement