LMK00301: LMK00301

Dave D'Epagnier

Part Number: LMK00301

The data sheet for the LMK00301 page 26 regarding AC coupling appears to be completely wrong and in need of a re-write. Contrary to what the whole page is about, the LVDS driver output DOES NOT NEED A DC CURRENT RETURN PATH. No LVDS driver does. As a result, the outputs of an LVDS clock driver can be ac coupled directly and terminated at the receiver end. The driver end needs no DC return path.

Please correct me if I'm wrong here and provide an equivalent circuit of the LMK output to back that up.

Dave

over 2 years ago

0 Jennifer Bernal over 2 years ago

TI__Genius 11315 points

Hi Dave,

That excerpt is unique to the LMK00301 due to internal circuitry requiring the DC path. It allows the LMK00301 to properly bias at start up and is valid in the case of Figure 33 and Figure 34.

Regards,

Jennifer

0 Dave D'Epagnier over 2 years ago in reply to Jennifer Bernal

Prodigy 180 points

Will you show me the internal circuitry that requires this? We've used this part in the past without a dc return path and without incident. We are about to employ this part in a new design and need to know for sure if the dc return path is required; we are very space constrained and do not wish to add resistors. If we DO have to add a resistor does it have to be 100 ohms? can it be say a 10k (referring to figure 34)?

0 Jennifer Bernal over 2 years ago in reply to Dave D'Epagnier

TI__Genius 11315 points

Hi Dave,

I will get back to you next week as I need to investigate further.

Regards,

Jennifer

0 Jennifer Bernal over 2 years ago in reply to Jennifer Bernal

TI__Genius 11315 points

Hi Dave,

I cannot share our internal circuitry.

If you've used this device without issues before and have undergone testing to ensure, then the DC path would not be necessary in your scenario. In the case that there was ever a startup issue or glitch observed, then I do recommend the DC return path to be included.

For Figure 34, DC coupling, the correct LVDS termination is needed on the driver side, typically 100 ohm differential.

0 Dave D'Epagnier over 2 years ago in reply to Jennifer Bernal

Prodigy 180 points

Ok Jennifer, thanks for looking into this. By the way, it's kind of interesting that the evaluation board comes "configured for LVDS output type" on bank B with ac coupling capacitors installed but no dc return path components installed.

0 Jennifer Bernal over 2 years ago in reply to Dave D'Epagnier

TI__Genius 11315 points

Hi Dave,

You make a good point. There should be a differential resistor between each clock output populated on Bank B.

0 Dave D'Epagnier over 2 years ago in reply to Jennifer Bernal

Prodigy 180 points

I rest my case.

Clock & timing

Clock & timing forum

LMK00301: LMK00301