CDCLVP111-SP: Output to LMX2694-SEP correct AC coupled termination

Spencer Wyman

Part Number: CDCLVP111-SP
Other Parts Discussed in Thread: LMX2694-SEP

LMX2694 datasheet calls for AC termination on the input, with the caps on the receiver side of the termination resistors. The recommended termination resistors on the driver side are 150 Ohm per figure 7b in the CDCLVP111-SP datasheet, but in this figure, the AC coupling caps are on the driver side of the transmission line. Below I have my initial thoughts on how to terminate each side, but I wanted to double check that this will work as intended, since the figures don't exactly match.

8 months ago

0 vicente flores prado 8 months ago

TI__Mastermind 20561 points

Hi Spener,
CDCLVP111-SP outputs are open emitters thus appropriate termination and biasing is important.

You will need the following as above ^ but with the AC coupling caps near the Rx - LMX2694-SEP has High-Z CMOS input pins.
You can place the 50 Ohm to GND resistors right before the coupling caps.

Regards,

Vicente

0 Spencer Wyman 8 months ago in reply to vicente flores prado

Prodigy 10 points

Yes, I have the 50 Ohm termination just before the AC coupling caps on the RX side, per the diagram I included above. I was wondering if my solution works without the AC coupling caps on the driver side as well, since they are already on the RX side

0 vicente flores prado 8 months ago in reply to Spencer Wyman

TI__Mastermind 20561 points

That is correct.

Remember you will still need to to bias CDC outputs given it's open emitter, and this needs to be done before the AC coupling caps.

Regards,

Vicente

0 Spencer Wyman 8 months ago in reply to vicente flores prado

Prodigy 10 points

So do I still need the 150 Ohm termination on the driver side? Or no because it's terminated before the AC coupling caps?

0 Noel Fung 8 months ago in reply to Spencer Wyman

TI__Guru* 93050 points

Hi Spencer,

150Ω is required for a LVPEVL driver. 50Ω termination is on the receiver side. AC-couple is the simplest way to connect a LVPECL clock to the receiver. LMX2694 input requires AC-couple. As a result, there are two DC-blocking capacitors per pin.

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CDCLVP111-SP: Output to LMX2694-SEP correct AC coupled termination