• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 2 answers
  • Subscribers 49 subscribers
  • Views 468 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS42LB69: ADS42LB69

E G
Intellectual 400 points

Part Number: ADS42LB69

Hi,

I'd like to feed ADS42LB69 clock input as LVDS from LMK 4828. I already have ac coupling capacitors of 100nF at the output of LMK; therefore, do I still need to put ac coupling capacitors at the clk inputs of the ADS42LB69 as shown in Figure 118 of the datasheet of the ADC revised on May 2016?

One other question is about the termination resistance of 100ohm placed at the clk inputs of ADC, In Figure 118 of the datasheet,  ac coupling capacitors are put after(closer to the ADC) the termination resistance of 100 ohm. Is this correct? Do we not need to put the termination after ac coupling capacitors?

My last question is about the 560 ohm termination resistance that needs to be put at the HSDS/LVDS output of LMK. Doesn't this resistance interfere with the 100ohm that's put at the ADC side clk input?

Thanks for your help in advance.

Regards.

  • 0
    TI__Mastermind 45670 points

    Hi,

    Only one set of AC coupling caps would be required between the outputs of the LMK to the inputs of the ADCs clock pins.

    Are the outputs of the LMK configured for LVDS? If so, a 100ohm termination would need to be used on the far end, near the inputs of the ADC's clock pins.

    If the LMK outputs are configured for PECL, then the 560ohm terminations would need to be used near the outputs of the LMK. If the connection is long, 2inches or more, between the two devices, we have found in the past that an additional 100ohm termination could be used to help with standing waves and better signal integrity.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Intellectual 400 points

    Hi Rob,

    I am planning to configure the output of LMK as LVDS thus I'll remove the 560 ohm. 

    For ac coupling capacitors, where do you think is a good place to put them, close to LMK or close to ADC? I am planning to put the 100ohm termination after ac coupling capacitors in either case, is this ok?

    For you last comment, ADC will be working at 250MHz; therefore, when I use a second 100ohm termination between _p and _n lets say close to LMK output and one close to ADC input, the combined resistance will be 50ohm, Isn't this a problem for LVDS termination?

    Finally in EVM schematic of LMK, PECL outputs are terminated with 240ohm to gnd at each _p and _n lines. In the same schematic HSDS/LVDS outputs are terminated with a 560ohm resistor between _p and _n lines. Is this not contradicting with what you are saying, I am a little confused. I'd appreciate if you can clearify it for me please.

    Regards.

  • 0 in reply to E G
    TI__Mastermind 45670 points

    HI EG,

    I would place the ac coupling caps nearest to the ADC clock inputs. I would also place the 100ohm termination near and before the ac coupling caps. The 100ohm diff termination on the LVDS is there to compete the circuit since effectively those outputs are current based.

    I wouldn't double terminate. Sorry for the confusion. I was only saying if you use a PECL type output, it is recommended to do the near and far end terminations.

    The schematic for the EVM is old, so the 560ohms might have been just a place holder. So you are correct, PECL use 240ohms to ground on each output leg, located near the output of the LMK outputs.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Intellectual 400 points

    Thanks Rob.