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TLK2501 Rref limitations and recommended oscillators.

Keith Nicholas
Mastermind 46310 points
Other Parts Discussed in Thread: TLK2501

Hello Support Team,

I have a customer interested in using TLK2501 over very short distances and would like to know the range/tradeoffs for the value of Rref.  They are also asking for suggested oscillators.  Specific questions:

We are specifically interested in the value of Rref.  In our 50 Ohm system, it is generically recommended to be 200 Ohm, which is exactly what we have installed.  The problem is, we are driving a very short cable in our customer’s system (like about 2 inches!).  The TLK2501 seems to be optimized for driving much more cable than that, and I would like to tune it down some to make it more amenable to driving such a short length.  The datasheet hints at doing this, but it doesn’t make any specific recommendations.  There is a table where they show Rref out to 300 Ohms.  But 300 Ohms is the recommended value for a 75 Ohm system.  I would like to know the following:

1.       What is the usable range of resistance of Rref?

2.       Can TI recommend a value for Rref that would work best with very, very short cabling?

3.       Are there any Cons to raising the value of Rref that I should know about?

How do I relate the TLK2501’s spec for input jitter of 40 psec pk-pk to an RMS jitter value that would be more commonly specified by an oscillator manufacturer?  Can TI recommend any oscillators?

Thank you!

 

  • 0
    TI__Intellectual 1500 points

    Hello Keith,

    According to the following curve (page-20 of datasheet), increase in RREF leads to decrease in differential peak-to-peak output voltage. If a short cable is being used, the RREF can be increased to a point, when the differential receiver input voltage requirement is reached (200mV minimum), present on the other side of the cable.

    The particular resistor value depends upon the attenuation characteristics of the cable and noise coupling to the environment.   

     

     

    Peak-to-peak Jitter (JitterP-P) =  40ps
    JitterRMS is approximated as JitterP-P / 14 = 40ps/14 = 2.86ps
     
    Any oscillator adhering to the reference clock timing requirements given in the datasheet on page 15 should work.

    Thanks,
    Ankur Verma
    High Speed Interface Applications