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DP83848I: A few questions about analog ground and crystal selection

Part Number: DP83848I

Datasheet: http://www.ti.com/lit/ds/symlink/dp83848vyb.pdf

Design & Layout Application Note: http://www.ti.com/lit/an/snla079d/snla079d.pdf 

1. What is the purpose of grounding the shield of the RJ45 connector since it is not connected to anything? 

2. Application Note: In figure 2.2, the bob-smith termination is grounded to signal ground - but in section 2.3 it says to ground the bob-smith termination to chassis ground, which one is correct ?

3. Application Note: in section 6.2 equation 1, what does 'crystal load spec' correspond to in the crystal datasheet? is it the 'load capacitance' of the crystal?

4. Application Note: in section 6.2, what value resistor should I use in series with the crystal if the crystal I want to use has drive level of 10 uW  (typical) - 200uW (max.)  ? Should I just pick a new crystal? 

5. Application Note: in section 10.1 table 2 how do we calculate parameter 'load capacitance' ? crystal load capacitance + C1 + C2 ?

I know it's alot of questions but anything helps, thank you guys so much!

  • Kevin,

    1. The RJ45 shield ground is usually connected to chassis ground by some passive components that allow energy to bleed off slowly rather than entering the system.

    2. Bob Smith should be terminated to the same ground as the RJ45 shield.

    3. Load capacitance of the crystal

    4. You should pick a new crystal that has an acceptable load capacitance and up to 500 uW drive level. Then no current limiting resistor is necessary.

    5. Load capacitance in that table refers to the values of CL1 and CL2 from section 6.2

    Best Regards,