0
\$\begingroup\$

I'm using this PHY

My questions:

  1. Is it OK, if I connect 1uF and 1000pF capacitor on the VDDCR pin instead of the recommendation of "1uF & 470pF"? Higher capacitance on the power rail should not be a problem, right?

  2. For the Bob-Smith termination, can I interchange the connection of resistor and capacitor like shown below? Will it make any difference?

enter image description here

\$\endgroup\$
3
  • \$\begingroup\$ Why do you want to change it? \$\endgroup\$
    – AKR
    Jun 27 at 6:28
  • 1
    \$\begingroup\$ I do not want to change it actually. But just asking, what would be the impact if I interchange? Just curious to know what would happen? \$\endgroup\$
    – Newbie
    Jun 27 at 6:35
  • \$\begingroup\$ 1) It depends on the capacitor properties and what the chip needs. It might be better or worse. Worst case is the regulator becomes unstable and does not work properly. Impossible to say without testing. 2) Regarding the Bob Smith, it depens if you plug it into passive PoE source or not. \$\endgroup\$
    – Justme
    Jun 27 at 11:00

1 Answer 1

1
\$\begingroup\$
  1. ... Higher capacitance on the power rail should not be a problem, right?

Depends on how high the capacitance is. For linear regulators higher capacitance is not always good as it may cause instability (e.g. putting 47u as opposed to the manufacturer recommendation of 1u). In your case, placing 1.01u instead of 1.0047u will not create an instability, of course. But I just wanted to point this out. Anyway, using 1n instead of 0n47 might be better for HF noise filtering. But it's always a good practice to follow the manufacturer's recommendation(s).

  1. ... Will it make any difference?

No it won't.


By the way... I don't know if this is stated anywhere in the community before but I'd like to say that the values given in Bob Smith's termination network are actually problematic because they don't bring a CM impedance matching for neither of CAT5, CAT5e and CAT6.

For resistance, some researches and works show that the proper termination should be around 50 Ohms for CAT5 and CAT5e, and around 65 Ohms for CAT6 (can't remember the exact numbers).

\$\endgroup\$
1
  • \$\begingroup\$ Thank you for the answer. So, the placement of the marked resistance and capacitors can be interchanged. Thank you. And regarding the Bob-Smith, I asked a couple of questions in this forum by I didn't get a proper answer on how it actually works. Only the effects are provided. Not the working of the termination. \$\endgroup\$
    – Newbie
    Jun 27 at 10:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.