Ethernet Centre tap capacitor on transformer

[FreshmanNewbie]

I'm using this IC with this Eval board as a reference

On page 5 of the Eval board, the centre tab capacitor on the transformer is 22nF.

When the value is changed to 100nF, it works OK. But if its 22nF, it doesn't work.

Can someone tell me what is the exact reason behind this and what would be the exact capacitor value?

 

[KlausST]

Hi,

Please tell which capacitor you talk about: Give the number shown in the schematic.
Aso please tell what "doesn´t work" exactly means and how you tested it.

Klaus

 

[FreshmanNewbie]

Hi,

Please tell which capacitor you talk about: Give the number shown in the schematic.
Aso please tell what "doesn´t work" exactly means and how you tested it.

Klaus

C32 and C37 capacitors in the schematic.
Doesn't work mean, When I connect this PHY to the other device, the link doesn't work. Connection is not established. Sometimes, connection is established with errors.

 

[KlausST]

Hi,

"the other device" is what?
I mean, maybe "the other device" is upset somehow.

From the schematic:
We talk about a 3.3V supply node.
on page4 left top, we see the distribution of the 3.3V supply to the VDD33TXRX1 and ...TXRX2 nodes.

So capacitor installed or not, there should be 3.3V.
Now if you install 100nF you just get a bit more stable supply = lower impedance, less ripple.

To debug this I´d use a scope and take some measurements at these nodes. Both with 22nF and 100nF to see the difference.
Indeed I can´t believe the system is that sensitive. I expect the root problem else where, maybe in a non stable power supply at all. Measurement should show.

Klaus