DP83620 AC coupled resistor parameters

rtos

Other Parts Discussed in Thread: DP83620

Hi,

Accroding to the DP83620 datasheet, AC coupling is neccesary on TD pair , but I want to know why the pullup resistor is 130Ω and pulldown resistor is 82Ω? When the power supply is 3.3V,the LVPECL common-mode voltage should be 2V, according to the value of resistors,the common-mode voltage is 3.3*82/(82+130) =1.3, Why?

I think the pullup resistor should be 82Ω and pulldown resistor 130Ω, the DP83620 datasheet or demo board is wrong. Is it?

thanks!

zchong

over 11 years ago

0 rtos over 11 years ago

Prodigy 130 points

Hi,

0 Patrick OFarrell over 11 years ago in reply to rtos

TI__Mastermind 31600 points

I have moved your post to the Ethernet forum so that it can get visibility from the experts.

Patrick

0 Patrick OFarrell over 11 years ago in reply to Patrick OFarrell

TI__Mastermind 31600 points

You are correct in noting that the terminations in the datasheet and on the EVM for the AC coupled TD+/- pair are the reverse of the typical recommendations. The terminations as shown would typically be used for DC coupled signals rather than AC coupled signals.

Having noted that, the existing solution has been tested extensively over time without any indication of issues. The typical terminations (with 130 Ohms to ground and 82 Ohms to 3.3V) have also been used in applications and would be acceptable.

Patrick

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DP83620 AC coupled resistor parameters