DS90UB941AS-Q1: AC coupling capacitor questions

Hi Betty,

Re: "1. how the select AC coupling capacitor? why choose  100nF for STP and 100nF/47nF for coaxial in 941 side?"

For the Coax cable option, the "DOUT+" will see two 100nF capacitors in series whereas "DOUT-" will see only one capacitor. Hence, this one capacitor value needs to be around half of that, which is why 47nF was selected.

Re: "2, in 948 side, why the AC-coupling capacitor is different when pair when different SER? any other requirement about it? how to select AC coupling when pair with 941? datasheet don't show it."

The reason for using different AC-coupling cap values with different SERs is because the design guidelines require to have the same value at both sides of the link. For example, 925 SER datasheet lists 100nF as the only valid value, then 948 DES has to match that. For 941 SER, you can check its datasheet and use the matching cap value in 948 side.

Re: "3. if in 941 side, use 100nF/47nF for coaxial, how about the 948 side? is it must be the same value in SER and DESER?"

Yes, we should use the same values in both sides of the link.

Re: "4. does the AC coupling capacitor will affect I2C and link status? I ask this because in coaxial, 941 use 100nF/47nF, will 948 use 33nF/15nF, they are can't link. can you share details about it?"

Yes, having the same cap values in both sides of the link is important, specially for the Coax cable option. So, please change 948 cap values to match that of 941 and let us know if that addresses the reported issue.

Best regards,

Solomon

Hi Betty,

Those are design decisions/requirements made based on the lowest back-channel supported rates (baseline wander) and the acceptable cap impedance at the target frequency.

Regards,
Solomon