• State TI Thinks Resolved
  • Locked Locked
  • Replies 2 replies
  • Answers 2 answers
  • Subscribers 34 subscribers
  • Views 454 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

DS80PCI102: DS80PCI102

DS80PCI102: DS80PCI102SQE, DS80PCI402SQ, DS80PCI402SQ

Swetha A M
Prodigy 70 points
Part Number: DS80PCI102

Hi, 

Iam using all three "DS80PCI102SQE, DS80PCI402SQ, DS80PCI402SQ" re-drives in my design. 

Iam stuck with placement of AC Coupling Capacitors, datasheet of Re-drivers says "Place AC-coupling capacitors near to the receiver end of each channel segment to minimize reflections"  but the FPGA (Part no: XCKU035-2FBVA676E) used in my design which is used to transmit and receive PCie signals says "Capacitors must be placed on the Transmitter side of an interface that permits adapters to be plugged and unplugged", both are contradicting statements.

Can i get help whether the caps should be placed near the re-drivers or not.

Regards,

Swetha A M

 

  • 0
    TI__Guru* 93670 points
    Hi Swetha,

    Both could be correct but let's dig into this a bit more.

    AC coupling capacitors pads provide parasitic that causes impedance drop. The degree of this impedance drop is related to the board pcb stackup. Ideally - to minimize reflections - you want this impedance drop to be very close to another impedance drop - less than quarter wavelength - so this would get swamped by larger impedance drop - caused by the adapter or device ESD structure. One thought is that AC coupling cap should be placed close to which ever has higher impedance drop (either adapter or the device). Another thought is that we should put this close to the receiver which has impedance drop due the device input capacitance and thus impedance drop. Additionally, putting the cap closer to the receiver introduces less reflection to the transmitter. So in summary, by placing anti-pad on adapter to minimize it's parasitic, i would vote for placing the AC coupling cap closer to the receiver since it has more advantages ( as noted earlier).

    Regards,,,nasser
  • 0
    TI__Mastermind 43795 points
    For most PCIe applications the decoupling capacitors are placed on the same PCB assembly as the PCIe Tx signals. To minimize any reflections caused by the coupling capacitor placement we recommend to leave space between the Tx and the capacitor footprint to give any reflected energy time to dissipate before it can interact with the PCIe Tx.