• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 18 subscribers
  • Views 253 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

SN74AXC8T245EVM: Channel B output level not at VCCB

Errol Leon6
Expert 5830 points
Part Number: SN74AXC8T245EVM

Hello team,

My customer purchased the SN74AXC8T245EVM and performed a quick test.

  1. Supplied VccA = 1.8V and VccB = 3.3V from a bench-top power supply.
  2. Fed A1 (SMB connector) with a 125MHz clock from an Agilent 81130A pattern generator at 1.8V and observed B1 (SMB connector) on a scope.
  3. Customer noticed that when they turned on the output of the pattern generator, the VccA rail would rise from 1.8V to about 2.34V, and the output high level would be around 2.0V.

Do you have any suggestions on why the VccA rail is rising and the output level of the B channel is not at the 3.3V? Lowering the frequency of the input signal did not seem to have any effect on the outcome.

Thank you for your support!

Errol

  • 0
    TI__Guru 56790 points

    Hi Errol,

    That is strange behavior. Is there scopeshot on this?

    What is the scopeprobe loading?

    are the OE pins enabled (gnd) with the right DIR setting( high)?

  • 0 in reply to ShreyasRao
    TI__Expert 5830 points

    Hi Shreyas,

    Thank you for supporting this! 

    The scope probe loading was 50ohms. When the customer changed it to 1Mohm it looked like the output signal levels improved. On the input side, the output had to be dialed down to avoid the VccA rail rising. Can you confirm if this is due to the high input impedance of the input channels?

     

    While the output was at the right levels, the customer was getting some severe reflections which he couldn’t tune as there are no source resistors on the EVB. Have you found that source terminations are not required for most applications?

     

    The customer did not change any jumper settings on the board. The OE jumper was already installed on the EVB. He was using A1 and B1 so based on the BOM populated on the EVB, it looks like he did not need to change DIR1/2 settings.

    Let me know if there is any other details you need.

    Thanks!

    Errol

  • 0 in reply to Errol Leon6
    TI__Guru 56790 points

    Errol,

    Yes. 1Mohm probe must be used as AXC cannot drive 50ohm load. The OE must be connected to gnd. I do not believe that there are jumpers which come prepopulated on the board. The DIR1 pins must be connected to Vcca and DIR2 must be connected to gnd.

    Let me know if this resolves..