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LMH6739: LMH6739 Output issue

Julian Chen
Expert 2550 points
Part Number: LMH6739

Dear E2E Member,

Our customer use LMH6739, and test block as below,

When they setting pattern gen to 640*480, the out level is normal.

But When setting to 1920*1200, we can find the out voltage had gap with input voltage @ high frequency

The voltage gap lead to  test fail.

The circuit as below, how can we improve the circuit to fix the issue? 

 

  • 0
    Guru 140200 points
    Hi Julian,

    has this post to do with this here:

    e2e.ti.com/.../717859

    Please don't make double posts. This will end in total confusion!

    All these filter caps, ferrite beads and protection diodes at the output of LMH6739 erode the characteristic impedance matching of cable and ruin the frequency respsonse. Does it work properly if you omit all these components?

    Kai
  • 0 in reply to kai klaas69
    Guru 140200 points
    Hi Julian,

    figure 21 of datasheet of LMH6739 recommends 6.8µ decoupling caps. You have only 100n?

    Kai
  • 0 in reply to kai klaas69
    Expert 2550 points

    Dear Kai.

    Thanks fot your support.

    I remove all bead, diode and cap, and also use 6.8uF to replace to C213 and C191, But no improve.

    Is it bandwidth issue? or  the drive current is not enough?

  • 0 in reply to Julian Chen
    Guru 140200 points
    Hi Julian,

    the leading edge of the pink pulse is not as steep as the blue pulse. So, something is wrong with the cabling to your test fixture. This also makes that the peak amplitude of pink pulse is too low. Have you provided a 75R cable for each color at the output of LMH6739? This looks like a typical mismatch of characteristic impedance.

    The output signal of each color must see an individual and continuous 75R cable from the 75R output resistor to the 75R load resistor. This must only be interrupted at the socket. Also keep in kind, that the three colors have individual signal ground terminals at the VGA socket. Don't intermix them!

    Kai