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LMH6733: LMH6733MQX HW design guide and output capability driving question

Leo Kuo
Prodigy 20 points
Part Number: LMH6733

Dear Support member:

I have a design using LMH6733MQX at BMC ASPEED 2600 VGA output side, I had studied spec application design, and now I follow spec Figure34. Recommended Non-Inverting Gain circuit to design, but I have some questions that need help.

1. LMH6733MQX RF & RG resistor value How do setting? now I set 549 Ohm.

2. BMC 2600 VGA guide recommends RGB signal output needs to add Ferrite Bead between connectors, but I have LMH6733MQX so I add on LMH6733 output side. about the Ferrite Bead value, how do setting? and the Ferrite Bead will affect LMH6733MQX output signal capability?

3. How does LMH6733MQX output capability? How about Max Length segment of layout routing? And LMH6733MQX need closed host side or device side? would you Provide detailed layout guide document to me?

4. LMH6733MQX & LMH6738MQX what's different ? can Substitute? Does HW design need modification?

 Thankful for your assistance.

2022-11-23

#Leo

  • 0
    Guru 140200 points

    Hi Leo,

    the datasheet mentions Rf = 340R for 5V single supply and Rf = 383R for +/-5V supply. See also the many figures in the "typical performance characteristics" section.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 20 points

    thanks, about question 2, 3, 4 do you have any good recommend ?

  • 0 in reply to Leo Kuo
    Guru 140200 points

    There's no general recommendation to install ferrite beads into the video lines because when taking improper values they can do more harm than good. Ferrite beads can cause nasty resonances in combination with stray capactiances and cable capacitances. And they do destroy the cable termination by adding unwanted impedances. They can do good when dosed very carefully but they will very probably harm, if you install "any good looking" ferrite bead prophylactically. Ferrite beads are only installed, if there are severe issues. Then they have to be designed very carefully in a further development phase. 

    The connectors of video signals must sit closest to the OPAmp. Keep the distance between OPAmp, cable termination resistor and connector minimal. Every millimeter counts. If you want to mount the connectors farer away from the OPAmp, then use microstrip or stripline technique to provide a signal track that shows the same impesance as the video cable. The termination resistor must still sit closest to the OPAmp.

    Look into the datasheets of both OPAmps to see the difference. The datasheet can better explain than me.

    Kai