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Original question:

LMH6722: Fanout analog signal

LMH6722: Layout suggestion for fanout input (opa V+)

DeWei Lan
Prodigy 250 points
Part Number: LMH6722
Other Parts Discussed in Thread: LMH6702, OPA3692

I adopt LMH6722*2 to implement analog fanout buffer.

Refer to here:

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1111959/lmh6722-fanout-analog-signal

Now I encounter a problem: how to layout buffer input?

Scheme as here

V+ of OPA Input is my problem.

I know that <90 deg angle is not good layout rult.

So I adopt Y-Shape(~135deg) to layout V+ of OPA (green rectangular)

Is this layout good?

Or others layout suggestion for 1:4?

Actually, I've think that maybe I need more buffer to do fanout.

1->2->4->8  but 15 OPAs are necessary.

I wish I could implement 1:8 fanout with LMH6722*2 and LMH6702*1.

  • 0
    Guru 140200 points

    Hi Dewei,

    I would locate the 50R series termination resistors close to the outputs of OPAmps. The copper tracks between the 50R resistor and the connector should be made from 50R microstrip lines. And the connectors should be mounted closest to the outputs of OPAmps. Sometimes this can mean that you do not take all the four OPAmps from the quad chip but only those which allow shortest connections.

    And I would avoid making this "tree" in the green rectangle. Just make simple and short connections. Short connections with 90° angles can be better than long and snake like connections with 45° angles.

    Kai

  • 0 in reply to kai klaas69
    TI__Expert 3620 points

    Excellent feedback Kai.

  • 0 in reply to kai klaas69
    Prodigy 250 points

    Many thanks @kai klaas69 !!

    I follow your suggestion to fix shape:

    As that picture, connector(J7~J9) seems to close OPA, longest path < 20mm (through 50 ohm resistor, OPA-> R18-> R25-> connector)

    But about the "tree" you mentioned,  I'm not sure my new layout whether correspond your suggestion.

  • 0 in reply to DeWei Lan
    Guru 140200 points
    Sam Burnett said:
    Excellent feedback Kai.

    Thank you, Sam Relaxed

    Hi Dewei,

    DeWei Lan said:
    As that picture, connector(J7~J9) seems to close OPA, longest path < 20mm (through 50 ohm resistor, OPA-> R18-> R25-> connector)

    Unfortunately, in this frequency range 20mm is not "close".

    Kai

  • 0 in reply to kai klaas69
    Prodigy 250 points

    Thanks @Kai's reply.

    OK!

    Maybe fixed gain and integrated product is a good choice for short trace, such as [OPA3692].

  • 0 in reply to DeWei Lan
    Guru 140200 points

    Dear Dewei,

    this is something which cannot be totally decided only by theory. You will need to fabricate a printed circuit board and carry out measurements. Then you will see how the board is behaving and whether the layout is already acceptable or needs some modifications.

    A good idea is to make two identical boards and make them differ in only the property in question. Then you can very clearly decide what version is better. Yes, I know this is time consuming, but that's how it is when designing a complex HF-circuit.

    Divide and conquer Relaxed

    Kai