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LMH1297: EQ Gain Adjustment for LM1297

user1475503
Prodigy 40 points
Part Number: LMH1297

We are currently evaluating LM1297 device with a 75m 12G-SDI compatible cable.

Usage mode:

SDI_IO operates as Cable Equalizer
SDI_OUT operates as Cable Driver

Measurement ①

DUT: LMH1297-EVM (Your evaluation module)
Measurement conditions: default
Measurement result:
- EQ gain value is 10.
- The eye is wide open. (Measured with Sigcon Architect)

Measurement ②
DUT: Our printed circuit board on which two LM1297s are.
Measurement conditions: default
Measurement result:
- The EQ gain value is 12.
- The eye is closed. (Measured with Sigcon Architect)

Measurement ③
DUT: Our printed circuit board on witch two LM1297s are.
Measurement conditions: Disabled the cable driver by SigCon Architect.
Measurement result:
- The EQ gain value will be 11.
- The eye is wide open. (Measured with Sigcon Architect)

My goal is that I make my printed circuit board stable with 75m cable.
I would like to know why the eye is opend in your evaluation module while the eye is closed in my board.
In measurement ③, if LM1297 adjust the EQ Gain value to 11, I would like to know if the eye open.

  • 0
    TI__Guru* 93670 points

    Greetings,

    Please note when SDI_OUT is enabled, we expect 10 to 15m of cable reach reduction. SDI-OUT causes cross talk and this reduces cable reach. 

    Given LMH1297EVM shows longer reach versus your own board, there are some points that could be checked:

    1). If possible please compare or check return loss of your LMH1297 design versus LMH1297EVM. If pcb layout is not optimum it is possible a poor return can cause cable reach reduction.

    2). Since you are seeing EQ gain of 11 versus 10 on the evaluation board, this shows the incoming eye diagram is more closed. Therefor the equalizer state machine applies more gain. This could be again due to return loss or maybe there is more cross talk on your board. 

    Suggestion is to use - to debug this issue - just one of the two LMH1297 devices on your board. Then re-run your test and compare with the LMH1297EVM.

    Additionally please check return loss - if possible - on your board and then compare with LMH1297EVM.

    Regards,, Nasser