LMH1239: Eye opening quality

Hi Luke,

Happy to review the schematic and layout. You can upload the files here or we can continue this in a private message space over E2E.

One question on my side: Are you checking the eye opening through the LMH1239 EOM, or are you probing the input pins with a scope?

• If you can share the eye opening waveform you're seeing that would help

Best,

Shane 

Hi Shane, thanks for your reply. Yes, we are checking the eye opening using the built-in EOM. I have reconstructed the eye using the output data and then importing it into an excel spreadsheet. The HEO = 0.48UI / VEO = 250mV.

This is the EOM output from the LMH1219 we have on another design.

I would prefer to share schematics and layout privately if that's ok?

Thanks,

Luke

Hi Luke,

I've sent you a friend request over E2E. If you can accept this request, it will allow me to create a private message space for sharing schematic and layout files:

Additionally, I found this E2E post with a presentation covering the EOM on our LMH12XX parts. On slide 4 this presentation mentions error-free operation with a VEO>150mV and HEO>0.45UI:

For my understanding, do you see errors in the recovered data or are we mostly looking to optimize the signal?

Best,

Shane

Hi Shane, yes I've seen that post. I am seeing occasional errors on the input, I think it is on the very edge of working. This is also with a 1m cable from a Phabrix Qx, so I am expecting it to work error free. As soon as I get the private space, I will upload design files. Thanks, Luke

Thanks for the context,

I've sent a message in our private message space. You should be able to access this from the top right corner chat bubble button:

Let me know if you have any issues seeing this space.

Best,

Shane

Hi Luke,

The LMH1239 schematic looks ok. Let me take a look at the layout files and follow up with my feedback later this week.

One question: Do you have an Altium or Cadence file you can share for the layout? The Gerber files show routing but do not have more specific info like trace lengths or symbol data.

Best,

Shane

Hi Shane,

Thanks for getting back to me on this. Glad the schematic looks good! We use ECADstar so not sure if the output from this is any good for you? I can send it over if it is but if not, then if you let me know which traces you want extra information on, then I can provide that no problem.

Thanks,

Luke

Hi Luke, I haven't worked with ECADstar before so I'll stick to the Gerbers you've sent. Will let you know if any traces or surface mount devices stand out.

Best,

Shane

Hi Luke,

Here are my comments on the layout:

1. I recommend reducing the size of surface mount devices on the high speed traces. Ideally it is best to keep the pad size on capacitors/resistors as close to the trace size as possible. When you use large pads on a small trace, the change in size can cause signal reflections near the pad.

2. Make sure the trace impedance is routed at a constant 75ohms near the BNCs and a constant 100ohm differential on OUT0+/-. Changes in trace impedance will cause reflections and degrade signal quality.

3. What trace width and lengths are used between the LMH1239 and the BNCs?

Since you're using both the SDI OUT BNC and the OUT0 differential output, do you notice the occasional errors on both outputs, or only one? 

• The EOM is only helpful for viewing the incoming signal, so I'm wondering if there is an output dependency to the errors.

Best,

Shane