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LMH0344: 50R-SE Input Termination

ROBERT BONINI
Intellectual 320 points
Part Number: LMH0344
Other Parts Discussed in Thread: DS30EA101

We have a design where we will be feeding an LMH0344 from a 50R single-ended source.  The LMH0344 specifies a return loss network coming from a 75R single-ended source.  If the LMH0344 inputs are not terminated internally, what is the purpose of this network?

My initial thought for terminating a T-line to a non-terminated IC input would look like this:

Am I missing something about the LMH0344 inputs that would require additional matching components for a single-ended 50R source?

  • 0
    TI__Guru* 88835 points

    Hi Robert,

    LMH0344 main application is 3G SDI compatible with different SMPTE standard. This standard requires certain return loss specification. The intention is to make sure device presents as close to 75-ohm impedance as possible across signal frequency band.  

    What you have should work as well. I would move 49.9-ohm R to the left side of the 1uF cap to prevent DC signal path to ground(if there is a potential difference between TX and RX).

    There is another part, DS30EA101 (  https://www.ti.com/lit/ds/symlink/ds30ea101.pdf?ts=1603827788297&ref_url=https%253A%252F%252Fwww.ti.com%252Fsitesearch%252Fdocs%252Funiversalsearch.tsp%253FsearchTerm%253Dds30ea101) - that could be of interest as well. You can take a look at this part as well.

    Regards,, Nasser 

  • 0 in reply to Nasser Mohammadi
    Intellectual 320 points

    Hi Nasser,

    Thanks for your help.  Do you have a resource explaining the effect/operation of the return loss network on page 11 of the LMH0344 datasheet?  If improved performance can be achieved in using the datasheet's approach then that seems a worthwhile endeavor.

    Correct me if I am wrong, but the 49R9 resistor is already on the left side of the 1uF capacitor, did you mean the right side?  From what I can tell, the SDI device datasheets from TI all seem to avoid a DC path to ground from the input/output pins.

  • 0 in reply to ROBERT BONINI
    TI__Guru* 88835 points

    Hi Robert,

    1). Return loss network is a better approach since it assures impedance is close to the idea value across spectrum. Having a 49.9-ohm R just assures DC return loss. However, at 49.9-ohm impedance we haven't done return loss optimization to achieve optimum. Also, different spec or standard may require different requirement. Normally the termination at 49.9-ohm is like what you have and this is what is typically used.

    2). My mistake, yes 49.9-ohm R should be on the right hand side of the AC coupling Cap.

    Regards,, Nasser

  • 0 in reply to Nasser Mohammadi
    Intellectual 320 points

    Hi Nasser,

    To compare the two, I ran the following simulation:

    With the following results:

    As you can see, the described network achieves a worse but more variable RL result.  Is there something missing in the simulation?

    Additionally, could you describe why only the 49R9 resistor should be on the right side of the AC coupling cap?  That would create a DC path to ground for the positive SDI input, but not the negative SDI input.

  • 0 in reply to ROBERT BONINI
    TI__Guru* 88835 points

    Hi Robert,

    There are a couple of observations:

    1). R11 or R7 for LMH0344 is normally 1.3K ohm single ended.

    2). At DC we should have optimum or best return loss. This is because L1 is shorted and essentially we have 75-ohm source(R10) driving 75-ohm(R12) load. Therefor return loss in negative infinite or optimum. In your graph, it shows close to 75-ohm at DC and then impedance drops (i.e return loss degrades) - perhaps capacitive load, at higher frequency.  Then there are two resonance at higher frequency. For a device like LMH0344 rated at 3Gbps, noramlly return loss is measured up to 3Gbps since most of the signal energy is at nyquist(i.e 1.5GHz).

    3). On the positive side, source may introduce DC shift. On the other hand, on negative side, termination is connected to the local GND. Also, you can connect negative side to resistor and then AC coupling Cap to ground on negative side as well.

    Regards,, Nasser 

  • 0 in reply to Nasser Mohammadi
    Intellectual 320 points

    Hi Nasser,

    Sorry, I think the graphs may have been a little confusing.  The vertical axis of the graphs is return loss (dB), not impedance.  Also, the bottom graph corresponds to the top circuit, and the top graph corresponds to the bottom circuit (see the axis labels at bottom-left in green).

    Sorry, I don't follow your reasoning/explanation regarding the DC offset issue.  Shouldn't both inputs be AC coupled and be driven by the same source impedance?  Wouldn't tying one to GND cause an imbalance that degrades CMRR and other input performance characteristics?

    Thanks, Robert

  • 0 in reply to ROBERT BONINI
    TI__Guru* 88835 points

    Robert,

    I forgot you aree using the part in 100-phm differential mode versus single ended 75-phm mode. For 100-phm differential input, you can do the same as what is done in DS30EA101 data sheet figure 3:

    https://www.ti.com/lit/ds/symlink/ds30ea101.pdf?ts=1604355528343&ref_url=https%253A%252F%252Fwww.ti.com%252Fsitesearch%252Fdocs%252Funiversalsearch.tsp%253FsearchTerm%253Dds30ea101

     Regards,, Nasser

  • 0 in reply to Nasser Mohammadi
    Intellectual 320 points

    Hi Nasser,

    Not using it in 100R differential mode.  We are using the part with a 50R single ended input.  Simulations above are for comparing the single-ended return loss network of the LMH0344 datasheet to the single-ended termination scheme of something like the DS30EA101 you mentioned above.  The question is why one technique is chosen over the other, and how the return loss network operates to provide said benefit.

  • 0 in reply to ROBERT BONINI
    TI__Guru* 88835 points

    Hi Robert,

    Return loss network of the LMH0344 is preferred over the scheme used in DS30EA101. This is because return loss network is tuned to make sure we get as close to the target impedance across signal bandwidth as possible. On the other hand, DS30EA101 scheme just sets the DC impedance - not across the signal bandwidth.

    Regards,, Nasser