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LMH0397: Capacitors for use in SMPTE applications

Conor
Guru 12185 points
Part Number: LMH0397

Tool/software:

Hi,

In the LMH0397 datasheet, the following is written in the OUT- and OUT+ terminals of pins 22 and 23 under Pin Functions.

Differential outputs to host video processor. On-chip 100-Ω differential termination.

Requires external 4.7-µF, AC-coupling capacitors for SMPTE applications.

We use the LMH0397 in SMPTE applications.

It says "4.7μF AC coupling is required," but if there are no problems with the device receiving this signal, would DC coupling be okay?

I think they just want to say that if you are AC-coupling in SMPTE applications, you need a capacitor of 4.7μ or more, but I would like to confirm just to be sure.

Thanks,

Conor

  • 0
    TI__Genius 9209 points

    Hi Conor, 

    A large AC coupling capacitor (4.7 µF) is typically used in transporting SDI serial bit stream to avoid low frequency DC wander. The idea is that SMPTE uses this large AC coupling cap to ensure that the bit stream has a heavy DC unbalance.

    DC wander can be detrimental to the bit stream, so it is recommended to not DC couple and use the 4.7uF cap on the OUT- and OUT+ pins.

    Best,

    Nick

  • 0 in reply to Nick Peabody
    Guru 12185 points

    Hi Nick,

    Nick Peabody said:
    DC wander can be detrimental to the bit stream, so it is recommended to not DC couple and use the 4.7uF cap on the OUT- and OUT+ pins.

    I would like to know more about the concerns when using DC coupling in SMPTE applications.
    It seems that AC coupling may have a negative effect on the bit stream, but is this ok?

    I looked up DC wander and found the following:

    --

    When using AC coupling, the DC balance, which means the even number of '0's and '1's in a special data pattern, is significantly disrupted, causing baseline wander (DC wander) and potentially data errors.
    --

    Thanks,

    Conor

  • 0 in reply to Conor
    TI__Genius 9209 points

    Hi Conor,

    AC coupling is recommended, but it is good to note that DC wander occurs from AC coupling. The capacitor value of 4.7uF is our recommended value that optimizes a combination of minimizing DC wander while maintaining AC coupling.

    Best,

    Nick

  • 0 in reply to Nick Peabody
    Guru 12185 points

    Hi Nick,

    I have one additional question. I am using the LMH0397 with the following internal route.

    HD-SDI (1.485Gbps) is connected to the "Rear SDI IN". A waveform monitor is connected to the "Rear SDI OUT" to observe the eye pattern.
    There seems to be no problem if I connect a signal source to the LMH0397 input via a short cable, but if I use a long cable (5C-FB cable 140m), the CDR seems to stop working.

    Is there any register setting that needs to be done to enable the CDR to lock when using this long cable (= input signal attenuated)?

    Thanks,

    Conor

  • 0 in reply to Conor
    TI__Mastermind 38735 points

    Hello,

    Due to a US Holiday, the response to your question may be delayed. We will look into your question when we return tomorrow, July 5th. Sorry for the wait and any inconvenience it may cause.

    Thanks,

    Ryan

  • 0 in reply to Ryan Kitto
    Guru 12185 points

    Hi Ryan,

    I understand. I look forward to hearing from you guys after the holidays.

  • 0 in reply to Conor
    TI__Genius 9209 points

    Hi Conor,

    I found the L-5CFB canare spec online and would like to confirm that I am looking at the correct data. See below for an image.

    According to the table, at 1.485Gbps a 140 meter L-5CFB cable would have (0.177db/m*140m) = 24.78 dB of loss, and the max transmission distance for the cable is 112m.

    Am I looking at the correct information?

    Best,

    Nick

  • 0 in reply to Nick Peabody
    Guru 12185 points

    Hi Nick,

    I understand that the maximum transmission distance of the cable is 112 meters.
    Going back to my original question:

    Is there any workaround (resistor setting, etc.) for the LMH0397 to be able to lock the CDR when using such a long cable (=input signal attenuation)?

    Thanks,

    Conor

  • 0 in reply to Conor
    TI__Genius 9209 points

    Hi Conor,

    Unfortunately, our register/resistors controls affect the signal after being relocked by the CDR. If the CDR is not able to lock to the incoming signal, it is too attenuated to work with at all.

    Best,

    Nick