LMH0074: LMH0074 input interface to CML

David Cole72

Part Number: LMH0074
Other Parts Discussed in Thread: SN65CML100, , DS15EA101

We are trying to interface the input of the LMH0074 to the output of the sn65cml100 driver. The output circuit is transformer coupled at the source driven into twisted shield pair cable. We A/C couple the inputs into the LMH0074 with a 100ohm diff term. We cannot seem to recover the signal correctly at the output of the LMH0074 and we hav ried adding 50ohm term to +3.3V on each leg of the CML input into our board with results the same. Any recommendations?

over 5 years ago

0 Nasser Mohammadi over 5 years ago

TI__Guru* 91860 points

Hi David,
For 100-ohm interface, normally we recommend DS15EA101 device. Having said this, LMH0074 should work as well - provided it is biased correctly or the same as what we recommend in DS15EA101 data sheet. Please check DS15EA101 input circuit and confirm you are following the same with LMH0073. Additionally if you put your high impedance differential low capacitance scope on 100-ohm termination, you should see your eye diagram. Otherwise, perhaps you have to look at your transformer requirements.
Regards,,nasser

0 David Cole72 over 5 years ago in reply to Nasser Mohammadi

Prodigy 150 points

Thank you for the reply. We will try the DS15EA101 part and see if we get the same results along with proper transformer selection.
Regards, David

0 David Cole72 over 5 years ago in reply to David Cole72

Prodigy 150 points

We noticed that on both the LMH0074 and the DS15EA101, it states "The input must be AC coupled. Transformer coupling is not supported"

The customer end is transformer coupled, but we are A/C coupled before we go into the device. I assume this will pose no issue. Not sure I understand why this device cannot use transformer coupling?

Thank you

0 Nasser Mohammadi over 5 years ago in reply to David Cole72

TI__Guru* 91860 points

Hi David,
As long as impedance and transformer bandwidth allows to recover the video, device would not know if this is a transformer or 100-ohm differential signal on it's input.
Regards,,nasser

0 David Cole72 over 5 years ago in reply to Nasser Mohammadi

Prodigy 150 points

Nasser, we have had good results with the DS15EA101 device. Do you have details of how the DS15EA101 is different than the LMH0074? Do they have different input stages or internal termination?

Thank you

0 Nasser Mohammadi over 5 years ago in reply to David Cole72

TI__Guru* 91860 points

Hi David,
They have the same internal impedance and common mode voltage. I don't see a difference.
Regards,,nasser

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LMH0074: LMH0074 input interface to CML