LMH1218: Differential input AC coupling

Eric Duong

Part Number: LMH1218

Hi,

The LMH1218 is required 4.7uF AC-coupling cap at differential input IN0+ and IN0- . Can it be place anywhere on the transmission line or it preferred to be closed to the LMH1218. Please advise. Below is my current configuration.

Thanks,

Eric

over 6 years ago

0 Nasser Mohammadi over 6 years ago

TI__Guru* 93570 points

Hi Eric,

It's better to put it very close - about 100mils - to the device or where there is a big impedance change. This is because there is impedance drop due to the package parasitic. Impedance drop due to the AC coupling cap could be swamped by the bigger impedance drop due to the device package.

Regards,,nasser

0 Eric Duong over 6 years ago in reply to Nasser Mohammadi

Prodigy 140 points

Hi Nasser,

Below are the eye plots captured at the LMH1218 differential input IN0+/IN0- (pin11 & pin12). #1 - without and #2 - with the device installed. You can see the distortion dramatically with the device installed. Do you think this could be the result of the 4.7uF AC coupling caps is placed far away from the device (approx. 6 inch)?

1- Without the lmh1218 installed (with external 100OHM termination)

2- With lmh1218 installed

Thanks,

Eric

0 Nasser Mohammadi over 6 years ago in reply to Eric Duong

TI__Guru* 93570 points

Hi Eric,

I don't think AC coupling caps being far away should cause this.

One question, under this condition, 75-ohm output of the LMH1218 provides clean video signal? If you connect LMH1218 75-ohm to a monitor or video analyzer all look good ?

Regards,,nasser

0 Eric Duong over 6 years ago in reply to Nasser Mohammadi

Prodigy 140 points

Hi Nasser,

Below is the output of LMH1218 OUT0+ (75 OHM) connects to PHABRIX

Thanks,

Eric

0 Nasser Mohammadi over 6 years ago in reply to Eric Duong

TI__Guru* 93570 points

Hi Eric,

1). I still cannot see the image. So I suppose under this condition video cannot be decoded by Phabirx?

2). Please confirm you are issuing or using complete initialization sequences noted in the data sheet and programming guide.

Regards,,nasser

0 Eric Duong over 6 years ago in reply to Nasser Mohammadi

Prodigy 140 points

Hi Nasse,

Hopefully, you can see the image this time.The video is decoded by Phabrix but it failed the jitter SMPTE standards 100Khz.

Thanks,

Eric

0 Eric Duong over 6 years ago in reply to Eric Duong

Prodigy 140 points

phabrix_sdi_100khz.pdf

0 Nasser Mohammadi over 6 years ago in reply to Eric Duong

TI__Guru* 93570 points

Hi Eric,

Something doesn't look right. You are getting 1.15UI of alignment jitter. I suggest if you can please get one of our LMH1218EVM for correlation and check/correlate with your own result. I expect far less jitter.

Another thing you can try, Do you see a similar result using a clean lab supply ? We should try to see where this extra jitter is coming from.

Regards,,nasser

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LMH1218: Differential input AC coupling