DS90UB948-Q1: DS90UB948-Q1 and DS15BR401

vlad123 vlad321

Part Number: DS90UB948-Q1
Other Parts Discussed in Thread: DS15BR401

Hello. I have a board with a repeater based on the DS90UB948 and DS90UB947 chips and DS15BR401 buffers. The board should display images on 2 displays and connect to the next board in repeater mode.
Display G286HAN01.0 has 2 lvds interface. Inside it, each LVDS pair is terminated with a 50 ohm resistor. (not 100 ohms).

I'm using DS15BR401 buffers, which have 100 ohm resistors inside the output. When the display is connected, the total resistance becomes 33 ohms.

The display shows information well, but I doubt the correctness of the termination of the lvds line. On the oscillogram, the lvds signals at the output of the buffers change as if by a wave. And the signal at the input of the buffers is clean. Maybe I incorrectly terminated lvds or selected buffers?

over 4 years ago

0 vlad123 vlad321 over 4 years ago

Prodigy 150 points

0 Casey McCrea over 4 years ago in reply to vlad123 vlad321

TI__Guru** 103215 points

Hey Vlad,

It doesn't look like any of the the pictures came through. Can you repost those?

Best Regards,

Casey

0 vlad123 vlad321 over 4 years ago in reply to Casey McCrea

Prodigy 150 points

The first measurements I made with respect to the ground, then I used a differential probe and the signal became normal.

Uploading an archive with photos.

I will reformulate the question as follows.
Can DS15BR401 be used to drive a 50 ohm load?

repeater.7z

0 Aaron Heng over 4 years ago in reply to vlad123 vlad321

TI__Mastermind 31950 points

Hi Vlad,

Can you provide me the block diagram of your setup? Having the buffer will remove the back channel signal features.

Aaron

0 vlad123 vlad321 over 4 years ago in reply to Aaron Heng

Prodigy 150 points

block_diagram.7z

0 Aaron Heng over 4 years ago in reply to vlad123 vlad321

TI__Mastermind 31950 points

Hi Vlad,

Please correct me if I am wrong. Based on your diagram, it looks like there will be 947 to the 948, and after the buffer you are also try to connect the 947 to 948. Let me investigate closer, and I will get back with you. The only issue I am seeing is that you have an issue in layout fan out all the OLDI signal to all three buffers.

Aaron

0 vlad123 vlad321 over 4 years ago in reply to Aaron Heng

Prodigy 150 points

The board should display a picture on two displays and transmit the picture to the next board using the 947.

The signal from 948 is terminated at one point and then goes simultaneously to three places: 2 connectors through buffers, and 947 through buffers.

And the output 947 can be connected to the input of the next 948 board.

I post screenshots of the printed circuit board with notes.

pcb_screen.7z

0 Aaron Heng over 4 years ago in reply to vlad123 vlad321

TI__Mastermind 31950 points

Hi Vlad,

Thanks for the attachment file. I will review the PCB.

Aaron

0 Aaron Heng over 4 years ago in reply to Aaron Heng

TI__Mastermind 31950 points

Hi Vlad,

I reviewed your layout based on the snapshot pictures. Here is my suggestion.

You have the differential signal vias. You might want to put the differential ground via for the return path. This will prevent any impedance mismatch.
You have many vias fan out to different buffers. Are you planning to do the back-drilling to avoid relfection?
Most of differential pair signal are too close, you might get the cross-talking.
The output of the OLDI traces seem to be very. I would suggest to put the signal traces inside the inner layer to the nearest layer on opposite side of the component preventing the noise coupling into the signal. You should take this advantage; especially, you have the vias at the buffers.
There are resistors across P and N of the OLDI output. Please make sure you have the anti-pad to the next layer in order to maintain 100Ω impedance.

Aaron

0 vlad123 vlad321 over 4 years ago in reply to Aaron Heng

Prodigy 150 points

Thanks for your reply/

0 Aaron Heng over 4 years ago in reply to vlad123 vlad321

TI__Mastermind 31950 points

Hi Vlad,

Please let me know if you have additional questions.

Aaron

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DS90UB948-Q1: DS90UB948-Q1 and DS15BR401