LMH0397: some questions about LMH0397 layout

Yuchen Zhang

Part Number: LMH0397

Hi, we are trying to develop a SDI repeater with LMH0397. But we have some question about LMH0397 layout：

# 1

The datasheet suggest us to place the power supply decoupling capacitors on the bottom layer. But won't the vias lead into more inductance? Is it OK to put the capacitors on the top layer?

# 2

We are trying to use LMH0397 as a SDI repeater, so we are gonna set LMH397 in EQ mode and use SDI_OUT+- and SDI_IO+-, but at EQ mode, the OUT+- is default enabled, and we are not gonna use it. Should we just leave it float on the PCB ?

# 3

If OUT0+- can be float, then can we use Layer2 as the reference of SDI_IO SDI_OUT 75Ω impedance ?

over 2 years ago

+1 Nasser Mohammadi over 2 years ago

TI__Guru* 93670 points

Greetings,

1). It should be fine to put the supply coupling Cap as close to the device pins as possible.

2). It would be a good idea to terminate with AC coupling Caps and 49.9-ohm Resistor versus leave them floating.

3). This depends on your board stack up. Layer 2 or layer reference plane is fine as long as you maintain 75-ohm impedance.

Regards,Nasser

0 Yuchen Zhang over 2 years ago in reply to Nasser Mohammadi

Prodigy 10 points

Hi. Thank you for your reply.

About Q2, the datasheet (Table 13. LMH0397 General Design Requirements) says "Input and output terminations provided internally. Do not add external terminations. "

Doesn't this description apply to this situation?

+1 Nasser Mohammadi over 2 years ago in reply to Yuchen Zhang

TI__Guru* 93670 points

Greetings,

There is internal termination. It is a good idea to provide external termination to complete impedance matching throughout the part and external to the part.

Regards,Nasser

0 Yuchen Zhang over 2 years ago in reply to Nasser Mohammadi

Prodigy 10 points

OK, I understand. Thank you so much for your reply and support !

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LMH0397: some questions about LMH0397 layout