LMH0387 used as an equaliser

Hi I am the FAE assigned to support BAE,

please could you answer their question, ie what tolerance and dielectric grade do we recommend?

There is also another question they have:

"I am running the LMH0387 as a cable equaliser on a 3.3V supply, and notice that the signal at its pin 8 (BNC_IO) has a dc level of approximately 3.3V. The LMH0387 must be setting the dc level, but is this what I should expect?"

many thanks

Dave

Hi Dave,

This is OK. The LMH0387 has the integrated return loss network and pull up termination inside the package. You are just measuring this high speed node from a DC perspective and seeing the node pulled up to VCC through the pull up termination.

Keep in mind that a high speed signal will view this node as having the return loss network plus the termination to an AC ground.

Mike Wolfe

DPS APPS / SVA

Mike,Thanks for the reply on the dc levels.
Do you have any information on the criticality of the 1uF and 220 ohm values on the TERM_RX pin?
Chris Dardry

Hi Chris,

Both the R and C are standard values, you should not have trouble finding those values. These components help to balance the differential receiver for single ended (coax) operation.

If you are asking about tolerances, up to 20% X7R should be fine for the cap. For the resistor up to 10% should be OK but it is easier to find 1% in the smaller SMT packages, which is what we typically recommend.

Mike Wolfe
DPS APPS / SVA

Mike,

 

Thanks for the answer.

 

Chris