LVDS Driver common mode feedback question

[IADanilov]

lvds cmfb

Hi all!

In my LVDS driver design used common mode feedback. When I modeling simple model which include LVDS driver and 100 Ohm termination amplitude-frequency characteristic of CMFB is good: gain is about 75 dB and phase margin is about 80 deg.

But when I modeling it with more complex model which include also package inductance, package capacitance, transmission lines etc. amplitude-frequency characteristic become bad: phase margin is about 20 deg.

Question: what model shall i use? Can i neglect package inductance, package capacitance, transmission lines etc.?

 

[FvM]

lvds commond mode usage

It's no good idea to ignore reality, I think.

 

[IADanilov]

common mode feedback

More exact: problem cause only when i use model with transmission line.

Model with all reality elements (parasitic and package capacitances, inductance etc) and without tline has good amplitude-frequency characteristic of CMFB. When I include tline it become bad.

And I don't know what to do. I try to adjust frequency correction circuit, adjust CMFB circuit - without result.

And now I think have I a right to neglect tline in my model when I simulate amplitude-frequency characteristic of CMFB?

 

[FvM]

lvds common mode

I don't know, how your simulation scircuit exactly looks like. It may be the case, that your transmission line model is simply incorrect. You can't model a differential transmission line with a single a TL instance. It would cause infinite common mode load impedance. Define differential and common mode transmission line impedances and use an appropriate model.