How to convert 1.8v common-mode voltage to 1.2v

[waosai]

0.5v common-mode level

I have met a problem in my circuit:
The input is a differential signal whose freq is 125MHz,common-mode voltage is 1.8v,single-end voltage swing is +- 0.5v,power supply is 1.8v.
I need an output differential signal whose common-mode voltage is 1.2v,
single-end voltage swing is +-0.35v.
How can I design this circuit?

BTW,who knows how to design auto cross interface in ethernet phy chip?
THX!

 

[flatulent]

One brute force method is to use a transformer with the turns ratio to make the differential levels you need and then apply the DC common mode level to the secondary center tap.

 

[yeewong_su]

Maybe, you can use the level shift

 

[lokeyh]

You can try to use a source follower to level shift the signal.

 

[JPR]

I see a couple of options for you.

The first is rude and crude, but might do what you need. If you use a resistor divider, you can shift your signal from 1.8V to 1.2V and your signal swing from 0.5V to 0.33V. This would not provide the exact specification you gave, but might be good enough for your application.

A second option (also crude) would be a combination of capacitor divider and resistor divider. This can bring the DC voltage division from 1.8 to 1.2, and the high frequency division from 0.5 to 0.35 by proper scaling of resistors and capacitors. The ratio of R's will provide the DC division. The ratio of 1/C's would provide the high frequency division, and the RC product will define the frequency response.

A more elegant solution would be to use a diff-in diff-out amplifier, with a differential gain of 0.7, and either a fixed DC common mode output voltage of 1.2V, or a common mode gain of 2/3.

I would not dismiss the rude and crude versions, if they do what you need, as they would provide more reliability, are much quicker to implement, and you can pretty quickly determine if they will work.

 

[sunking]

you can use full diffierical opa to realize it, whose common mode voltage is 1.2V