Electronic – From where does the Common Mode 1.2V of LVDS comes from

lvds

I am trying to understand the LVDS signalling.

But from where do we get the 1.2V common mode voltage?

Thanks

But from where do we get the 1.2V common mode voltage?

That depends, there are several ways to generate an LVDS signal with a common mode voltage of 1.2 V.

Generally an LVDS signal can be generated like this:

Image from here.

However, here the common mode voltage is actually undefined!

If the top current source is actually not 3.5 mA but 3.499 mA and the bottom current source is 3.500 mA then the bottom current source "pulls harder" and that will pull down the common mode level of the LVDS signal. In the real world current sources never have exactly identical values so with this circuit we cannot set the commonmode level easily. (There is a solution though, a common mode feedback loop which would adjust the value of one of the 3.5 mA current sources such that the common mode level becomes 1.2 V. For an LVDS circuit, that's a bit overkill and too complicated, there are simpler solutions).

A simple way to set the common mode voltage is by using some resistors. Here's an example:

Image from here.

The two 250 ohm resistors to a 1.2 V reference voltage set the common mode voltage.

As these resistors are for "DC only" they need to have such a value that they do not influence the impedance of the LVDS lines too much. This means the resistors need to have a value that is significantly larger than 100 ohm which is the value of the termination resistor. For the LVDS signal those two 250 ohm resistors behave like they are in series so that makes then 500 ohms, which is significantly higher than 100 ohms.

The circuit to set the common mode voltage can be at the receiver side or the transmitter side. Since it is DC, the location does not matter much.