How to decide what should be output common mode voltage in an LVDS

[circuitking]

Hello, while designing LVDS driver, there is a need for CMFB circuit to stabilize the output common mode voltage. how do I decide on what should be the output common mode voltage value? what factor impact in choosing the value.

 

[Dominik Przyborowski]

This value is specified by standard and you have to follow it.

 

[FvM]

All LVDS standards specify 1.2 V nominal, range is different, e.g. 1125 mV min, 1275 mV max in IEEE 1596.3, 1.2 to 1.4 V in mini-LVDS,

 

[circuitking]

All LVDS standards specify 1.2 V nominal, range is different, e.g. 1125 mV min, 1275 mV max in IEEE 1596.3, 1.2 to 1.4 V in mini-LVDS,

My supply is limited to 1.2 V. Is it still possible to get 1.2 V for output common-mode voltage?

 

[FvM]

1.2V supplied logic can hardly comply with LVDS standard. There are other high speed differential IO standards better suited for low supply voltage.

 

[BradtheRad]

My supply is limited to 1.2 V. Is it still possible to get 1.2 V for output common-mode voltage?

A PNP transistor can provide a low-ohm path to the positive supply rail. Just like an NPN provides a low-ohm path to ground.

Or, reverse operation so that it pulls varying amounts of current from a pullup resistor connected to positive supply. Suppose you draw no current, then the node gives entire 1.2 V.

 

[KlausST]

Hi,

There are LVDS buffers or translator ICs. They may be used to get true LVDS levels. But usually they need to be supplied with the specified voltage.
(Don't know whether the 1.2V requirement is just from your IC or from the complete application)

Klaus

 

[circuitking]

(Don't know whether the 1.2V requirement is just from your IC or from the complete application)

Klaus

We are using 1.2 V only for all our application.s I could go and use 3.3 V also but we just want to use only one supply i.e., 1.2 V

 

[FvM]

As previously stated you can use differential signalling with 1.2V supply, but it's not compliant with LVDS standard. AC coupling may be a simple way to get 1.2V CM though if your data stream is bit balanced.