# How to Measure the Open Loop Gain of an LVDS Driver?!

1. ## How to Measure the Open Loop Gain of an LVDS Driver?!

Hello,

I am very much pleased to know about the benefits of Negative Feedback and how it is used in conventional LVDS Driver Circuit. I am trying to design one for myself. But as per the negative feedback thery, the Open Loop Gain of the Amplifier/Core Module is normally very high. With feedback it comes down near unity to enforce very high linearity. Can anyone tell me how to measure the Open Loop Gain for an LVDS Driver System. I am very confused becasue if I check the differential gain i.e. (OP-ON)/(IP-IN) for the figure give in the attachment. I get -30dB gain????!! While measuring Open Loop Gain what should be the common mode input of the Driver?! Where am I going wrong with this?! Please give your suggestions

•

2. ## Re: How to Measure the Open Loop Gain of an LVDS Driver?!

Not quite clear what you are trying to achieve. There's negative feedback implemented for the common mode circuit but not for the differential mode driver path itself.

It's unusual to specify the differential driver behavior by an open loop gain, but you can expect maximum gain at the differential output zero crossing. The input stage of the LVDS driver isn't shown in the schematic, respectively we don't know what the real measurement setup is. Common mode output should be simply set by the existing CM control loop. CM input - if applicable. What's the input stage interface standard?

•

3. ## Re: How to Measure the Open Loop Gain of an LVDS Driver?!

You will have high gain only about the logic threshold,
at the normal resting logic level gain should be nil.

If you care about signal path gain then you need to
put the driver in a loop that holds the input near
threshold. Look to comparator testbenches, maybe
use a vcvs to turn differential into the normal single
ended output for such test setups. Then you can
run two simulations for two output voltages (one
output dV) and measure the input "gain proxy"
(one input dV) and there's your gain (the docs for
comparator / op amp test loops will give formulae
and "switch positions").

1 members found this post helpful.

--[[ ]]--