Interfacing LVDS Driver With a Sub-LVDS Receiver

Abstract

Sub-LVDS is a reduced voltage version of the LVDS electrical specification. Sub-LVDS varies from LVDS in that its common mode and differential signal levels are reduced, but are still able to drive an LVDS receiver. The problem arises when an LVDS driver needs to interface with a sub-LVDS receiver. Since the drivers’ signal voltage levels are not in a suitable range to ensure proper communication with the receiver, it is necessary to alter the drivers output to guarantee appropriate voltage levels. This application report discusses how to interface between an LVDS driver and sub-LVDS receiver and how a simple resistor network can be used to modify the signal voltage levels in a way to ensure compatibility.

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1 Introduction

In order to use an LVDS driver such as the TI DS90LV011A and pair it with a Sub-LVDS receiver, a step-down resistor network must be used in order to provide the proper interface between the LVDS driver and Sub-LVDS receiver. Since an LVDS driver has a typical fixed common mode voltage output of 1.2 V and typical differential voltage swing of 350 mV, the goal of the step-down resistor network is to achieve a fixed common mode voltage of 0.9 V as well as output differential voltage swing of 150 mV. Table 1-1 shows the differences between LVDS driver output levels and Sub-LVDS receiver input levels.

There are two methods in which you could implement this step-down resistor network. The method chosen depends on whether or not your receiver has internal termination integrated into it or not. The first section will focus on the formulas and simulations in a device without internal termination, while the second portion will focus on a device that includes internal termination.

Table 1-1 LVDS Driver Output and Sub-LVDS Receiver Input

	LVDS Driver Output Levels			SubLVDS Receiver Input Levels
Parameter	Min	Typ	Max	Min	Typ	Max	Unit
V_CMF Fixed Common Mode Voltage	0.05	1.2	2.35	0.5	0.9	1.3	V
V_OD Differential Voltage Swing	247	350	454	100	150	200	mV
V_TH Threshold Voltage				-25		25	mV