LVDS is a differential signal system that is typically used for high speed data rate transfer. LVDS is not an interface protocol, but rather the hardware mechanism that is used to transmit the data across wires. This system can be used with either serial or parallel or a hybrid interface protocol.
The operation basically has the LCD controller transmitting two logic voltage levels that are compared in the LCD module. LVDS uses the difference in voltage between the two lines to encode information. These two lines are typically twisted pairs. The transmitter sends a small current into one line or the other, depending on the logic level to be sent. The current passes through a resistor which is matched to the impedance of the line at the LCD module, then returns in the opposite direction along the other wire. The LCD module senses the polarity of the voltage across the resistor to determine the logic level.
This type of signal minimizes the amount of radiated electromagnetic noise and power lost to conductor resistance. LVDS is less susceptible to noise than single wire data systems because it uses two wires with opposite current/voltage swings instead of the one wire used in single-ended methods to convey data information. The advantage of the differential approach is that if noise is coupled onto the two wires, the noise appears on both lines equally, and is mostly self-cancelling. The op amp at the receiver/LCD module end basically only looks at the difference between the two signals. Using this differential convention, the signals also tend to radiate less noise than single wire data signals due to the canceling of the induced magnetic fields. Constant current drivers are also not prone to ringing or switching spikes, which further reduces noise.
How LVDS Works
The schematic diagram shows a simple LVDS driver and receiver. It consists of a current source that drives the differential pair lines and an op amp receiver with high input impedance. This causes most of the driver current to flow through the termination resistor, which generates a voltage across the receiver inputs. When the driver switches, it changes the direction of current flow across the resistor, which changes the voltage sense from positive to negative, which creates a logical "one" or "zero".