The standard defines the input voltage range of the receiver, VIN, to be 0 to 2.4 V. This input voltage range is significantly larger than the range of expected voltages from the driver. This difference provides the ability to absorb and reject common-mode noise, noise that is present on both lines of the differential pair, and allow for offsets between the driver and the receiver. Refer to Figure 3 for an illustration of this common-mode noise rejection.
The differential nature of LVDS has many inherent advantages. The most fundamental of these advantages is the ability to reject common-mode noise. When the two lines of a differential pair run adjacent and in close proximity to one another, environmental noise, such as EMI (electromagnetic interference), is induced upon each line in approximately equal amounts. Because the signal is read as the difference between two voltages, any noise common to both lines of the differential pair is subtracted out at the receiver. The ability to reject common-mode noise in this manner makes LVDS less sensitive to environmental noise and reduces the risk of noise related problems, such as crosstalk from neighboring lines. As a result, LVDS can use a much lower voltage swing compared with traditional single-ended schemes that rely on higher voltage swings to maintain an adequate threshold for noise tolerance.
The differential nature of LVDS not only reduces the effects of common-mode noise, it also results in a reduced amount of noise emission. When the two adjacent lines of a differential pair transmit data, current flows in equal and opposite directions, creating equal and opposite electromagnetic fields that cancel one another. The strength of these fields is proportional to the flow of current through the lines. Thus the lower current flow in an LVDS transmission line produces a weaker electromagnetic field than other technologies.
At first glance it may seem that one of the drawbacks of using LVDS in an application rather than a traditional single-ended data transmission method is that it requires twice as many wires to transmit the same number of channels. In reality, an LVDS application can easily reduce wires between the transmitter and receiver. With the higher data rates available in LVDS, the same amount of data can be transmitted serially across a single channel, avoiding the necessity of transmitting multiple bits in parallel at slower data rates to achieve the same throughput. Multiple channels of slower parallel data can be serialized onto a single high-speed LVDS channel and transmitted from one point to another. At the receiver the data can then be deserialized and separated into the slower parallel channels. The combination of a serializer and deserializer (SerDes) is a common architecture found in many applications today including CameraLINK and PCI Express.
Another major benefit of LVDS is the low power consumption of LVDS devices. The current-mode driver of LVDS provides a constant 3.5 mA of current through the differential pair. The power consumption at the load can be calculated using the power equation, P = I2R, which states that power is equal to electrical current squared times resistance. Given the 3.5 mA of current through the 100 Ω termination resistor, the power equation results in (3.5 mA)2 x 100 Ω = 1.2 mW. In comparison, another differential data transmission technology, RS422, dissipates 90 mW of power at the load. Other differential signaling technologies, such as RS485, ECL, and PECL, also dissipate significantly more power than LVDS.