In 1994, the United States National Semiconductor (National Semiconductor) proposed a new high-speed signal transmission level standard: LVDS, and still in use today.
So, what are LVDS?
● V-Voltage (voltage)
As the name implies, LVDS is “low-voltage differential signal”, so its main characteristics can be divided into two points: “low voltage” and “differential”.
LVDS signals have a small voltage swing, typically around 350mV, while current-driven output is only 3.5mA (100Ω terminal).
Different from the traditional “single end” transmission of “signal wire + ground wire”, a pair of differential PCB wiring or balancing cables complete the transmission of data signals through the “difference” transmission of “signal wire + signal wire”.
Compared with single-ended signal, after using differential signal, one signal line will be changed into a pair of signal lines. The wiring area and interface complexity of LVDS will inevitably increase, and special terminal resistance needs to be matched, so the occupied space and production cost will inevitably increase.
But the advantages of LVDS over more complex environments are also obvious.
01 Low power consumption
An LVDS driver consists of a current source that drives a differential line pair, usually 3.5mA;
LVDS receivers have a very high input impedance, with a matching terminal resistance of 100Ω, resulting in a voltage of approximately 350mV at the input end of the receiver.
LVDS working diagram
02 Low requirements on “ground”
Ideally, we usually use “ground” as the zero potential point, but in practical environmental applications and industrial production is not the case, “ground” is a highly variable thing.
For example, the two ends of the transmission are regarded as two points A and B. Due to certain changes in the current (such as large and small), and the structure layout of the line is not invariable (due to the movement of the equipment, far and near), the “ground” will produce certain voltage fluctuations.
So from A’s point of view it looks like 1.2V, from B’s point of view it looks like it’s only 1V, and then you’re going to have an error in there, and that’s what the difference in ground potential does to the single ended signal.
LVDS is a “differential signal” of a pair of signal lines. Since the two signals are relative to the ground, when the local potential changes, the two signals will go up and down at the same time, but the voltage difference between the two lines rarely changes.
So as long as the ground potential changes within a certain range, the differential signal can be almost unaffected.
03 Low noise
If there is always a 0.2V electromagnetic interference source in the external environment, the single-ended signal will always be affected by this electromagnetic interference.
In the case of a differential signal, because both signal lines are subject to this electromagnetic interference, the electromagnetic interference is subtracted after the difference is made. Therefore, LVDS can effectively offset common-mode noise and electromagnetic interference, with strong anti-noise ability.
tips: In the transmission process of image data, the lower the noise, the better the image quality.
LVDS is defined in two international standards: IEEE P1596.3 (adopted in March 1996), mainly for SCI (ScalableCoherent Interface), defines the electrical characteristics of LVDS, but also defines the encoding of packet switching in SCI protocol; ANSI/EIA-644 (adopted in November 1995), primarily defines the electrical characteristics of LVDS and recommends a maximum rate of 655 Mb /s and a theoretical limit rate of 1.823Gb/s on distort-free media.
Physical media-independent features are specified in both standards, ensuring that LVDS can be used as a multipurpose interface standard.
< 12G-SDI code board – connected LVDS wire >
Forward and reverse LVDS lines
The KEL protocol in Japan does not distinguish the forward and reverse LVDS lines. But in the practical application of industry, LVDS lines are divided into positive and reverse.
So there are a lot of door-to-door consultation customers, do not know how to distinguish the positive and reverse LVDS line.
The following Xuan exhibition Wang Gong, teach you how to quickly distinguish LVDS line of the forward and reverse video, interested may wish to click to see.