Low‐Voltage Differential Signaling - 180D-FW-2023/Knowledge-Base-Wiki GitHub Wiki
What is LVDS?
Low Voltage Differential Signaling (LVDS) is a versatile interface to send and receive signals. It is a fast interface as well as low voltage making it useful across a wide array of industries and applications. At the core of LVDS is a transmitter with a non-inverting and inverting output and a receiver with the same inverting and non-inverting input. Instead of measuring the difference between the signal and ground, like with single-ended signaling, LVDS measures the difference between the inverting and non-inverting levels of the signals.
Before LVDS...
In the old days, a signal would be sent with a high voltage or ground. This was a simple way to send over data but it had noise limitations. As noise over the ground signal increased, our signal would be directly affected, greatly decreasing how much data could be sent before losing information.
The old solution to this problem was to increase the voltage of our signal was so noise would be proportionally smaller. This wasn’t a free solution. Higher voltages meant that it took our system longer to switch between high and low, limiting how much data we could send. Once that limit was reached, designers started using these cables in parallel but that meant we needed to have more complex systems to handle these signals. We would need to be able to receive multiple signals at the same time as well as handle the noise induced from neighboring wires. LVDS was developed to overcome limitations that came with these single ended systems.
How LVDS works
(source https://hackaday.com/2016/03/29/when-difference-matters/)
LVDS takes advantage of differential signaling to lower the voltage potential we need to send a bit. Low voltage means that we'd have low power costs and fast switching speeds. A 1 or 0 is measured by the difference between the two input voltages. This creates means that most noise induced in one wire, the same noise will be induced in the other wire, making it so that the difference in voltages is still the same that’s outputted by the transmitter.
LVDS Components and Implementation
The interface has a transmitter and a receiver. It is assumed that the sender and receiver would share a ground so that the relative difference between the transceivers is minimal.
To send a bit it is first encoded in differentially by sending a high voltage through one wire and a low voltage through the other wire. What this looks like is that we would have 2 wires named P and N. To read a 1, P would be driven up to 1.4V and N would be driven down to 1V. The receiver would then have a 400mv differential across the terminals and read a 1. To read a 0, N would be high and P would be low. At the same time clock signals are sent along with the data so that the transmitter and receiver are synched up.
A current driver is used to in conjunction with a matched load to prevent reflections. A termination resistor is used to generate the voltage needed for the receiver input to read. To minimize board costs it is assumed every receiver provide's it's own termination resistor as well. There exist more exotic ways to terminate the load but for most applications simplicity is king as it's effective and cheap to implement.
There are two main standards used in industry for LVDS, the industry standard ANSI/TIA/EIA-644 and the IEEE standard SCI (Scalable Coherent Interface). The ANSI standard is more generic than the IEEE standard so it sees more use. ANSI focuses on the electrical characteristics of LVDS like the voltage swing, timing requirements, and physical properties so that it is up to the implementer to decide how they want to send and receive information. SCI is more restrictive because it defines electrical characteristics of the interface as well as a data encoding protocol. SCI encoding specifies how packet types, packet lengths, and idle symbols are uniquely identified. For example, there are specific packets that signifies to the receiver how it should sync up with the sender. A flag signal is sent before and after each packet to signify the starts and end of each packet. One symbol is sent in one transfer period. The SCI protocol was designed with the intention of replacing positive emitter coupled logic in existing multiprocessing systems.
Applications of LVDS
As the standard is a relatively simple electrical standard, LVDS is used all the time for electronics.
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Automotive Industry: The standard's resistance to noise allows LVDS to shine in a high noise environment. The signal format doesn't induce a lot of noise and is low voltage as well. In tight spaces like a car, this works very well for video transmission car navigation displays.
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Medical Field: The medical industry has particularly high standards when building devices so ensuring a reliable signal is critical to patient safety. It is especially useful for live devices like ultrasound scanners that need to switch between sending control signals, receiving input, and distributing clock data.
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Communication Systems: When sending large amounts of data, having a simpler standard is useful for implementation and cost. With a single pair of LVDS wires, one can easily achieve gigabit transfer speeds.
References
https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=503756
https://www.ti.com/lit/an/snla165/snla165.pdf
https://www.ti.com/lit/an/slla038b/slla038b.pdf
https://hackaday.com/2016/03/29/when-difference-matters
https://resources.pcb.cadence.com/blog/2023-the-lvds-interface
https://www.quadrangleproducts.com/what-is-lvds-used-for/
https://www.analog.com/media/en/technical-documentation/application-notes/AN-1177.pdf