LVDS: Low Voltage Differential Signaling

Table of Contents

LVDS-low voltage differential signaling

what is LVDS?

LVDS (Low Voltage Differential Signaling) is a small-amplitude differential signaling technology that uses very low-amplitude signals (250mV~450mv) to transmit data through a pair of parallel PCB traces or balanced cables.

Benefits of LVDS Technology

Reduce EMI

The transmission standard of LVDS uses a current mode to drive the output, which does not generate ringing and peak signals caused by signal switching, and has good EMI characteristics.

Low power consumption

Since the LVDS differential signaling technique reduces noise concerns, lower signal voltage amplitudes can be used. This feature is very important, it makes it possible to increase the data transfer rate and reduce power consumption. Low drive amplitude means data can be reversed faster.

High efficient transfer

Since the driver is a constant current source mode, the power consumption hardly changes with the frequency, and the power consumption of a single channel is very low. Therefore, after adopting this technology, as long as the length of a pair of parallel transmission lines is consistent enough, and a good matching termination impedance technology is provided at the receiving end to reduce the generation of reflected signals, a very high data transmission rate can be provided.

how does LVDS work?

The schematic diagram below explains how LVDS work. Its driver consists of a constant current source (usually 3.5mA) driving a pair of differential signal lines. The current and voltage amplitudes flowing through the two parallel differential signal lines are opposite, and the noise signal is coupled to the two lines at the same time. The receiver only cares about the difference between the two signals, so the noise is canceled out. Since the electromagnetic fields around the two signal lines also cancel each other out, differential signal transmission has much less electromagnetic radiation than single-line signal transmission. There is a high DC input impedance at the receiver (which draws almost no current), so almost all of the drive current will flow through the 100 ohm terminating resistor to produce about 350mV at the receiver input.

LVDS driver and receiver circuit
LVDS signal circuit schematic diagram (source: wikipedia.org)

When the driving state is reversed, the direction of the current flow through the resistor is changed, thus producing an effective “0” or “1” logic state at the receiving end.

Features

  • High-speed transmission capability up to 2Gbps;
  • Low voltage, low power consumption;
  • LVDS adopts CMOS process;
  • Low static power consumption;
  • Low noise radiation;
  • The use of differential transmission mode has strong anti-interference ability.

LVDS Signal Design Rules

In the process of designing lvds signals, please consider the following factors:

1. Electromagnetic interference

LVDS signal filter design is mainly for filter design such as clock signal and bus signal. The clock signal is added with an RC filter design at the sending end to reduce the external radiation interference of the clock. For differential signals, the filter design needs to add common-mode inductors at the port to filter and suppress common-mode noise.

2. Interference with fixed paths

The interference path is generally the power supply or signal line, so the LVDS circuit design only needs to add a protection design to the interface, increase the capacitance to the ground after adding magnetic beads to absorb the interface, so that the interference can be released through the fastest path.

3. Environmental interference

This interference is caused by electromagnetic radiation from external sources in the environment, and protective measures such as adding ferrite beads and capacitors are often used to reduce the effects of this interference.

To reduce crosstalk between single-ended signals and LVDS signals, follow:

  • Single-ended signals at least 12 mm away from LVDS signals on the same PCB layer;
  • The distance between the differential lines should not exceed twice the width of the signal lines, and the thickness of the circuit board should be greater than the distance between the signal lines;
  • The distance between two adjacent differential pairs should be greater than or equal to 2 times the distance between independent signal lines.

4. Impedance matching

When designing impedance matching for LVDS signals, the following should be followed:

  • The PCB should be at least 4 layers, and LVDS signals and TTL/CMOS signals need to be isolated with power planes or ground planes; LVDS drivers and receivers should be placed as close to the connector as possible;
  • Place a 4.7μF or 10μF capacitor close to the Vcc pin of the driver or receiver, and consider the matching between the operating frequency of the signal and the optimal operating frequency of the capacitor;
  • Place at least one 0.1µF and one 0.001µF capacitor close to a driver or receiver Vcc pin;
  • Power and ground traces should be as wide as possible to reduce power return impedance.

Share:

Facebook
Twitter
Pinterest
LinkedIn

Leave a Comment

Your email address will not be published. Required fields are marked *

About Author

Aidan Taylor
Aidan Taylor

I am Aidan Taylor and I have over 10 years of experience in the field of PCB Reverse Engineering, PCB design and IC Unlock.

Need Help?

Scroll to Top
welldone pcb manufacturer

Start to Achieve Your PCB Project

Open chat
Scan the code
Hello 👋
Can we help you? Chat with us on WhatsApp↓, or send an email to us: info@reversepcb.com