Blog Post:

Mastering Differential Signals

Mastering Differential Signals

Our world is inherently electrically noisy, yet electronics are a part of our everyday life in everything we do. Sending data from device to device is integral to this infrastructure and ensuring that data makes it from point A to point B is even more important. Let’s look at what makes a wired signal more noise immune and how can we best protect our signals from the noisy world they exist in.  

Two Types of Signals

With wired communication leaving one electrical device and going to another, there are two main types of signals: single-ended and differential signals. The two are often confused since they can both sometimes use 2 wires, but they are quite different. Most signals are single-ended and not differential. A single-ended signal has 2 wires: 1 signal and 1 return wire. A differential signal has 3 wires: 2 signal wires and 1 return wire. 

Single-ended signal schematic

Differential signal schematic

Definitions of Single-Ended vs Differential Signals

For the single-ended signal, data is communicated as the voltage difference between the signal wire and the return wire. The return wire is typically 0V or referenced to circuit ground. However, for differential signaling, data is communicated as the voltage difference between the 2 signal wires with reference to return. The signal wires in a differential pair are inverses of one another, meaning if we are communicating a signal differentially the first signal wire will be +1V and the second signal wire will be (–1V) producing a 2V peak to peak differential signal. A single-ended signal would only be 1V – 0V = 1V peak to peak. The third wire, the return wire, is used as the return current path for both signals and it keeps the voltage difference between the two devices’ circuit grounds equal.

Single vs Differential waveforms

The Return Wire

Since a differential signal is usually received by a differential op-amp, an instrumentation amplifier, or a CMOS receiver with high input impedance (MΩ to GΩ range), the current on the return wire will be small. There is debate on the necessity of the return wire since very small currents flow in this wire. This is a large topic to discuss and will need to be discussed in situation-specific scenarios where the system is reviewed to maximize signal integrity, minimize ground loops that cause noise or inefficiencies, and minimize complexity. For example, some systems connect the differential transceiver’s ground to chassis and chassis makes a continuous connection throughout the system. This can be a good approach for many applications to not use a ground wire, since return currents flow through chassis. In any approach, make sure to understand the return path. If this is not well understood, electricity will create interesting return current paths within a system.

Signal Voltage Range

Differential waveforms

Why would you choose single-end signal over a differential signal or vice versa? One reason is improved noise immunity. For example, if we have a 5V single-ended signal, the maximum voltage potential between signal and return is 5V. However, if we have a 5V differential signal, you can send 5V on the first signal wire and 0V on the second signal wire or 0V on the first signal wire and 5V on the second signal wire. Since it is a differential signal we take the difference between the 2 signal wires to get +5V in the first case and –5V in the second case. Now our differential signal has a signal range from –5V to 5V (10V) vs our single-ended signal’s range of 0V to 5V (5V). This gives us twice the signal voltage range which gives us twice the noise immunity because a bigger signal will have a smaller percentage of distortion due to noise compared to a smaller signal exposed to the same noise. In noisy environments like a factory or warehouse, differential signals can improve the overall electrical system’s integrity by providing signals that will be less corrupted by noise. It is also good to note that differential signals can work like the description in this paragraph, or they can also have the negative signal wire be the inverse of the positive signal wire like the example towards the beginning of this discussion.

For long cable runs and high-speed data, differential signals can be especially important.

Noise Rejection

Common Mode Noise

Another excellent feature of differential signals is their increased noise rejection. When noise is coupled onto your signal wires it adds random noise that distorts the signal and can alter the data’s value if it is large enough. If it is analog data, your data could no longer be accurate and if it is digital data, it can be hard to determine if a 1 or a 0 is being received or if anything is being received. (It is important to note for our discussion we have been looking at examples of digital signaling, but analog single-ended and differential signals work similarly.) Differential signaling typically uses twisted pairs of signals. Since the two wires are closely twisted signals, they will be exposed to similar electric and magnetic field strengths, resulting in coupled noise added equally to both signal lines. When the differential signal reaches the receiver, we subtract the first signal wire from the second signal wire to get our differential voltage reading. Since the noise was coupled to both wires and affects both wires the same, the noise is effectively subtracted out by the receiver, and we get a clean signal. The receiver acts as a comparator circuit, comparing signal positive to signal negative. See the image below for an example of the waveforms. Notice that the waveforms are numbered and correspond to the numbers in the image above to show where in the circuit the waveform is from.

Common Mode rejection

Common Mode Rejection Ratio (CMRR)

This type of noise is called Common Mode noise, which is noise seen at the inputs of your device referenced to the circuit’s ground. You will also see something called Common Mode Rejection Ratio (CMRR) and this refers to how well the device can reject this common mode voltage. The higher a receiver’s CMRR value is, the better the receiver is at rejecting common mode noise. A receiver with a CMRR of 80dB or more is generally considered good according to a white paper by Analog Devices, but this also depends on the application.

Balanced Signals

The Common Mode noise is only perfectly removed from a differential signal if the signals are perfectly impedance balanced between each other. A signal is balanced when the impedance between the signal wire(s) to the circuit’s ground is equal. The picture below shows a differential signal depicted as 2 single ended signals with a source, characteristic, and load impedance. Each impedance pair must be equal to make the differential signal balanced.

Balanced Signal schematics

The Importance of a Balanced Signal and High CMRR

This is important because if the impedances are not equal (unbalanced) then the noise coupled onto your differential signal wires will not be equal and then will not get completely subtracted out. Noise causes current and / or voltages to be induced on a wire. Similarity in the signal path impedance will cause noise coupled equally on both wires. Impedance balancing your signal is fundamental to maintaining good differential noise immunity. Even so, no circuit path can ever be perfectly balanced so there will always be some level of noise in your signal, but we can greatly reduce this by impedance balancing source, line, and loads.

Going Further

It is especially important to consider which signal type an electronic device uses when designing electronics or integrating an off the shelf device into your system. In addition to the discussion above, there are many other types of ways to protect a system from noise like using wire or cable shielding, isolation, preventing or breaking ground loops and much more. Check out these 2 articles by MultiCable and Phoenix Contact for further material on cable noise protection.

Sparx has experience in electrical component design and system integration across many industries (aerospace, medical, oil & gas, consumer products, etc.). Sparx can help meet your needs when designing an electrical system or adding a device to an existing system. Reach out to Sparx today to see how we can help design and implement solutions to fit your needs.

Leave a Reply

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

Get in Touch

If you have a product design that you would like to discuss, a technical problem in need of a solution, or if you just wish you could add more capabilities to your existing engineering team, please contact us.