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22 September 2009
SIGNAL CHAIN BASICS (Part 29): Digital interfaces - Single-ended versus differential interfaces

Understand the key attributes and tradeoffs of these basic signaling interfaces

By Thomas Kugelstadt, Senior Applications Engineer, Texas Instruments
Planet Analog
May 1, 2009 (7:00 AM EST)

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(Editor's note: there is a complete, linked list of previous installments of this series below the About the Author section at the end.)

This installment of Signal Chain Basics introduces you to the digital interfaces required to transmit the digital conversion results from an analog-to-digital converter (ADC) to a system controller, as well as transmit any digital configuration data from the controller to a digital-to-analog converter (DAC). The two main signaling schemes used are single-ended and differential signaling.

Single-ended data transmission uses only one signal line, with its voltage potential is referred to ground. While the signal line provides the forward path for signal currents, ground provides the return current path. Figure 1 shows the basic schematic of a single-ended transmission path.

Figure 1: Single-ended transmission path
(Click on image to enlarge)

Single-ended interfaces benefit from their simplicity and their low implementation cost, but have three main drawbacks:

1) They are highly sensitive to noise pick-up, because noise induced into the signal or ground paths adds directly to the receiver input, thus causing false receiver triggering.

2) Another concern is crosstalk, which is the capacitive and inductive coupling between adjacent signal and control lines, particularly at higher frequencies.

3) Finally, due to the physical differences between the signal trace and the ground plane, the transversal electromagnetic waves (TEM) generated in single-ended systems can radiate into the circuit environment, thus representing a significant source of electromagnetic interference (EMI) to adjacent circuits.

Differential signaling uses signal pair consisting of two conductors: one for the forward, the other for the return current to flow. Each signal conductor possesses a common-mode voltage, VCM, superimposed with 50 percent of the differential-driver output VOD, but of opposite polarity to one another (see Figure 2).

Figure 2: Differential transmission path
(Click on image to enlarge)

When the conductors of a differential pair are close to each other, electrically coupled external noise induced into both conductors equally appears as common-mode noise at the receiver input. Receivers with differential inputs are sensitive to signal differences only, but immune to common-mode signals. The receiver, therefore, rejects common-mode noise and signal integrity is maintained.

Figure 3: TEM wave radiation from the large fringing fields around a single conductor and the small fringing fields outside the closely coupled conductor loop of a differential-signal pair
(Click on image to enlarge)

Close electric coupling provides another benefit. The currents in the two conductors, being of equal amplitude but opposite polarity, create magnetic fields that cancel each other. The TEM waves of the two conductors, now being robbed of their magnetic fields, cannot radiate into the environment. Only the far smaller fringing fields outside the conductor loop can radiate, thus yielding significantly lower EMI.

Applications
Single-ended interfaces allow for relatively high frequencies (up to 70 MHz) when applied in close proximity to a system controller. Differential interfaces possess significant higher noise immunity and drastically reduced EMI and can, therefore, transmit data at frequencies of up to 500 MHz and above.

The most commonly implemented interfaces in data converters are the inter-integrated circuit bus (I2C), the serial-peripheral interface bus (SPI), and the low-voltage differential signaling interface (LVDS). Look for these topics to be discussed in future Signal Chain Basics articles.

For more in-depth information about the above interfaces, click here.

About the Author

Thomas Kugelstadt is a Senior Systems Engineer at Texas Instruments, where he is responsible for defining new, high-performance analog products and developing complete system solutions that detect and condition low-level analog signals in industrial systems.
During his 20 years with TI, he has been assigned to various international application positions in Europe, Asia and the U.S. Thomas is a Graduate Engineer from the Frankfurt University of Applied Science. You can contact Thomas about this article at: [email-masked].

Previous installments of this series:

SIGNAL CHAIN BASICS (Part 28): Building (Electrical) Bridges, click here
SIGNAL CHAIN BASICS (Part 27): Control EMI resulting from board-level clock distribution, click here
SIGNAL CHAIN BASICS (Part 26): How to close timing on High-Speed ADCs, click here
SIGNAL CHAIN BASICS (Part 25): Designing the audio-signal chain for non-audio experts, Part 1, click here
SIGNAL CHAIN BASICS (Part 24): Basic networking using the IEEE 802.15.4 PHY/MAC protocol, click here
SIGNAL CHAIN BASICS (Part 23): EIA-485: Receiver equalization boosts networking performance, click here
SIGNAL CHAIN BASICS (Part 22): Phantom microphone power--the ghost in the machine, click here
SIGNAL CHAIN BASICS (Part 21): Understand and configure analog and digital grounds, click here
SIGNAL CHAIN BASICS (Part 20): Understand the basics of op amps and speed, click here
SIGNAL CHAIN BASICS (Part 19): Exploring and understanding linear voltage regulators, click here
SIGNAL CHAIN BASICS (Part 18): The op amp as integrator, click here
SIGNAL CHAIN BASICS (Part 17): Hysteresis--Understanding more about the analog voltage comparator, click here
SIGNAL CHAIN BASICS (Part 16): Understanding the analog voltage comparator, click here
SIGNAL CHAIN BASICS (Part 15): Analog/digital converter--dynamic parameters, click here
SIGNAL CHAIN BASICS (Part 14): Analog/digital converter--static parameters, click here
SIGNAL CHAIN BASICS (Part 13): Putting the Bode plot to use, click here
SIGNAL CHAIN BASICS (Part 12): The Bode plot, an essential ac-parameter display tool, click here
SIGNAL CHAIN BASICS (Part 11): Introducing voltage- and power-conditioning circuits, click here
SIGNAL CHAIN BASICS (Part 10): Exploring the Delta-Sigma Converter, click here
SIGNAL CHAIN BASICS (Part 9): SAR Converter Operation Explored, click here
SIGNAL CHAIN BASICS (Part 8): Flash- and Pipeline-Converter Operation Explored, click here
SIGNAL CHAIN BASICS (Part 7): Op Amp Performance Specification--Bias Current, click here
SIGNAL CHAIN BASICS (Part 6): Op Amp Input Voltage Offset, click here
SIGNAL CHAIN BASICS (Part 5): Introduction to the Instrumentation Amplifier, click here
SIGNAL CHAIN BASICS (Part 4): Introduction to analog/digital converter (ADC) types, click here
SIGNAL CHAIN BASICS (Part 3): Analog and the digital world, click here
SIGNAL CHAIN BASICS (Part 2): Op Amp--Basic operations, click here
SIGNAL CHAIN BASICS: Operational Amplifier--The Basic Building Block, click here

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