Peak Differential Output Voltages: Differential Signaling, Techniques, and Output Voltages

December 5, 2019 Cadence PCB Solutions

Circuit diagram of an operational amplifier

 

The word differences, depending on the connotation, can be a rallying source or a divisional wedge. We all seek like-minded individuals, and overall it is human nature to do so. The saying “Birds of a feather, flock together,” has more than a bit of truth to it. However, it is also our differences that make humankind more resilient, and I dare say stronger.

In the world of electronics, differences in voltage, current, or the reference to ground, can be the difference in a device operating correctly and being in desperate need of a Class C (CO2) fire extinguisher.

Also, within the field of electronics, there is a variation of the word difference (differential) that encompasses a type of measurement that assists in circuit analysis, gauges circuit noise characteristics, and output performance.

What is a Differential Output Voltage?

Firstly, a differential voltage is “floating,” or in other words, it has no reference to ground. Also, a differential output voltage in electronics is the difference between the values of two AC voltages, 180° out of phase, present at the output terminals of an amplifier when you apply a differential input voltage to the input terminals of an amplifier.

Furthermore, a differential voltage measurement is the difference between the voltages, such as the voltage across a resistor. Also, typically, a differential voltage measurement is useful in determining the voltage that exists across an individual element of a circuit, or if the signal sources are noisy.

To further clarify, the measurement you take is the voltage difference between the two wires. Also, differential measurements afford you the benefit of noise rejection since both wires have the added noise, and the data acquisition system can filter it out with its common-mode rejection. We typically use differential measurements if the sensor is in a noisy environment or for sensors with output voltages that are susceptible to noise interference.

Differential output sensor

Measuring differential output voltage properly can save your device. 

 

Differential Output Voltage Measurements Versus Single-ended Measurements

As I am sure you are aware, a differential voltage is a voltage that has no reference to ground. The measurement is of the voltage difference between two wires and not the wires and ground. Also, as mentioned earlier, the benefit of these types of measurements is noise rejection. In other words, it is beneficial to use this type of analysis within noise inducing environments or with circuits (components) with output voltages that are susceptible to noise interference.

However, a single-ended measurement is one in which the voltage difference is between a wire and ground. Since the noise is only on the positive wire, you can still take its measurement along with the output voltage from the sensor. However, some sensors, for example, only have a single output anyway and must be wired into a single-ended channel.

Furthermore, you should note that you can wire a sensor with a differential output for single-ended measurements by wiring the low side to ground. However, this is usually done to reduce the number of channels needed to measure the sensors. You should also be aware that some sensors are capable of outputting a negative voltage. This, in turn, means that the data acquisition system you use needs to be able to measure a negative voltage.

The decision to take a single-ended measurement versus a differential measurement can depend on the data acquisition system you use or the cabling. One final note, you should always take a differential measurement if there are sufficient datalogger channels available or if the data acquisition system cannot measure negative voltages.

Why There is a Need for Differential Signaling in Electronics

The use of two complementary signals for electrically transmitting information is called differential signaling. Also, the receiving circuit responds to the electrical difference between the two signals, rather than the difference between a single wire and ground.

In summary, differential signaling allows us to transmit information with lower voltages, better signal-to-noise ratio (SNR), improved immunity to noise, and higher data rates. Also, this promotes an increase in the conductor count, and the system will also need specialized transmitters and receivers instead of standard digital ICs.

This beneficial technique sends the same electrical signal as a differential pair of signals, each on its own conductor. Also, the pair of conductors can be wires (generally twisted) or traces on a PCB. The receiving circuit responds to the electrical difference between the two signals, rather than the difference between a single wire and ground. The inverse of this technique, of course, is called single-ended signaling. Furthermore, you typically find differential pairs in use on printed circuit boards, in twisted-pair and ribbon cables, and in connectors.

Advantages and Suitability of the Differential Technique

In electronics, whenever the source and the receiver’s impedances are equal, so are the effects of external electromagnetic interferences. Furthermore, since the receiving circuit only detects the difference between the wires, using the differential technique, it helps the circuit to resist electromagnetic noise. The method also works for both analog signaling and digital signaling.

Today’s electronics trend favors smaller, more portable, and mobile devices. The current landscape forecast seems to point to an increase in the current demand for more portable devices. Therefore, to keep up with these demands, designers are continually striving to lower supply voltages to save power and reduce emitted electromagnetic radiation. However, a lower supply voltage reduces noise immunity, which is a critical problem for device functionality.

However, the use of the differential signaling technique helps to reduce these problems. Overall, differential signaling provides twice the noise immunity of a single-ended system for a given voltage. As a whole, the differential technique minimizes noise emission, noise acceptance, electronic crosstalk, electromagnetic interference, and it can achieve a constant or known characteristic impedance. This, in turn, allows impedance matching techniques necessary in a high-speed signal transmission line or a high-quality balanced-line, and balanced circuit audio signal path.

Printed circuit board at an angle with an array of components

When looking to maintain production expectations, don’t skimp on differential measurements. 

 

In conclusion, differential techniques offer superior noise immunity over single-ended systems. Today’s mobile market continues to demand increased functionality, performance, and reduced size. The only way to continue to meet today’s demands is through the use of differential techniques. 

Using differentials and measuring peak differential output voltages in your designs is easy with Cadence’s suite of design and analysis tools. If you’re looking for noise immunity, than consider Allegro PCB Designer for its incredible array of SI capabilities and component placement opportunities.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

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