Differential signals are the wave of the future for high-speed, high-volume data transmissions. There's no doubt about it. The prime focus of the convergence industries—television, personal computers, and communications—is not only to transmit more data at faster speeds, but to do it more accurately and more economically. As transmission speeds have increased, digital signal transition levels have correspondingly decreased, making signal integrity and clarity critical concerns. Also, in accordance with Moore's Law, as computer microprocessor speeds continue to double every 18 months, the maximum rate at which ground-referenced signals can reliably transmit data is quickly approaching.
In order to limit the disturbance of signals and reduce power dissipation, more and more designs are turning to differential circuitry. This move to improved differential-signal technology for signal transmission, though, faces challenges—not only at the basic measurement levels, but at the technological-breakthrough level as well. Designers of personal-computer architectures are expanding their use of differential signals to achieve faster speeds. Rambus memory is a prime example, due to its differential clock and data lines. Digital signals aren't just "1s" and "0s" any longer. They behave like analog signals at RF and higher frequencies and require high-frequency analog measurement techniques.
Circuit designers and measurement-instrumentation designers both must invent and improve differential design and measurement techniques to allow technology to jump to the next performance level. The need to understand and use technology-breakthrough measurement tools and proper measurement techniques is of key importance. Many designers still use a pair of probes (single-ended passive or active probes) to measure differential signals. But this technique can be full of hazards, including rapid signal degradation, producing unreliable measurements.
So what's the right way to measure differential signals? We will examine a few key elements that, hopefully, will provide the insight and information required to help define or refine the type of differential measurements required by today's circuitry. We'll also try to identify the pitfalls as well as some of the tools needed to design and verify new products from this differential world.
All voltage measurements are two-point measurements—therefore, they are inherently differential. The measurement is made between two nodes in the circuit. One node is at a potential while the other may be at ground reference or at an elevated voltage level. Single-ended signals are referenced to ground, while differential signals are the difference between two signal lines or test points, neither of which are at ground potential. Many of today's signals fall into the category of true differential or pseudodifferential. These include common telephone lines (balanced nongrounded), battery-powered communication equipment, battery-powered computational devices, disk-drive read-write channel signals, and RF communication ICs.
Many modern RF ICs use differential-signal pairs to provide balanced transmit-and-receive intermediate signals. To ease the matching of these high-frequency signal pairs, the termination impedances sometimes exceed traditional 50-Ω levels. Measuring these impedance levels differentially is easier with a high-impedance differential probe.
While today's signals are becoming more differential in nature, the signal levels themselves are decreasing. Driven by applications that need to draw lower power, such as battery-powered products, this decrease in signal level can result in lower signal-to-noise ratios. And with a lower signal-to-noise ratio, there is a greater need for a measurement technique that can reject the noise or common-mode signals.
An ideal differential amplifier amplifies the difference signal between its two inputs, rejecting signals common to both inputs. Having a high impedance from these two inputs to a common ground helps to eliminate ground loops and their associated problems. The measure of a differential amplifier's ability to eliminate the undesirable common-mode signal is referred to as the common-mode rejection ratio, or CMRR.
CMRR can be degraded by a multitude of factors: amplifier mismatches, poor input connections, long lead lengths, incorrect ground connections, attenuation mismatches, changing source-impedance levels, and increases in signal frequency. The presence of one or more of these factors will degrade the performance of any differential-amplifier design to some extent. It also will determine the amount of common-mode signal that it will be able to reject. Figure 1 shows the effect of unequal source impedance on CMRR, while Figure 2 shows the common-mode error from a differential amplifier with a 10,000:1 CMRR.
So how do you limit the number of errors? Here are a few methods for decreasing the amount of common-mode error introduced.
First, make sure that the measurement tool's bandwidth is sufficient to capture the signal and the noise components, and that it has sufficient CMRR for the signals targeted for capture. Next, keep all signal interconnects to the device under test as short as possible to avoid parasitic inductance and capacitance. If you need to use extended leads, twist them several times to reduce the line pickup. Be careful, though, because this increases the capacitance and inductance to the probe's input. For high-frequency measurements, make ground connections as short as possible (yes, differential probes still have a ground connector). Remember, you may need to connect the differential probes to ground to prevent damage to the device under test. Finally, measure at test points where source-impedance changes are minimal.
Here's a tip for estimating the CMRR error of nonsinusoidal signals: Connect both inputs to the measurement source. The scope will display the common-mode error. Unfortunately, though, this doesn't catch any changes in source impedance at the two measurement points.