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Subtleties count in wide-dynamic-range analog interfaces

Subtleties count in wide-dynamic-range analog interfaces

Transporting high-dynamic-range analog signals from one piece of equipment to another is not a trivial task. Even subtle design variations can make huge differences in the equipment’s ability to reject interference from the ac power line and other sources when the equipment connects to a real-world system.
Noise is pervasive

The fundamental interface problem stems from the fact that once noise contaminates a signal, it’s nearly impossible to remove the noise. Dynamic range quantifies the ratio of the maximum undistorted signal to the noise floor, whereas SNR quantifies the ratio of the reference signal to the noise floor. Dynamic range equals SNR plus “head room”–the ratio of the maximum undistorted signal to the reference signal. These values are generally ex-pressed in decibels.

System-dynamic-range requirements depend on the application and on user expectations. The human ear has about 140 dB of dynamic range, whereas a high-performance audio-reproduction system in a typical home listening environment may require as much as 120 dB (Reference 1). Video systems generally accept 50 dB of dynamic range as the limit beyond which expert viewers perceive no further improvement.

Both basic types of interfaces–unbalanced and balanced–use a pair of wires to carry the signal; the impedances of these wires–with respect to a reference point, usually ground–define them. In an ideal unbalanced interface, one wire has zero impedance, and the other signal-carrying wire has nonzero impedance to ground. In the ideal balanced interface, both wires have equal and nonzero impedances to ground.

When you are dealing with any ac-line-powered system, you must accept the existence of significant ground-voltage differences between system components. Although you can sometimes reduce these voltages by carefully designing and executing system-grounding schemes, they are virtually impossible to eliminate. In most systems, these voltages are the dominant noise source, entering unbalanced signal paths through common-impedance coupling and balanced paths through common-mode conversion. Common symptoms are hums, buzzes, pops, clicks, and other noises in audio systems; hum bars or bands of “sparkles” in video systems; and unexplained data errors or crashes in data systems.

All internal and external power transformers have unavoidable parasitic capacitances from their power-line-connected primary windings to their equipment-ground-connected secondary windings. These parasitic capacitances never appear on schematic diagrams, and you cannot eliminate them in a practical way. Power-line RFI/EMI filters generally have even larger capacitances from their lines to their chassis. The periodic charge and discharge of these capacitances cause small but significant ac-power-line currents to flow from the power line to each chassis. System devices are either “grounded” or “floating.”

Grounded devices use three-wire power cords. Parasitic currents flow through the safety ground wire to the ac outlet ground. Because this wire has both resistance and inductance, each chassis assumes a small voltage with respect to the outlet ground. The series-coupling capacitance and shunt-wire resistance/inductance effectively form a highpass filter, so the resulting chassis voltage generally is a rich mixture of high-frequency power-line noise and distortion components, which you hear as a buzz rather than the more fundamental rich hum in an audio system. Nonlinear loads on the power line can generate these high frequencies. Such loads can include electronic equipment with a capacitor input or switching power supplies; fluorescent or dimmer-controlled lights; and intermittent or sparking loads, such as switches, relays, or brush motors.

Even if you plug both devices into one outlet, because of differing parasitic capacitances, the chassis voltages will likely be different. Because “grounded” devices connect the chassis to safety ground at low frequencies, each device effectively acts as a voltage source with an impedance less than a few tens of ohms. If you plug two devices into different branch circuits of the ac power system, the voltage differences generally increase, often reaching several volts. If you connect the devices by a cable shield, for example, 100-mA currents may flow. Even higher voltages and resulting current flows can result if you connect one of the devices to a nonpower ground, such as the earth ground for lightning safety with a cable-TV or a satellite-broadcast receiver.

Note: Never lift or disconnect safety grounds; it’s not only illegal, but also dangerous. You use a ground adapter to provide a safety ground for three-conductor power cords with two-prong outlets, not to defeat the safety ground a three-prong outlet provides. Defeating a safety ground could allow lethal voltages to appear on all equipment in an interconnected system should a power-line fault develop in the lifted device.

“Floating” devices use two-wire power cords, and each chassis assumes an open-circuit voltage as high as 120V ac with respect to safety ground. If you externally ground any accessible point, including signal connectors, current flow is limited to about 1 mA. This current can cause you an unpleasant but harmless shock. If you leave it ungrounded, the parasitic power-line current flows only in any cables you use to connect the devices. You do not eliminate–only reroute–the parasitic current flow. In unbalanced interfaces, even a few microamperes of interchassis current can significantly degrade dynamic range. At low frequencies, floating devices are effectively high-impedance current sources with short-circuit currents as high as 1 mA and open-circuit voltages as high as 120V.

In larger systems, the flow of power-line parasitic currents becomes more complicated. In any case, assume that significant currents flow in any interchassis connections and that significant voltages exist between the local-device grounds.

Unbalanced, or single-ended, interfaces are common, presumably because they are inexpensive and often perform acceptably well in small systems. They prevail in consumer audio systems, most video and RF systems, many data systems, and, unfortunately, in most electronic instruments.

All unbalanced interfaces suffer from common-impedance coupling, in which the grounded conductor of the interconnecting cable, as well as the contact resistance of any connectors, becomes the offending common impedance (Figure 1). Because the cable shield is effectively connecting the device chassis, the shield has either noisy interchassis current–from floating devices–flowing through it or noisy interchassis voltage–from grounded devices–impressed across it. The resulting noise voltage across the shield directly adds to the signal. Consider 20 ft of cable, having 25-mohm/ft shield resistance, connecting two floating devices between which a 500-µA interchassis current flows. This resistance adds 250-µV noise, which is only 61 dB lower than a 300-mV consumer-audio reference signal.

You can reduce this noise coupling by shortening the cable, which reduces both the resistance and the inductance of the shield. Using cable with a heavier gauge shield improves matters at low frequencies but has little effect at higher frequencies. At power frequencies, wire impedance roughly equals dc resistance, which decreases by a factor of 10 for every 10 AWG decrease in gauge. For example, if you replace a #12 with a #2 AWG–measuring 0.26 in. in diameter–you reduce hum by about 20 dB. However, at RF, inductance determines wire impedance. Diameter has little effect on inductance, which is proportional to length. For example, 8 ft of #10 AWG has an impedance of about 22ohm at 1 MHz–the AM broadcast band. Replacing this #10 with #0000 AWG–measuring about 1/2 in. in diameter–reduces the impedance only to about 18ohm.

Because the shield impedance rises with frequency and the power-line noise is essentially capacitively coupled, common-impedance coupling is generally efficient at coupling power-line “trash” at 100 kHz to 10 MHz. Most devices’ performance suffers when you couple such noise to their inputs. For example, audio systems sometimes demodulate this conducted RFI–producing clicks, pops, or buzzes. More often, the RFI results in subtle intermodulation distortions, in which listeners describe the reproduced audio as having a veiled or grainy quality (Reference 2).

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