When designing or installing the cables for a video transmission system, one assumption that engineers commonly make is that the local earth grounds of both ends of the cable are the same. When circumstances don’t support this assumption, the video performance can exhibit gross distortions. This article looks at the situations in which ground may have a non-zero potential, and reviews the basic options for grounding connectivity. Finally, it shows a solution that ensures quality video transmission, even when any ground-potential differences are present.
In a perfect world, a low-impedance ground connection serves as a reference for all electronic signals. This assumption is reasonable for systems with ground planes or even those with short, wired connections. When comparing the earth-ground potential between houses in a subdivision, the difference is typically in the microvolt range. Within a building, the measurements are similar. However, in the top floor of a 50-story high-rise building, the earth-ground connection has been measured at 10 volts above the first floor value.
A separate issue is the use of ground potential as a third terminal in power connections. Many buildings in the United States that predate current electrical codes do not have a local ground connection other than the cold water pipe. The situation in Asia is exacerbated and a multitude of sites lack ground references. With the absence of ground, all signals are floating with respect to a stable reference like earth ground.
Figure 1 illustrates this concept of a ground loop with a simple driver and receiver circuit.
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Figure 1. Ground loop in Differential Video Transmission System
Of course, in the ideal situation, VED is zero volts and the earth-ground connections (EGND1 and EGND2 ) both equal zero volts. Since the driver and receiver enjoy a low-impedance connection to earth ground, R1 and R2 also equal zero in the ideal case. However, when potential differences exist, a loop of V1 +VED -V2 is formed between the references of the driver and receiver.
Sometimes you can’t control all aspects of a system. In buildings with excessive height or age, one side of the transmission set may already exist—and be prohibitively difficult to reach or upgrade. Consider an example where the driver circuitry is isolated and floating with respect to earth ground 1 (R1 approaches infinity). As referenced earlier, V1 may be equal to 10 V. Let’s assume that VED is 1 volt for simplicity. If, out of habit, the receiver is installed with its reference (GND2) tied to earth ground (EGND2 ), there is a 9-V common-mode error between GND1 and GND2. The input circuitry of the receiver is overwhelmed and the system is not operational. Instead, the receiver ground must also be left floating, to match the driver circuitry and avoid large offset errors.
For minimizing dc errors, the driver and receiver ground should be treated the same. Either both are tied to earth ground or both are left floating. Common-mode signals with frequency are introduced through the antenna capabilities of the wires and the power supply circuitry, and require specific consideration to reduce their effect. Even with the use of differential signals, any difference in the ground reference between the driver and receiver results in a common-mode signal on each line of the received pair. Figure 2 presents a solution for a receiver with common-mode cancellation circuitry.
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Figure 2. Common-Mode Noise Cancellation Circuit
For simplicity, composite video signals are assumed. (This technique works equally well for other transmission systems.)
Proper termination of each must be maintained with transmission lines. The twisted-pair transmission line connected at B+ and B- must be terminated in 100 Ω to a common-mode point, to prevent reflections of the differential signal. This is provided by the resistors, R27 -R30 . R45 provides a location for the common-mode voltage. All resistors in the schematic are 1% tolerance to guarantee common-mode rejection within 50 mV for 5 V common-mode signals on the line.
Equalization for the high frequency components of the differential signal are provided by C7 and C8 with R31 and R32 . However, it is crucial that these signals remain free from common-mode noise. Therefore, the outputs are not referenced to the local ground. Instead, they use the reference provided by an op amp circuit.
This reference is a common-mode compensation voltage created by summing the signals of the differential transmission lines. The sum cancels the differential component and doubles the common-mode contribution from each line. This signal is connected to the input of the op amp, in this case, an Intersil ISL55001.
Since dc signals are already reduced by matching the reference of the driver and receiver as discussed above, 10 μF capacitors couple the input and output of the amplifier. Similar-size capacitors are used for supply bypass. These values are large enough to compensate 60-Hz signals. Larger capacitors (or multiple values in parallel) may be used at the designer’s discretion.
The output of the summing amp returns the common-mode voltage on the output side of the receiver network (Bout+ and Bout- ). The 30 kΩ matched pair of resistors (R33 and R34 ) connects the differential signal at Bout+ and Bout- to the common-mode voltage. Basically, the circuit provides a common-mode signal so that the input signals referenced to GND2:
B+ = Vdiff /2 + Vcm
B- = -Vdiff /2 + Vcm
Bout+ = Vdiff /2 +Vcm –Vcm created = Vdiff /2
Bout- = -Vdiff /2 + Vcm – Vcm created = -Vdiff /2
The quality of this common-mode removal is demonstrated in Figures 3 and 4 . Figure 3 shows a one-volt peak sine wave (in yellow) on B- to represent the differential signal.
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Figure 3. Common-Mode Signal on Input (yellow) and Feedback (pink) lines
In this example, both nodes B+ and B- share a 2-volt peak common-mode signal (in pink).
Figure 4 shows the success of the circuit in Figure 2.
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Figure 4. Common-Mode Signal Improvement from Input (yellow) and Output (pink)
The differential signal (in yellow) has not changed. The common-mode signal (in pink) is measured at Bout- , displaying the successful reduction of the common-mode signal by a factor of 20. Complete removal is limited by delay mismatch in the passive network and the accuracy of the resistor values.
With the techniques discussed, ground loops are overcome and common-mode signals are removed to restore video transmission in long-distance or old-building applications. Simply put, the ground references of the driver and receiver must be identical. Secondly, the addition of common-mode cancellation circuitry preserves the quality of differential component video and ensures that the common-mode input range of the receiver is not exceeded.
About the authors
Both Mike Wong and Tamara Papalias are with Intersil Corp., Milpitas, CA.
Wong is the director of application engineering focusing on high speed analog applications. He has previously worked on power supplies at ASTEC. He graduated from UC Davis with a BSEE in 1989.
Papalias is a principle application engineer for analog applications. She is also a full time professor of Electrical Engineering at San Jose State University. She has a BSEE, MSEE, and PhD in RF CMOS design from Stanford University.