In electronic equipment and installations, balanced cables and unbalanced cables are used to connect, for example, sensors of all kinds (analog and digital), digital communication lines, audio lines, telephone lines, etc. The two types of cables make up two theoretically separate and incompatible worlds. But in practice they mix confusingly, causing interference (EMI) problems. Balanced cables are also called differential cables or lines. In an unbalanced line, the signal is carried through a cable with two conductors: the "live" and the ground in the form of a screen or shield (in English it is called "single-ended line").
The unbalanced signal connectors have two pins, just like the typical RCA audio connector, usually used in home hi-fi equipment, or 1/4” (6,35mm) or 1/8” jack connectors. (3,5 mm) unbalanced used in musical instruments and headphone outputs (in this case in its stereo version). Sometimes these connectors are also used to connect signals from unbalanced sensors. Connectors with more pins can also carry unbalanced signals. For example, a professional audio XLR-3 (Cannon) connector (figure 3) with three pins (for unbalanced audio signals) could carry an unbalanced signal, leaving one pin unused. Home audio equipment uses almost all unbalanced connections. In a balanced cable there are two signals. One signal has the polarity inverted with respect to the other. It is also called the differential line.
To carry a balanced or differential signal at least we will need a three-pin connector and a cable with two twisted conductors and the ground, which is normally the shield of the cable. EMIs that are not rejected by the cable shield affect both cables carrying the signal equally. The input of the device to which we carry the signal performs what is known as unbalanced, which consists of adding the two signals that arrive after inverting one of them. As one signal has been inverted with respect to the other in the cable, the balanced one manages to reinforce (double) the original signal and cancel the EMI that was captured in the cable. Differential inputs provide a higher level of immunity against external EMI and are therefore recommended for use in the event of an EMI problem. This is especially true when measuring thermocouples, or strain gauges, as they generate very small signals, very susceptible to EMI.
Figure 4b shows an unbalanced line. The use of balanced cables using a twisted pair provides a much higher level of immunity to magnetic field EMI and the additional shielding improves immunity against electric fields. Twisted pair cable should not be confused with a balanced circuit. They are two different things, although they are often used together. The performance of any interconnection system between equipment depends on the topologies of the input/output circuits (balanced or unbalanced schemes), the design of the printed circuit board (TCI) and the installation of the cables and their connectors. . Here we are only going to consider the wiring. Considering the topologies of the I/O circuits ideal for this discussion to focus on the problems of the connections. We are not going to go into the description and problems of standard communications systems (Ethernet, HDMI, DBI, VGA, Firewire, LVDS, HDMI or USB, etc.) because having specific connectors make mixing difficult. On the other hand, the most widespread standards in the industry such as the old RS-232, or RS-422 and RS-485, I2C and SPI do not have specific connectors assigned and in the design of the I / O in the TCI it is often confusing if you don't have clear ideas about balanced and unbalanced lines.
EMI problems
Mixing balanced and unbalanced cables in installations can cause EMI problems. In this case isolation techniques should be used. A common solution to EMI problems is to disconnect one side of the shield, although you cannot buy cables on the market with the shield disconnected on one side. The best side to disconnect the shield is not important in this explanation. The fact that many installers follow the one-sided shield connection rule indicates that this solution is sometimes acceptable, although the use of digital technology and the corresponding increase in frequency has increased the likelihood of major problems. of EMI. It should be considered that the shield connected to only one side of the cable does not shield against magnetic fields and only protects the cable against electric fields. A trade-off is to provide a path for EMI through a capacitor (≈ 33 nF) connected from the unconnected side of the display to the chassis.
In this case, the shield is connected on both sides at high frequency and on only one side at low frequency. It is recommended that a cable joint be made with a 360° joint (sometimes called a peripheral joint) on both sides to generally achieve the best possible performance at the lowest cost.
Technical justification
The use of balanced cables is a very useful technique for reducing EMI. The purpose of balancing on a balanced line is to make the pickup of external EMI on both conductors the same, so that this EMI can be canceled out at the load. Balanced cables can be used in conjunction with other techniques for EMI suppression. They are also useful for minimizing radiated emissions. The balance of a differential circuit is defined with respect to the impedance of the two signal leads relative to a reference point, which is usually signal ground. If these impedances are equal and are not zero, the set is balanced. Thus, in figure 5, to have the entire circuit balanced, Rf1=Rf2 and Cf1=Cf2 must be fulfilled in the source, and RC1=RC1 and CC1=CC2 in the load. The cable must also have the same Z impedance value on both conductors.
The value of the voltages Vf1 and Vf2 does not affect the balance of the circuit and it is not necessary that they be equal. If the impedances at any point in the circuit are uneven, the circuit is unbalanced. In figure 6, as an example, we see that there is an imbalance due to a variation of the resistance (ΔRf) of the source in the upper conductor. The CMRR parameter ("Common Mode Rejection Ratio": Rejection Ratio in Common Mode) expresses the level of imbalance of the circuit or the effectiveness of a balanced circuit to reject EMI in common mode. The formula for its calculation is shown in the same figure 6 for the example presented. In the formula, the impedance value of the conductors is neglected because it is very small and is the same on both sides of the cable.
Its value in dB is: CMRR(dB) = 20 log(CMRR) and it usually ranges between 60 and 80 dB. An excellent example of the effectiveness of using a balanced or balanced system in reducing EMI is telephone wiring, where signal levels are typically in the tens to hundredths of millivolts. Telephone cables (analogue) usually work well even if they are installed for many kilometers parallel to the mains lines of high voltage (kilovolts) without hearing any 50 Hz hum on the telephone. This is because telephone cables are balanced by preferentially using twisted pairs. On the rare occasion that you hear 50Hz hum on the phone, it is because something has caused an imbalance in the lines.
Twisted pair cabling, even when not shielded, is very effective in reducing low-frequency magnetic field coupling if both of these conditions are met:
- The signal must flow equally and in opposite directions on the two conductors.
- The twist pitch of the twisted pair should be less than 1/20 of the wavelength (40 turns/meter is effective up to 500 MHz). This is true whether the terminations (matching impedances) are balanced or not. If the terminations are balanced, the twisted pair will also be effective in reducing electrical field coupling. In an ideal balanced system, EMI is greatly reduced in the circuit. In the real world, however, small imbalances will limit EMI suppression. These imbalances can be:
- load imbalance
- Imbalance at the source
- Cable Unbalance: o Resistive Unbalance (Usually Negligible) o Capacitive Unbalance (typically 3 to 5%) o Inductive Unbalance:
- If the shield connection uses pigtails)
- Typical in aluminum foil armored cables due to drain wire
- Virtually non-existent at frequencies > 100 kHz in copper braided shielded cables if properly connected 360º
The chassis and the mass
Let's examine the chassis ground connection (metal case) of a piece of equipment and the signal ground connection. The chassis ground is the conductor that connects the chassis of an equipment to the building ground. This connection is made for electrical safety reasons for human protection against accidental electrical shock. On home products with a 2-conductor network cable only, the chassis is not grounded, although the chassis is normally connected to signal ground. In products with a plastic case, there is no chassis or ground connection. The signal ground is the internal conductor used as a 0 V reference potential for the internal electronics.
It's easy to confuse chassis ground and signal ground, since they're usually connected together. The key to having a good ground and ground connection, with good electromagnetic behavior, is knowing where and how to connect the ground signal to the chassis. One of the reasons for connecting signal ground to the chassis is to reduce the effects of capacitive coupling between the chassis and internal circuitry. Figure 7a shows a circuit inside a totally closed metallic box (chassis), without external connections. The chassis will therefore isolate the circuit from external electrostatic influences.
The circuit is isolated from the chassis. Then some parasitic capacitances of the circuit with respect to the chassis appear. Three of them have been represented in the figure: Ce at the input, Cs at the output, and Cm at the 0V supply conductor, with respect to which the input and output signals are referenced. Figure 7b is the same previous circuit schematized in another way, where it can be seen that the parasitic capacitances have formed a feedback structure from the output to the input of the circuit. The only efficient way to completely eliminate this feedback effect is to join the 0V lead to the chassis, shorting the Cm capacitance. The internal circuit needs input and output connections. Any wire entering or leaving the equipment will violate the rule that the chassis must completely enclose the protected circuit and may cause problems due to the possibility of picking up and radiating EMI. The way to avoid violating this rule is to extend the chassis by using shielded cables, connecting their shields on both sides to the chassis of each equipment.
Equipment connected together with unbalanced cables connect successive signals together directly through each interconnect cable. This, and the fact that the chassis is connected to the ground of the signal, keeps its impedance very low. If the chassis of each equipment are connected to ground, there may be a potential difference of multiple return paths, since there are several return paths (ground loops). Low signal ground impedance between equipment is essential for acceptable operation of all balanced and unbalanced transformerless isolation systems. Balanced interconnects do not directly connect signal grounds. The negative lead of the balanced line provides the required signal return current. To avoid loss of dynamic range, balanced systems use a different method to keep signal ground potentials small.
Since the cable shield connects the two chassis together, simply connecting the signal ground to the chassis in each box maintains signal ground potentials between small equipment. The key is how to connect them. Since inter-equipment cables also provide the shortest path (and therefore lowest impedance) between two pieces of equipment, the use of cable shielding to minimize signal ground potentials between pieces of equipment is quite effective. Now that we know why the signal ground must be connected to the chassis, we are going to see how to connect them correctly. Close attention should be paid to analyzing where the currents flow. EMI currents from the shield flow through the chassis and are grounded in equipment with a 3-conductor network cable. The key point is that these EMI currents must not be shared with any signal current. The signal ground connection to the chassis can only be made at a single point on each unit.
This point is called the junction point of the signal ground and star ground. If they are connected at two different points, this leaves the possibility open for EMI currents to flow through a path shared by the signal, forming an unwanted ground loop that can cause problems. The best practice for connecting between signal ground and ground is to connect a wire directly from the power supply ground terminal (supply negative terminal) to the chassis ground point (figure 8). It is important not to allow other currents to flow through this connection path. This wire must not be allowed to share other return currents from other circuit points connected to the signal, such as input or output circuit ground. This prevents EMI currents from the chassis from flowing through the same wire, which is a return path for the signal. It should also be noted that EMI currents can flow through this cable and it should be kept away from sensitive circuitry. This is a star grounding scheme that uses a point originating from the power supply output as the star center of ground. There are two common locations on the power supply for the center of the star: the negative output terminal of the power supply or the point between the mains input filter capacitors.
Combinations of balanced and unbalanced connections
Let's see how the problem of incompatibility between balanced and unbalanced cables can be solved. Isolation transformers and active interface boxes are the best solutions. Figure 9 shows the correct method specified by the AES48 standard for connections between equipment with balanced cables. It shows how the connector housings (the cable shield) must be connected to the chassis and the signal ground pin that must also be connected to the connector housing. The cable shield must be connected to the chassis of the equipment on both sides. Figures 10 through 13 show recommended wiring for all combinations of balanced and unbalanced input and output interconnects when using 2-conductor shielded cable. It also includes the two most common shield grounding schemes: shield connection to chassis (ground) and signal ground. Identifying these schemes for each piece of equipment in a system is essential to lowering EMI.
This is not a simple task, since the chassis and signal ground are connected together. The dashed lines in the figures represent the chassis of the equipment. In these figures, the wiring diagrams are arranged in such a way that the upper figure is the "best" theoretical way to connect the equipment to obtain optimal results. As we move down the figures, we should expect a degradation in connection performance. The focus is the quality of the wiring, not the configuration of the input and output circuits. I/O circuitry is assumed to be ideal.
Fully balanced connection
Fully balanced connections between two pieces of equipment provide the best performance when both sides of the cable shield are connected to the equipment chassis. When for some reason, out of our control, the equipment with shields connected directly to the signal ground must be connected, without being able to connect to the chassis and, above all, in the case of having low level signals, we must disconnect the cable shield on the side connected to signal ground. This keeps induced currents in the cable shield out of the signal ground. If both teams involved have difficulty connecting the shield to the chassis and are only connected to signal ground, this is the worst case (figure 10d) and should be avoided if possible.
This is a fairly common but incorrect scheme. Most disconnect one side of the cable shield and there is always a debate about which side should be disconnected. As already mentioned, a compromise solution is to connect the ungrounded side with a capacitor (≈ 33 nF) between the shield and the signal ground. In this case, the shield is “connected” on both sides at high frequency and on one side at low frequency. In any case, both sides of a cable shield should never be disconnected. It is worse to float the shield than to use an unshielded cable.
Unbalanced output driving a balanced input
Figure 11 shows unbalanced outputs that are connected to balanced inputs. Again, only a two-conductor shielded cable is used. The best case has both sides of the shield connected to the equipment whose shield is connected to chassis ground (figure 9a). It could be argued that most likely the EMI induced in the signal conductors can be injected into the equipment sending the signal through the unbalanced output stage. This depends on the system and the output circuit. Disconnecting the cable shield on the unbalanced output could reduce this problem. When you have equipment with shields connected to signal ground, you must disconnect the shield on the signal grounded side. This keeps noisy screen currents out of the low level signal ground. If both teams involved have the shields connected to the signal ground, there is a problem again, as in the previous section, and you must choose which side to connect the shield on (figure 9d). Here too the capacitor solution on the unconnected side can be used to enhance at high frequencies.
Balanced output driving an unbalanced input
This is the most problematic configuration. In it the balanced outputs are connected to unbalanced inputs. Since the input stage is unbalanced, induced EMI on the signal leads cannot be rejected. This configuration should be avoided, but if it is necessary to use it, a cable that is as short as possible should be used to reduce induced EMI. This is why it is hard to find unbalanced RCA cables longer than 3 meters on the market.
This configuration does not support very long cables. Figure 12a shows both sides of the cable shield connected to equipment with shields connected to chassis ground. If the equipment is far apart, the likelihood of shield currents inducing EMI on the signal conductors is greater. Keeping the cable too short reduces shield current and therefore reduces EMI that is not rejected by the unbalanced input stage. Many systems may require the disconnection of one side of the shield for the case of figure 12a.
Even a small EMI current in the shield can be too high for an unbalanced input stage. Once again, consider connecting the shield on only one side or using a capacitor on the other side as well, as in the previous sections. It must be disconnected on one side in the shield of equipment with signal ground shields. If both sides have shields connected to signal ground, the shield on one side must also be disconnected (figure 12d). This schematic connects the negative output of the balanced output to signal ground, rather than a high-impedance input. Many balanced output circuits will try to control this ground of the signal, causing gross distortion and potentially damaging the output stage. If this scheme is used, it must be ensured that the balanced output stage can correctly handle the ground signal at its negative output.
Fully balanced connection
Completely unbalanced systems often do not have connectors with 3 conductors or more to allow proper use of a shield connection. In these cases it becomes difficult to properly connect the cable shield. Figure 13 shows its possible configurations. Once again keeping cable lengths short will reduce EMI problems, with or without shielding. For example, most home audio systems are totally unbalanced. Millions of these systems work practically without interference every day, due to the use of short cables and 2-conductor network cables, without grounding.
Also because they work in a home environment, electromagnetically quiet, problems would arise when trying to add professional equipment with balanced connections to a home system. Something similar often happens when trying to connect a new sensor to industrial equipment that is not prepared to manage it properly.
Conclusions
Balanced and unbalanced interconnects have two very different behaviors. The incompatibility between these two configurations, for both analog and digital signals, must be taken into account when designing, specifying, installing, or upgrading equipment. It is important to consider how the two types of interconnections are connected. The same care should be taken when connecting the input and output cable shields to chassis ground or signal ground.
