1.4.2 Techniques for metallic communications
Single-ended signal communication techniques have very poor EMC performance for both emissions and immunity, and are best restricted to low frequency, low data rate, or short distance applications. They are usually all right as long as they remain on a PCB with a solid ground plane under all the tracks and don’t go through any connectors or cables, which means that the single-PCB product is often the most cost-effective.
High-frequency or long-distance signals should be sent/ received as balanced signals (sometimes even on PCBs) for good signal integrity and EMC, and this is going to be a main issue in this sub-section.
Figure 8 shows examples of good and bad practices when connecting a millivolt output transducer to an amplifier via a cable.
In general, connecting a cable shield to a circuit’s 0V is very bad practice, as is the use of pigtails and grounding cable screens at one end only. Some older textbooks divide cables up into low and high frequency types, with different shield-bonding rules for each. But the electromagnetic environment is now so polluted with RF threats (and as was shown earlier, even ‘slow’ opamps will demodulate >500MHz), and so many signals are polluted with RF common-mode noise from digital processors inside their products, that all cables should now be treated as high-frequency.
The three schemes in figure 8 show a hierarchy from a poor system for connecting to a transducer, through a better one, to a good system. Fitting an A/D converter in the transducer enclosure and sending high-level encoded data (with error-correction) over the cable to the product for decoding would be better than the best shown opposite. A perfect system would send the digital data over a fibre-optic instead of a metallic cable, and such systems are increasingly used in industry.
Concerns about cable shield heating in large or industrial premises are best dealt with by running the communications cable over a parallel earth conductor (PEC) to divert the majority of the heavy low-frequency currents (which will prefer to follow paths with lower resistance) and not by ‘lifting’ a shield connection at one end – which ruins the cable’s shielding benefits at that end. Fitting a capacitor in series with the shield at one end is also not recommended as a design technique, although it may be useful as a remedial technique, because it is very difficult to make a capacitive bond work effectively over the full range of frequencies. PECs and other installation cabling and earthing techniques are discussed in detail in [2] [3] and [4].
For low frequency signals (say, under 100kHz) higher voltage levels in the communication link are better, for reasons of immunity. Where signal frequencies are above 10MHz (say) high voltages can lead to high levels of emissions – lower voltages are often preferred as the best compromise (e.g. as used by ECL, LVDS, USB). The signal frequency at which lower voltages are preferred depends on the length of cable and its type and EMC performance (especially its longitudinal conversion loss) and the design of the transmit and receive circuits.
Transmission line techniques may be essential for high-speed analogue or digital signals, depending on the length of their connection and the highest frequency to be communicated (see Part 5 of this series). Even for low-frequency signals, immunity will be improved by using transmission line techniques for their interconnections.
The best type of cable for EMC usually has a dedicated return conductor associated with each signal conductor, and any cable shields are used only to control interference. Co-axial cable is generally not preferred. Some cables need individually shielded signal pairs. It is very important to achieve a good balance over the whole frequency range, as this means a good common-mode rejection ratio (CMRR) and hence improved emissions and immunity. Balanced send / receive ICs are good, but isolation transformers have the benefit of adding galvanic isolation (up to the point where they flash-over) and also extending the common-mode range well beyond the DC supply rails.
Balanced construction twisted-pair or twinaxial cables usually give the best and most cost-effective emissions and immunity performance and very small differences in twist (and even the dielectric constants of the pigments used to colour their insulation) can be important. Balance is so important that in high-performance circuits even a physically balanced (mirror-image) PCB layout will be needed, using the same PCB layers.
Transformers and balanced send/receive ICs all suffer from degraded balance at RF. They generally require a common-mode choke in series to maintain good balance over the whole frequency range of interest. The CM choke always goes closest to the cable or connector at the boundary of the product.
Transformer isolation, balanced drive and receive, and CM chokes, all help to get the best EMC performance from a cable.
Figure 9 shows two examples, both equally applicable to providing good emissions and immunity for digital or analogue signalling (communications) of any speed or frequency range.
These circuits are ideal, in that a balanced send or receive circuit (in one case from a transformer, in the other an IC with balanced output or input) is connected to a balanced communications medium (the twin-axial or twisted-pair cable) via a CM choke.
Figure 10 shows how the CMRR of the choke is tailored to suit the transformer to give good balance over the whole frequency range, for a high-speed data example such as Ethernet. A similar design technique is used for the balanced IC.
For a professional audio communication link the signal frequencies extend to 20Hz or less, so the isolating transformer will be large. Its large interwinding capacitance rolls its CMRR off to zero before 1MHz, so the CM choke then needs to be larger to provide CMRR down to 100 kHz or less. It is difficult to find a choke that has good CMRR from 100kHz to 1,000 MHz, so two chokes with different specifications may be needed in series to cover the range.
Where co-axial cables are used instead of twisted-pairs or twin-ax, EMC and signal integrity will suffer and the techniques shown in Figure 11 will help to achieve the best possible performance from the cables used.
The circuit without the isolation transformer will generally suffer from poorer immunity at lower frequencies.
Many communications are still low frequency or low rate, and their signals are not particularly prone to causing emissions or suffering from interference. E.g. analogue to/from 8-bit converters will not be as sensitive as that from 12-bit converters, whereas 16 and higher number of bits will be very sensitive indeed.
Such signals are often sent down single wires in multiconductor cables to save cost, as shown by Figure 12 (an example of an RS232 application).
Where a conductor has N cores, it is best to connect it to the electronics at each end with a CM choke with N windings. Figure 12 shows a seven winding choke used for an eight-core cable, because one of the conductors is dedicated to “frame ground” according to the RS232 standard. (The frame ground lead is not likely to carry heavy currents and require a PEC because RS232 is only used for short-distances.)
RS232 only suits short distances because its single-ended signals lose their integrity rapidly as they radiate their energy as emissions. So although figure 12 (and the bottom circuit in figure 11) looks easy enough, the use of single-ended signals will require attention to CM choke and/or cable and/or connector quality. (Cable and connector types and qualities are discussed in the 2nd part of this series.)
Using drivers with very slow output edges (preferably slew-rate limited) can ease emissions problems significantly. Alternatively, standard drivers can be passively filtered to reduce their high-frequency content.
1.4.3 Opto-isolation
Opto-isolation is a common technique for digital signals, but the input-output capacitance of typical opto-coupler is around 1 pF – this creates a low enough impedance at frequencies above 10MHz to interact with the circuit impedances and destroy the balance of the signals in the cable.
As before, the selection of a suitable common-mode choke will restore the balance at high frequencies, allowing fast-edged signals to be communicated with fewer emissions or immunity problems.
Figure 13 shows an example of good EMC practices in a high-speed optically isolated link.
Similar to the previous examples, the CMRR of the CM choke is chosen to compensate for the fall-off in the balance of the opto-isolator at high frequencies, so that a good balance (equal to a good CMRR) is maintained across the full frequency range (DC to 1GHz in this example).
In many cases the CM choke can be replaced by two individual ferrite beads, and sometimes no choke or ferrites at all prove to be necessary.
But if they are not placed and routed on the PCB Murphy’s Law predicts that they will be needed, and furthermore it is likely that there will be no room for them, no doubt making a wholesale redesign of the product necessary, including its plastic enclosure.
If the cable needs to be shielded, it must be 360° bonding via a shielded connector or gland to enclosure shield at both ends, using a PEC if necessary (see IEC 61000-5-2). But where galvanic isolation is needed bonding the shield at both ends may be forbidden. In this case a capacitive bond at one end may be used (the capacitor rated for the full voltage, and probably safety-approved too) - or the shield left unterminated at one end, which is liable to have poor EMC performance.
Analogue signals can now benefit also from opto-isolation with up to 0.1% linearity (e.g. using IL300 and the like). This can save having to use voltage-frequency converters (and vice-versa) in many opto-coupled applications.
Because of the common drawing practice of not showing power rails in full, it sometimes happens that both sides of an opto-isolator are powered form the same DC power rails, seriously compromising the isolation achieved and the RF performance. The RF performance of opto-isolators can only be as good as the RF isolation between their power supplies.
1.4.4 External I/O protection
External I/O is exposed to the full range of electromagnetic phenomena. The better circuits in the above figures should need less filtering or protection, for a given signal and semiconductors.
All the above communication circuits may need additional filtering for emissions or immunity with continuous EMC phenomena.
For ESD, transient, and surge phenomena the upper circuits of figures 9 and 11, and figure 13, are well-protected – providing their isolating transformers or opto-couplers will withstand the voltage stresses applied. RF filtering can also give some protection against ESD or fast transients.
The above circuits without isolating transformers or opto-couplers will almost certainly need overvoltage protection with diodes or transient suppressors, although heavy filtering might be adequate if data rates or frequencies are very low. For control signals a series 10k or 100k resistor closest to the connector followed by a 100 nF or 10 nF capacitor to the PCB ground plane makes a marvellous barrier against almost all EMC phenomena, but does not allow rapid changes in logic state.
Digital communications generally need a robust digital protocol (see below) to prevent data corruption, as protection devices only prevent actual damage to the semiconductors.
Allow for additional protection devices on a prototype board, and test it as early as possible to see which are needed.
1.4.5 “Earth – free” and “floating” communications
Another name for galvanic isolation is “earth free” or “floating”, but these terms are often misunderstood or misused.
The above circuits using isolating transformers or opto-couplers are all “earth-free” and “floating”, because no currents from the communications devices are assumed to flow between Tx and Rx via the 0V or chassis. This is true even though their cable screens are bonded at both ends to local chassis (enclosure shield). In fact, leakage currents flow through parasitic capacitances, and when CMRR is poor they can reach surprisingly large values.
The terms “earth-free” and “floating” are also sometimes applied to electronically balanced inputs or outputs, such as the lower circuit of figure 9. Although good CMRR performance will still give low leakage via 0V or chassis, such circuits are not galvanically isolated and are intrinsically more vulnerable to surges. Electronically balanced circuits also have a reputation for suffering from instability when one of the two lines is accidentally connected to ground.
Don't forget that the quality of the isolation achieved in practice is limited by the isolation performance of the power supplies supplying each side.
Never try to achieve “earth-free” operation by removing the protective earth from any equipment – this creates serious safety hazards and immediately contravenes several mandatory laws. If “ground loops” are a problem, use the proper circuit and installation techniques (e.g. PECs) and never compromise safety.
It is best to avoid jargon phrases like “earth-free” and “floating”, instead state what is actually required or meant in plain circuit terms.
When screens cannot be connected at both ends
In some applications it is mandatory not to connect equipment grounds via cable screens or other conductors. The equipment concerned is still connected to main supply system’s earth, but the earthing system is controlled in a special way. This does not help to achieve EMC at low cost. A screen connection at only one end will make the balance of the circuit and its conductors more important, and it will be more difficult and expensive to achieve a given EMC performance for a given signal.
Attention to creepage and clearances will also be important for safety reasons. In larger installations: when screens are not bonded at both ends, surges can cause arcing at the unconnected end possibly causing fire or toxic fumes. People can also receive shocks if they happen to be touching the screen and other equipment when a surge arrives. Clearly, not connecting the screens at both ends must place extra electrical and EMC stresses on some of the circuit components and cables, making surge, transient, and ESD damage more likely.
1.4.6 Hazardous area and intrinsically safe communications
Special barrier devices to limit the maximum power available in normal and fault conditions, and other restrictions, may be required. The EMC performance of these devices, which are made by specialist companies, is crucial. Further discussion is beyond the scope of this series.
1.4.7 Communication protocols
The data protocols used for digital communications are vital for both emissions and immunity, and it is much better to purchase chips that implement proven protocols than to try to develop them yourself. Simple protocols are easy, but they are very poor for EMC. Chips implementing CAN, MIL-STD-1553, LONWORKS, etc, have hundreds of man-years experience with interference control built into them, which no normal project team can ever hope to equal. Spend the extra few dollars on robust protocols, it will be worth it. Protocols are not discussed further in this series.
1.5 Choosing passive components
All passive components contain parasitic resistance, capacitance, and inductance. At the high frequencies at which many EMC problems occur these parasitic elements often dominate, making the components behave completely differently. E.g.: at high frequencies a film resistor becomes either a capacitor (due to its shunt C of around 0.2pF) or an inductor (due to its lead inductance and spiral tolerancing). These two can even resonate to give even more complex behaviour. Wire-wound resistors are useless above a few kHz, whereas film resistors under 1k usually remain resistive up to a few hundred MHz. A capacitor will resonate due to the effect of its internal and lead inductances, and above its first resonance it will have a predominantly inductive impedance.
Surface mounted components are preferred for good EMC because their parasitic elements are much lower and they provide their nominal value up to a much higher frequency. E.g. SMD resistors under 1k are usually still resistive at 1,000 MHz.
All components are also limited by their power handling capacity (especially for surges handling), dV/dt capacity (solid tantalum capacitors go short-circuit if their dV/dt is exceeded), dI/dt, etc. Passive components can also suffer severe temperature coefficients, or need de-rating. SMD parts have lower power ratings than leaded, but since most power occurs at lower frequencies it is often possible to use leaded parts in those areas, although taking care to minimise lead length.
For capacitors, ceramic dielectrics usually give the best high frequency performance, so SMD ceramics are often excellent. Some ceramic dielectrics have strong temperature or voltage coefficients, but COG or NPO dielectric materials have no tempco or voltco to speak of and make very stable and rugged high-quality high-frequency capacitors. They tend to be larger and cost more than other types, for values above 1nF.
Magnetic parts should have closed magnetic circuits, as has been described above. This is important for immunity as well as for emissions. Rod-cored chokes or inductors must be used with great care, if they cannot be avoided altogether (what shape is the ferrite antenna of a radio receiver?). Even the mains transformers used in linear power supplies can give better EMC performance if they have an interwinding screen connected to protective earth.
All these imperfections in passive components makes filter design very much more complicated than the circuits in textbooks and on simulator screens might suggest. Where a passive component is to be used with high frequencies (e.g. to decouple interfering currents up to 1,000MHz to a ground plane) it helps to know all about its parasitic elements and to do a few simple sums to work out their effects. Helpful manufacturers of quality components publish parasitic data, even sometimes impedance performance over a broad range of frequencies (often revealing their self-resonances).
Some passive components will need to be rated for safety, especially all those connected to hazardous voltages, of which the AC supply is often the worst case. It is best to only use parts here which have been approved to the correct safety standard(s) at the correct ratings by an accredited third-party laboratory and allowed to carry their distinguishing mark (SEMKO, DEMKO, VDE, UL, CSA, etc.). But the presence of the mark on the component means nothing. Much better is to get a copy of all the test labs’ certificates for the safety approved parts and check they cover all they should.
The use of components with unknown parasitics for high-speed signals and/or EMC purposes makes it more likely that the number of product design iterations will be high and time-to-market delayed.
1.6 References:
[1] Tim Williams, EMC for Product Designers 3rd edition, Newnes 2001, ISBN: 0-7506-4930-5, www.newnespress.com
[2] IEC 61000-5-2:1997 Electromagnetic Compatibility (EMC) – Part 5: Installation and mitigation guidelines – Section 2: Earthing and cabling, www.iec.ch.
[3] Tim Williams and Keith Armstrong, EMC for Systems and Installations, Newnes 2000, ISBN 0 7506 4167 3 www.newnespress.com, RS Components Part No. 377-6463. |
