12 Methods of Increasing Immunity to Interfering Signals “Rest, rest, perturbed spirit” (Shakespeare: Cymbeline) Despite careful design and protections to limit the emission of interfering sig- nals by disturbing circuits, some residual noise will still contaminate sensitive circuits by conduction or field coupling. Therefore, the next task is to improve the immunity of perturbed circuits, in order to make them more noise tol- erant without sacrificing their electrical performance. Traditional methods include balancing, filtering, and grounding. Although these were discussed in Chap. 10 (in the context of reducing emission from sources of interferences), they will be revisited here with focus on how to use them in order to increase electromagnetic immunity. 12.1 Balancing Definition. A balanced circuit is a two-conductor circuit that is symmetric with respect to a longitudinal axis (ultimately passing through the grounds of its terminal ports). The basic condition required is symmetry, which also applies to the trans- mission path and to the source and load. Balanced Line. In a balanced line, the fields around the conductors are symmetric and there is no special ground conductor. For instance, two-wire lines are balanced, but a coaxial line is unbalanced. Explanation. The property of symmetry is essential because the ultimate goal of balancing is to make the noise pickup equal in the two longitudinal circuit halves. When this condition is achieved, any external noise is trans- formed into a common-mode signal, which cancels in a load which has a symmetric configuration with respect to ground. 390 12 Methods of Increasing Immunity to Interfering Signals Fig. 12.1. Generic configuration of a balanced circuit For instance, in Fig. 12.1 suppose that Vp1,Vp2 are the inductive noise voltages picked up by the conductors, which generate common-mode currents Ip1,Ip2, respectively. The useful signal is delivered by the symmetric source (E1,E2) and causes a current (Is) through the loop passing through the terminals A–B–C–D; hence, it is a differential-mode current (which is not influenced by load sym- metry). It follows that the total output voltage across the load is VAB =RL1 Ip1 − RL2 Ip2 +IS(RL1 +RL2) (12.1) The first two terms represent the external noise contributions; they cancel provided that RL1 =RL2 and Ip1 =Ip2. The former condition implies that the load must have a symmetric configuration with respect to ground; the latter requires identical conductors equally exposed to interfering signals (which is a critical condition). To conclude, perfect balancing requires: – A signal source that is symmetric with respect to ground (E1 =E2, Rs1 =Rs2). – A load that is symmetric with respect to ground (RL1 =RL2). – Identical conductors, equally exposed to noise. Example. To illustrate the difficulties of achieving perfect balancing, con- sider a system (Fig. 12.2) of two metal traces on a board, denoted (A), (B), coupled to a third one (P), which is the source of interference (perturbing system). The cross-coupling has two components. Consider the capacitive coupling: the signal (Vp) carried by conductor P is transmitted through CPA ,CPB to 12.1 Balancing 391 Fig. 12.2. Capacitive coupling in a quasi-balanced system its neighbors and becomes a perturbation. Let us denote by VPA the fraction of VP reaching conductor A; thus RA VPA =VP (12.2a) RA +1/jωCPA At low frequencies, we may suppose that RA |1/jωCPA |, and (12.2a) be- comes ∼ VPA = VP (jωCPA RA) (12.2b) Similarly ∼ VPB = VP (jωCPBRB) (12.2c) When the system is symmetric, RA =RB, but due to the location of the disturbing conductor (closer to A than to B), CPA > CPB, and unequal contamination results. If inductive coupling is considered, the effect of the magnetic flux gener- ated by the current flowing through conductor P is stronger on conductor A than on conductor B (due to different separation distances). Note that this effect, rather than cancelling the unbalance of capacitive coupling, adds to it instead. This shows that even when the circuit has perfectly symmetric structure with respect to ground, it is not perfectly balanced until the coupling to the source of interference is the same throughout. Possible solutions are: – modify location of traces to obtain, as much as possible, equal exposure to interfering signals; – when wire conductors are used instead of traces, twisting represents an effective way to reduce unbalance. Balance Ratio. This concept is employed in communication systems, which are designed for a minimum of stray pickup from the outside. The degree to which a transmission device deviates from an ideal balanced condi- tion is expressed by the balance ratio or simply balance [193]. Note that this 392 12 Methods of Increasing Immunity to Interfering Signals Fig. 12.3. a Measuring the unbalance of a transmission system; b equivalent load for well-balanced systems corresponds to the common-mode rejection ratio (CMRR) in the theory of operational amplifiers. In practice, this parameter can be measured by using the circuit in Fig. 12.3, where we apply a common-mode voltage Vc and measure the re- sulting differential-mode voltage Vd on the load. For a perfectly balanced configuration, Vd should be zero. The greater the value of Vd,themore unbalanced the circuit. Often expressed in decibels, the balance ratio (BR) is V BR = 20 log c [dB] (12.3) Vd Whenever the source internal resistance is low, Vc is roughly equal to the voltage (V) measured between each conductor and ground; definition (12.3) can be expressed as V BR = 20 log [dB] (12.4) Vd Equation (12.4) is better suited when the distance separating the source and the load is significant, since both measurements are made at the same end. For circuits with high BR, it is advisable to replace the load with the equivalent circuit proposed in Fig. 12.3b, where X is a highly balanced reactor (for instance, a transformer). Concluding Remarks – A well-balanced circuit exhibits a BR factor of the order of 60 to 80 dB. – In operational amplifiers, a high BR guarantees a high rejection of common- mode interfering signals and therefore increased immunity. – As a general rule, balancing is employed when shielding has been imple- mented but fails to sufficiently increase circuit immunity. 12.2 Filtering 393 12.2 Filtering As shown in Chap. 9, filters can be employed either to suppress the con- ducted emission of perturbations from noisy circuits or to improve immunity of critical circuits. The former aspect has been addressed in Chap. 10; now, we shall focus on the latter. Overview. Depending on the particular type of action, filters are used in two situations: 1) When the DC power supply is distributed by lines (planes) having a non- negligible internal resistance and inductance, abrupt current demand due to a large number of simultaneous switching events can give rise to consid- erable inductive noise. As a result, a transient voltage drop in the power supply results, which may introduce logic failures and/or degrade the drive capability of transistors. This noise, propagating along the power and ground planes, may contaminate critical circuits sharing the same power/ground system. In this case, decoupling filters must be provided at all sensitive circuit power pins. 2) In electronic equipment, shields are employed to protect critical circuits; however, the cables and wires passing through the shield can transport not only useful signals, but also picked-up interfering signals. To avoid contamination of the protected circuits, careful filtering of wires and grounding of interconnect screens must be provided. 12.2.1 Decoupling Filters Decoupling the DC Supply. This is mandatory whenever the user has no control over the design of the DC supply and its associated distribution system. In practice, two kinds of filters are used to decouple circuits: RC filters and LC filters. Both are presented in Fig. 12.4, where filter ground should be connected to the ground of the protected circuit. Essentially, they are low-pass pi-section configurations intended to sup- press voltage fluctuations due to abrupt transient currents in the DC supply system. To be effective, the HF series impedance of the filter must be as high as possible, but the DC series impedance must be negligible (to avoid losses in the DC supply). This compromise is rather difficult to obtain with RC filters, since the value of R is not frequency-dependent. From this point of view, LC filters are better suited, but they have the following drawbacks: – their inductance is sensitive to spurious magnetic fields; – they present a self-resonant frequency. Provisions have to be made to ensure that the normal operation of the filter will not be affected by it. 394 12 Methods of Increasing Immunity to Interfering Signals Fig. 12.4. Decoupling the DC power supply input of every protected circuit with: a RC-filters; b LC-filters All in all, RC filters are preferred, mainly because they are less suscepti- ble to electromagnetic interference. Contrary to common wisdom, the value adopted for capacitor C should not be taken much higher than the calculated value. As a general rule, adopt the smallest decoupling capacitor that will do the job (since the parasitic series inductance of any capacitor increases substantially with its value, and this will impair filter performance). Amplifier Decoupling. To illustrate the necessity of decoupling, consider the single stage amplifier of Fig. 12.5a, where the parasitic inductance of the +VCC trace (between nodes 4 and 5) is denoted by Lp. It is common practice to draw the AC equivalent circuit of the amplifier by considering node 4 to be at ground potential (since any ideal voltage source has zero internal AC resistance, and the trace inductance is neglected).