Put The Whammy On RF Noise, EMI Without Hurting Performance

At some point, you've probably wondered why some airlines require you to turn off all electronic equipment for 10 minutes or so prior to takeoff and landing.

In fact, some airlines don't allow such equipment to be turned on during any part of the flight. In reality, neither your CD player nor your child's video game is likely to disrupt the communication or navigation signals because the cockpit is shielded. The real concern involves the large amount of windows. If 300 cell phones were turned on at the same time, the airplane would become a huge source of interference to other planes. That problem worsens when planes are aligned in parallel awaiting takeoff.

Similarly, most circuits and systems are subject to interference from internal "unwanted" transmitters and receivers intrinsic to the design. That interference can radiate to other systems. Needless to say, electromagnetic interference (EMI) has received lots of attention over the past few years. Regulatory bodies (the FCC in the U.S. and the EC, or Conformite Europeenne, in Europe) govern higher-frequency products so they comply.

One of the most popular directives, the EMC (electromagnetic compatibility) directive (code 89/336/eec), prevents radiation that can interfere with neighboring applications. But while it's crucial to pass all mandatory certification tests, it's also important to consider optimizing the design for performance.

This article describes methods and tips on how to suppress EMI in your design and still maintain high performance levels. It underscores the importance of component selection. Also discussed is the use of multilayer boards to reduce loop areas that cause radiation to be propagated and transmitted.

HIGHLIGHTS

The Most Suitable Devices TTL and ECL (bipolar) devices are preferable in terms of RFI and radiation. Ringing does occur, though, particularly if there's a heavy capacitive load at the output of an op amp. Discussed are two methods that can eliminate such noise. Preventing Rectification Interference should be kept to a minimum to avoid rectification. The most direct approach is to use filters at the op amp's output. Amplifier oscillation can be a problem, however, so a couple of quick fixes are presented. Board-Level Protection EMI is propagated in the presence of electric and magnetic fields. The most effective way to reduce these fields is to enclose the board in a metal box. Also, using a multilayer board reduces the loop areas that can cause radiation to propagate and transmit. Choosing The Op Amp If the design requires a precision amplifier, and the source impedance is relatively low, it's preferable to choose an amplifier with low voltage noise, high common-mode rejection ratio (CMRR) versus frequency, high power-supply rejection ratio (PSRR) versus frequency, wide dynamic range, and high phase margin.

At some point you've probably wondered why some airlines require you to turn off all electronic equipment for 10 minutes or so prior to takeoff and landing. In fact, some European airlines don't allow such equipment to be turned on during any part of the flight. In reality, neither your CD player nor your child's video game is likely to disrupt the communication or navigation signals, or cause damage in the cockpit. After all, the cockpit is shielded, and the chance of a portable device interfering is minimal. The real concern comes from the fact that airplanes have lots of windows. If 300 cell phones were turned on at the same time, the airplane would become a huge source of interference to other planes. This is particularly true in airports, where airplanes are aligned in parallel awaiting takeoff.

Similarly, most circuits and systems are subject to interference from internal "unwanted" transmitters and receivers that are intrinsic to the design. Moreover, that interference can be radiated to other systems.

So it should come as no surprise that the subject of electromagnetic interference, or EMI, has received lots of attention over the past few years. Yet for many engineers, it remains a quasi-mystical phenomenon. Most electronic products with frequencies of 10 kHz and higher are subject to strict rules and regulations. In the U.S., the Federal Communications Commission (FCC) controls such rules, and manufacturers must test their products for electromagnetic compatibility (EMC) before marketing them.

Products destined for Europe also are subject to mandatory regulations. Currently, 18 European countries require a wide variety of products, including electronic and video games, to bear the label CE. This label stands for "Conformite Europeenne," or European Conformity, and implies that the product complies with European directives.

Among the several directives that exist, the most popular (and probably the most feared) is the EMC directive (code 89/336/eec), which applies to electrical and electronic equipment with potential electromagnetic emissions. Such directives are meant to prevent radiation that can interfere with neighboring appliances. Most manufacturers design their products for CE certification even when they're not required, as the CE label sets a benchmark for the product.

Electromagnetic compatibility refers to a system's proper functionality with regard to itself and to its environment. Whether conducted or radiated, energy must be constrained to a minimum level. Fast-switching devices and sharp edges generate EMI, the frequency of interference can be calculated by the formula:

fEMI = 1/(π × tRISE)

Contrary to general belief, most analog circuits are prone to radio-frequency interference (RFI). The fact that these circuits have a relatively low operating frequency doesn't mean that they're immune to interference emanating from high-frequency sources. In general, analog circuits are more susceptible to interference than their digital counterparts. Digital circuits are usually sources of emission because of high-frequency signals. But in both cases, care must be taken at the board level early on to prevent what could lead to a possible redesign.

While it's crucial for a circuit board to pass all mandatory certification tests, it also is important to consider optimizing the design for performance—particularly in high-precision circuits. EMI can transmit and propagate throughout the board by cables, current pulses, and sometimes even passive components.

Radiation affects temperature, which in turn affects the accuracy of electronic circuits. As much as 40°C of temperature drift can occur for every 1 krad (1 rad = 1 × 10^-5 joule/kg) of radiation. In addition, a circuit can easily pick up and amplify noise, causing serious interference. Thus, it's important for the designer to consider decoupling noise before it enters the circuit. With this in mind, engineers should take every precaution to minimize and prevent radiation through every step of their designs, starting with the choice of components.

THE MOST-SUITABLE DEVICES CMOS devices are usually worse than TTL and ECL (bipolar) in terms of RFI and radiation due to higher shoot-through current, which results from higher speed. When the output of a logic gate switches states, a large current flows to discharge the stray capacitance. This produces a noise voltage that can be coupled into the next gate and onto the rest of the circuit. This noise appears as ringing. Placing a small resistor at the output of the gate reduces the ringing.

Capacitive loads draw higher currents than resistive loads. A heavy capacitive load at the output of an op amp can cause severe ringing in much the same way as in digital gates. A simple interpretation is that the output impedance of the amplifier forms a pole with the load capacitance—causing a high Q or peaking in the frequency response of the op amp. This degrades the phase margin and can create an oscillation.

Figure 1 shows a square wave at the output of an op amp with a 2-nF capacitive load. Note the readily discernable ringing on the signal.

Using one of two alternatives can eliminate the ringing. The first circuit is best suited for applications that need a wide dynamic range (Fig. 2a). The added RC network, R_S and C_S, doesn't reduce the output swing. This implementation is recommended for op amps with non-rail-to-rail outputs.

Figure 2b illustrates another method. Besides handling heavy capacitive loads, this circuit offers the advantage of filtering the noise at the op amp's output. However, because the resistor (R_S) is inserted in the feedback loop, the output swing is reduced. Thus, this method is more suitable for op amps with rail-to-rail outputs. Figure 3 shows a photograph of a square wave after eliminating the ringing using the methods described.

Analog circuits are susceptible to interference because op amps can rectify high-frequency signals—causing dc errors—as well as amplify noise coupled into the circuit. To make matters worse, these signals can then be conducted throughout the circuit board, resulting in yet more interference. Generally, op amps with bipolar inputs are more susceptible to RFI rectification than their JFET and CMOS counterparts. This is because usually bipolar junction transistor inputs have lower impedance than CMOS and JFET devices.

Figure 4 shows a BJT differential pair. Capacitor C represents the substrate capacitance. As it starts to charge up, Q1 pulls up with low impedance, making the current flowing through the collectors of Q1 and Q2 to become unbalanced. This can lead to rectification of the signal. Low-power designs, however, are usually less susceptible to RFI rectification, because the input impedance of the input stage increases as the tail current decreases.

PREVENTING RECTIFICATION Interference should be kept to a minimum to avoid rectification. The most direct approach is to use filters at the op amp's input. Filters, though, can cause other anomalies. One of the most common problems is oscillation of the amplifier.

Figure 5a depicts a low-pass filter at the input of an inverting configuration. Capacitor C1 introduces a phase lag, which causes the phase margin to deteriorate, resulting in instability. To compensate for C1, another capacitor (C_F) is placed in the feedback loop of the op amp. Isolating C1 from the inverting input of the amplifier is also recommended, as shown in Figure 5b. The gain resistor (R4 in Figure 5a) is split into two resistors of equal value and C1 is doubled to maintain the same time constant of the output step response. Figure 6 shows the compensated and uncompensated output response.

In a similar fashion, a low-pass filter (R1 and C1) can be implemented at the noninverting input of the op amp (Fig. 7). A small capacitor (C2) is inserted in the feedback loop for proper compensation.

BOARD-LEVEL PROTECTION EMI is propagated in the presence of electric and magnetic fields. The most effective way to reduce these fields is to enclose the board in a metal box. Radiation is proportional to the currents flowing through the board and to the loop area. Using a multilayer board reduces the loop areas that can cause radiation to be propagated and transmitted. Reducing the loop area minimizes the inductance throughout the board. Designers must also keep traces and wires as short as possible. At higher frequencies, these wires become inductive and can be a source of EMI.

Passive components should be chosen carefully to help optimize the design. For example, to minimize noise on the power supplies, decoupling capacitors are necessary. These capacitors effectively reduce the loop area, thus minimizing radiated emissions. The most important thing to remember when choosing a capacitor is the desirable frequency range. This will help determine the appropriate capacitor size.

To determine the useful frequency range, two equations can be used: f_POLE = 1/(2π √ LC) and f_ZERO = 1/(2πRC), where L stands for the internal inductance and R represents the capacitor's shunt resistance. Suppose a capacitor has C = 0.01 µF, L = 1 µH, and R = 1012 Ω. Using the above equations, we find that the frequency for this capacitor ranges from 1.6 × 10^-6 Hz (f_POLE) to 1.6 MHz (f_ZERO). Beyond 1.6 MHz, the capacitor becomes inductive.

A second important factor to consider when choosing decoupling capacitors is the transient current required by the ICs, and the maximum supply voltage to be limited. For example, if 100 mA of transient current is needed over a period of 10 ns, and if the supply-voltage transient must be limited to a maximum of 100 mV, the capacitor's value can be determined by applying the formula: C = I × dt/dV. In this case, the minimum value of the capacitor is 0.01 µF. Due to their low inductance, ceramic capacitors are usually recommended for power-supply decoupling.

Inductors also will generate unwanted interference. Inductors can be grouped into two categories: air core and magnetic core. Air-core inductors tend to cause more interference because the flux lines are greater. On the other hand, these flux lines are reduced in magnetic-core inductors, as they tend to be contained within the core.

Resistors have several important parameters and specifications. Besides their value and tolerance, power rating is often crucial in determining the most- suitable resistor for a given design. The power rating of a resistor determines the maximum voltage that it can withstand without incurring damage. This voltage is calculated by √PR, where P is the power rating and R is the resistor value. It's also good practice to check the power-derating curves for the various types of resistors to be used in the design.

CHOOSING THE OP AMP If the design requires a precision amplifier, and the source impedance is relatively low, it's preferable to choose an amplifier with low voltage noise, high common-mode rejection ratio (CMRR) versus frequency, high power-supply rejection ratio (PSRR) versus frequency, wide dynamic range, and high phase margin. As a figure of merit, the voltage noise density of the op amp should be under 10 n √Hz. In this case, current noise matters little.

Also, connecting two or more amplifiers in a package in parallel can further decrease voltage noise. The tradeoff of this technique is an increase in the input current noise. For example, the voltage noise of the AD8512 at 1 kHz is about 8.5 n √Hz . When the two op amps from the dual package are connected in parallel, the output noise decreases by √2. Total noise is now about 6 n √Hz. However, if the source impedance is high, the current noise is of greater concern. In that case, the designer should terminate the unused op amps in dual or quad packages by grounding the positive input and connecting the negative input to the output.

POINTS TO REMEMBER Whenever possible, use multilayer boards. They help reduce the loop area and contain the magnetic field. Also, keep the decoupling capacitors close to the vicinity of the supplies, with the traces as short as possible throughout the boards. A long wire can act as an antenna for transmitting electromagnetic fields.

When measuring, twist the ground lead of a probe, and keep it close to the actual probe. This will reduce the dc error reading. In the presence of fast switching devices, traces should not be in parallel. This avoids creating greater loop areas for radiation.