Many factors are conspiring to increase the amount of “noise” or interference that can disturb the functionality and even damage electronic devices, starting with the sheer number used in our vicinity at any given time. Today’s Wi-Fi or Bluetooth-enabled medical instruments and even medical implants provide a prime example.
Industry has typically employed electromagnetic interference (EMI) shielding along with EMI filters in various configurations to eliminate unwanted noise. However, even some of the traditional solutions for eliminating EMI and radiofrequency interference (RFI) are no longer sufficient given increases in operating circuit frequency, noises of higher frequencies that expand the affected frequency range, and the miniaturization of electronic devices that shrinks the distance between source and victim.
Because circuits today often operate at lower voltages, many medical electronic devices are more easily affected by noise, even with less energy. For this reason, many OEMs avoid options such as 2-capacitor differential, 3-capacitor (one X-cap and 2 Y-caps), feed-through filters, common mode chokes, or combinations of these, opting instead for more ideal solutions such as monolithic EMI filters that deliver superior noise suppression in a substantially smaller package.
Eliminating Unwanted Noise
When medical electronics receive strong electromagnetic waves, unwanted electric currents can be induced in the circuit and cause unintended operations — or interfere with intended operations. EMI/RFI can be in the form of conducted or radiated emissions. When EMI is conducted, it means that the noise travels along the electrical conductors. Radiated EMI occurs when noise travels through the air as magnetic fields or radio waves.
Even if the energy applied from the outside is small, if it is mixed with the radio waves used for broadcasting and communication, it can cause loss of reception, abnormal noise in sound, or disrupted video at places where the radio waves for broadcasting and communication are weak. If the energy is too powerful, electronic devices can be damaged.
Sources of noise include natural, such as electrostatic discharge and lighting; and artificial such as contact noise, leaking from devices that use high frequencies, unwanted emissions (e.g., harmonic emissions from digital circuits or emissions from switching power supplies), and others. Even a circuit inside an medical electronic device can generate noise that can cause interference with another circuit in the same device.
Usually, EMI/RFI noise is common mode noise, so the solution is to all but eliminate unwanted high frequencies with an EMI filter, either as a separate device, or embedded in circuit boards. This also helps medical OEMs meet regulations that exist in most countries limiting emissions.
Traditional EMI Filters
EMI filters normally consist of passive components, such as capacitors and inductors, connected together to form circuits. The inductors allow dc or low-frequency currents to pass through, while blocking the harmful unwanted high-frequency currents. The capacitors provide a low impedance path to divert the high frequency noise away from the input of the filter, either back into the power supply or to the ground connection.
Traditional common mode filtering approaches include low pass filters comprised of capacitors that pass signals with a frequency lower than a selected cutoff frequency and that attenuates signals with frequencies higher than the cutoff frequency. It is common to apply a pair of capacitors in a differential configuration, with one capacitor between each trace and ground of the differential input. The capacitive filter in each leg diverts EMI/RFI to ground above a specified cutoff frequency. Because this configuration involves sending a signal that is opposite in phase through two wires, the signal-to-noise ratio is improved while unwanted noise is sent to ground.
Unfortunately, the capacitance value of an MLCC with X7R dielectric (typically used for this function), varies significantly with time, bias voltage, and temperature. So even if the two capacitors are tightly matched at room temperature, with a low voltage, at a given time, it very likely that they end up with a very different value once time, or voltage, or temperature have changed. This mismatch between the two lines will cause the response near the filter cutoff to be unequal and, therefore, it will convert common mode noise to differential noise. Another solution is to bridge a large value X capacitor across the two Y capacitors. The X capacitor shunt delivers the desired effect of common mode balancing, however, with the undesired side effect of differential signal filtering.
Perhaps the most common solution — and an alternative to low-pass filters — is to use a common mode choke. A common mode choke is a 1:1 transformer where both windings act as both primary and secondary. The current through one winding induces an opposing current in the other winding. Unfortunately, common mode chokes are also large, heavy, expensive, and subject to vibration-induced failure.
An ideal common mode choke with perfect matching and coupling between the windings is completely transparent to differential signals and presents very high impedance to common mode noise. One disadvantage of common mode chokes is the limited frequency range due to parasitic capacitance. For a given core material, the higher the inductance used to obtain lower frequency filtering, the greater the number of turns required and consequent parasitic capacitance that defeats high-frequency filtering.
Mismatch between windings from mechanical manufacturing tolerance can cause mode conversion, where a percentage of the signal energy converts to common mode noise and vice versa, leading to EMI. Mismatches also reduce the effective inductance in each leg. When differential signals (to pass) operate in the same frequency range as the common mode noise that must be suppressed, common mode chokes offer an advantage over other options because the signal pass band can extend into the common mode reject band.
Monolithic EMI Filters
Monolithic EMI filters, which are multilayer ceramic components (MLCCs), offer an ideal alternative to common-mode chokes. When properly laid out, these MLCCs are actually better for rejecting common mode noise. They combine two balanced shunt capacitors in a single package, with mutual inductance cancelation and shielding effect. Monolithic EMI filters utilize two separate electrical pathways within a single device attached to four external connections.
It is important to note that a monolithic EMI filter is not a traditional feed-thru capacitor. Although they look identical (same package and external look), their design is very different, and they are not connected in the same way. Like other EMI filters, monolithic EMI filters attenuate all energy above a specified cutoff frequency, passing only required signal energy while diverting unwanted noise to ground. The key, however, is the very low inductance and matched impedance. With monolithic EMI filters, the terminations connect internally to a common reference (shield) electrode within the device, and the plates are separated by the reference electrode. Electrostatically, the three electrical nodes are formed by two capacitive halves that share common reference electrodes all contained in a single ceramic body.
A monolithic EMI filter introduces almost no conversion of common mode noise to differential signals, or vice versa. Having a very low inductance makes it particularly effective at high frequencies. The balance between capacitor halves also means piezoelectric effects are equal and opposite, canceling out. This also affects temperature and voltage variation, so components age equally on both lines. Compared to the common mode choke, a monolithic device provides significantly more RFI suppression in a substantially smaller package and rejects a much wider frequency band.
If there is a downside to monolithic EMI filters, it is that they cannot be used if the common mode noise is at the same frequency as the differential signal. When this is the case, the common mode choke is a better solution. Although monolithic EMI filters initially cost more than equivalent ordinary capacitors, the cost is a fraction of the cost of a common mode choke alone.
This article was written by Christophe Cambrelin, Product Manager for Johanson Dielectrics, Camarillo, CA, a company that manufactures a variety of multi-layer ceramic capacitors and monolithic EMI filters. For more information, click here.