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EMI Noise, Part 1: Don’t Be the Problem

Electromagnetic interference (EMI) is a problem in all electrical and electronic circuitry. This six-part series will discuss the available component solutions for mitigating EMI noise emissions; how to make your circuitry less susceptible to EMI; and specific EMI considerations for automotive, medical, implantable, and space applications. In this first blog, I'll address EMI and the available component solutions for reducing EMI noise emissions.

Electromagnetic interference — which typically consists of a source, a path, and a victim — is a problem in all electrical and electronic circuitry. Some circuitry emits noise, others are susceptible to noise, and some both emit and are affected by noise. So, how do you know if your design is emitting too much noise? Government agencies, such as the Federal Communications Commission (FCC) in the US, set standards that dictate how much noise electrical and electronic systems are allowed to radiate and require proof of compliance before issuing approval for market introduction.

To ensure compliance with EMI standards, designs can be submitted to independent testing laboratories or tested in-house by the designer prior to applying for government approval. If a system's noise emissions are above the established limit (typically measured in µV/m), the entire design might have to be reconsidered.

Fortunately, however, there are a couple of effective ways to suppress radiated system noise. One option is to shield your system in a metal box so that nothing gets in or out. This method is often costly, though, and may not always work as effectively as hoped since even the smallest gaps in the shield will allow radio waves in or out. The other, more effective option is to implement EMI filtering.

EMI filtering requires that you use a device like a capacitor, an inductor, or a combination of the two to filter high frequency noise out of your circuit. A capacitor at self-resonant frequency (SRF) will become a short if connected in parallel (with respect to the ground), which is useful since any energy with frequency in the range of the capacitor's SRF will be attenuated. Consequently, for commercial, industrial, and automotive applications, a capacitor will generally act as an effective EMI filter.

More critical/high reliability applications — like medical, space, and aerospace applications — may require more specialized filters, though. Typically, when implementing EMI filtering, you'll want to choose a capacitor with a high enough SRF and enough attenuation at the noise frequency to filter out the noise.

Standard multilayer ceramic capacitors (MLCCs) are most commonly used in EMI filtering applications since they're often the least expensive devices and provide relatively good filtering. However, they tend to filter out a relatively narrow frequency band when compared to other available EMI filtering technologies. For example, surface mount feedthrough capacitors have been shown to significantly improve filtering bandwidth and attenuation. (See Figure 1 below for a comparison.)

Figure 1

100 pF feedthrough capacitor vs. 100 pF SMT capacitor S21 comparison.

100 pF feedthrough capacitor vs. 100 pF SMT capacitor S21 comparison.

Feedthrough capacitors are three-terminal devices that connect in series to the line that is being filtered, the middle terminal of which connects to ground (Figure 2a). Due to their construction, feedthrough capacitors exhibit a lower parallel inductance than standard MLCCs, which helps broaden its filtering bandwidth, as well as exhibit higher series inductance, which further attenuates noise signals at high frequencies (Figure 2a and Figure 2b).

Figure 2a

Feedthrough capacitor schematic vs. standard SMT capacitor schematic.

Feedthrough capacitor schematic vs. standard SMT capacitor schematic.

Figure 2b

Feedthrough capacitor device model vs. standard SMT capacitor device model.

Feedthrough capacitor device model vs. standard SMT capacitor device model.

Further, an interesting new development in EMI filtering device technology, the TransFeed multilayer varistor is a device that not only filters unwanted noise, but also protects circuits from the destructive transient voltages that are often the cause of electrostatic discharge. TransFeed multilayer varistors have a similar construction to feedthrough capacitors, with the dielectric containing zinc-oxide grains that become conductive when voltage spikes appear across the line they're connected to.

The main advantage of using TransFeed multilayer varistors in place of standard feedthrough capacitors or MLCCs is that they significantly reduce component count by incorporating the transient suppressor into the T-filter design of the feedhtrough capacitor, thereby providing the filtering capabilities of a feedthrough capacitor in addition to the transient voltage suppression (TVS) capability of a standalone TVS device (like a multilayer varistor or a TVS diode). See Figure 3a and Figure 3b.

Figure 3a

Feedthrough varistor schematic

Feedthrough varistor schematic

Figure 3b

Feedthrough varistor device model.

Feedthrough varistor device model.

All of the aforementioned filtering solutions — MLCCs, feedthrough capacitors, and TransFeed varistors — are ideal for use in automotive applications, but would have to be qualified to AEC-Q200, a common automotive qualification that is offered by many capacitor manufacturers. Alternatively, medical, space, and aerospace applications require the use of more sophisticated and more robust EMI filters to satisfy the high safety and performance standards of these more critical applications. Fortunately, however, several circuit and physical topologies for filtering in critical applications exist and will be covered in detail in future posts.

7 comments on “EMI Noise, Part 1: Don’t Be the Problem

  1. lukeic
    September 4, 2014

    I have a question, how to filter out noise from a switching power supply.

    I am using a switching power supply as the power source for my board. I need to output small signal. Will a cap you mentioned be enough to filter out the noise? or a Pi structure will be better? Do I need to put inductor on the ground line also? Will a common mode choke be good also?

  2. exmenendez
    September 4, 2014

    Hi Lukeic,

    It depends on the switching frequency, operating voltage, current and noise frequency of the switching power supply in question. Can you provide these parameters?

    Thanks,

  3. lukeic
    September 4, 2014

    It’s an AC-DC power supply. The input is the normal 87-264VAC, 47-63Hz The output is 18.7V, 1A, it has a noise level of about 190mV. The switching frequency is 119kHz, it has a noise frequency of 35MHz. I need to output 1mV analog signal out of this.

  4. JamesBryant
    October 30, 2014

    You may find some useful ideas in my article on the Analog Devices' website “Low Noise from DC/DC Switching Supplies” which is associated with their “Rarely Asked Questions” column RAQ 37. I cannot post a URL here, but if you visit the Analog Devices' website and use their search engine you should find them easily enough.

  5. lukeic
    October 30, 2014

    I found it, thanks a lot.

  6. lukeic
    October 30, 2014

    I found it, thanks a lot.

  7. lukeic
    October 30, 2014

    I found it, thanks a lot.

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