Next-generation high-speed differential data transmission EMI low-pass filter

The latest consumer electronics products, such as TVs, set-top boxes or mobile phones, are all interconnected or at least equipped with high-speed data interfaces. The most common interfaces for the end consumer are USB or HDMI/MHL, but there are also internal interfaces that are invisible to the end consumer but equally important for designers, such as the emerging MDDI or MIPI interfaces, which are used to connect different modules or functions on the circuit board. For example, we cannot ignore the trend of using differential data lines instead of parallel data lines to connect the camera sensor or AMOLED display panel of the mobile phone to the motherboard, because the advantage of differential data transmission is that it can increase the data rate and display resolution...

Obviously, these new interfaces are creating a new challenging environment, while increasing the threat of ESD and EMI phenomena to electronic product design.

Of course, traditional EMI low-pass filters, such as RC filters or LC filters, will soon reach the filter limit, so to adapt to these new application trends, a new generation of protection chips must be developed. The new ECMF series of protection chips are a new generation of protection products designed by STMicroelectronics, which is well known in the industry for its high-performance filters, to meet the latest ultra-high-speed differential signal filtering and ESD protection requirements.

In the past, ESD protection or EMI filtering functions were mainly based on the use of RC or LC solutions, such as LTTC or silicon chips. However, the advent of higher data rate buses and the trend of differential signal transmission instead of parallel buses naturally forced designers to improve the EMC immunity of the entire system and seek new solutions. Needless to say, considering that LC or RC filters are composed of inductors or resistors and grounded capacitors, especially the inherent capacitance effect itself will affect the integrity of the signal, these two types of filters will not be able to adapt to the trend of increasing data bus speeds. Therefore, the capacitance effect of the filter can be avoided by simply suppressing the capacitance; but this approach means losing the filtering properties of the filter chip. This approach is a dilemma when data rates increase to hundreds of megabits per second.

CMF filters, also known as common-mode filters, are a good way to solve this dilemma, not only supporting the highest data rates, but also providing the best protection for differential signaling technologies such as USB, HDMI and MIPI.

The high-speed USB 2.0 interface uses differential signaling to transmit data on two data lines, with a maximum transmission rate of 480 Mbps. Differential signaling means that the signal is not referenced to ground, but one signal is referenced to the other signal. Differential signaling is transmitted on two lines with a 180-degree phase difference on each line, which means that an appropriate filtering topology must be used to correctly filter out unwanted frequencies without destroying the integrity of the target differential signal.

The new CMF filters allow the target differential signal to pass through the filter without destroying the integrity of the differential signal, while also filtering out common-mode signals. The inductive characteristics of the common-mode filter produce a wide frequency band of up to 7 GHz for the differential signal, while producing a narrow frequency band of less than 100 MHz for the common-mode signal.

An ideal common-mode filter can selectively suppress common-mode signals while releasing differential signals without any impact on differential signals.



The current directions of the differential mode are opposite, and the polarities of the magnetic fields generated are opposite, and the magnetic fields are canceled. In this case, the signal passing through the CMF filter does not encounter any impedance, let alone signal attenuation. The



current of the common-mode signal flows in the same direction, generating an in-phase magnetic field on the filter, and the two magnetic fields are superimposed on each other. As a result, the yoke current coil is a large impedance for the common-mode signal, thus reducing the integrity of the common-mode signal.

The SCC21 standard describes the basic characteristics of common-mode attenuation, as shown in the figure below:






The USB 480 Mbps signal can generate a 240 MHz base frequency. Because the signal itself is a square wave, it is not difficult to estimate the bandwidth required to transmit the signal. Using the Fourier series approximation algorithm, the final required bandwidth is about three times the base frequency. Therefore, differential signal transmission requires at least 720 MHz bandwidth. It is not difficult to see from the common-mode filtering diagram of the SCC21 standard that in order to allow the third harmonic to pass through the filter, some frequencies that meet the requirements are filtered out.

We tested a USB interface with an STMicroelectronics ECMF filter built in. From the eye diagram of USB 480Mb/s, we can see that the interface design is 100% compliant with the USB high-speed data transmission standard, and the signal attenuation measured at 900MHz reaches 30db.

This principle also applies to higher speed interfaces, such as MIPI or HDMI/MHL interfaces.



The test results shown in the figure above are exactly the same as those done on USB 2.0. Now, we will do another test on an interface with a higher data transmission rate such as HDMI720p.

The figure below is a harmonic measurement result of a MIPI interface. The filter allows the highest frequency of the MIPI signal to be 800Mhz (the fourth harmonic of the 200Mhz clock signal) while filtering out noise from 900Mhz to 2.2Ghz.



Given that common-mode filters support data rates up to Gb/s and can suppress common-mode noise up to GHz, we conclude that common-mode filters are the best filtering solution for differential signal transmission technology.

While filtering out EMI/RFI noise is important, it is equally important to prevent ESD from damaging or even destroying the internal circuits. Obviously, all of the aforementioned interfaces are installed in external ports, and ESD events can easily damage these interfaces when the user inserts or removes the plug. Usually, CMF filters are used in conjunction with additional external devices such as protection chips to effectively prevent the main chip from being broken down, as shown in the figure below (15kV contact discharge voltage produces a melting point in the chip oxide layer).





STMicroelectronics' new ECMF filter integrates ESD protection circuits on the same chip, as shown in the figure below:

Like STMicroelectronics' ECMF, common-mode filters with integrated low-capacitance ESD protection circuits can provide perfect and safe ESD protection for high-speed data interfaces, allowing the final product design to meet the most stringent IEC61000-4-2 semiconductor component 15kV air discharge and 8kV contact discharge standards.


When it comes to ESD protection, there is an important factor to note. When an electrostatic discharge pulse is applied, the protected chip directly behind the protection circuit cannot accept excessive voltage. This parameter is called clamping voltage.



To evaluate different common mode filter topologies and measure ESD protection performance, we performed several ESD tests around the clamping voltage values. The following diagrams describe the test process. We used an electrostatic gun to test the device under test with an 8kV contact discharge, while connecting an oscilloscope after the CMF filter to capture the voltage waveform changes.

This experimental method accurately simulates the voltage values ​​that the printed circuit board will be subjected to after an ESD electrostatic discharge event. Under these conditions, the output voltage was measured to be 50V after an 8kV electrostatic discharge, which is the lowest clamping voltage currently available, making ECMF the safest CMF filter + TVS transient voltage suppression solution on the market.



To achieve the best RFI/EMI noise attenuation using common mode filters, designers must consider several important parameters, among which bandwidth is a critical parameter. This value is an estimate of the harmonic frequency, corresponding to the maximum signal frequency allowed to pass by the filter. At least 3-4 GHz bandwidth is required to transmit the third harmonic of a 1 GHz signal, which is a factor in avoiding data integrity damage.

The following table compares the bandwidth of ECMF filters with other brands of filters on the market. We assume that the bandwidth is obtained at the maximum attenuation of -3db of the SDD21 parameter.

Solutions 1 and 2 show that adding a varistor to an LTTC structure will increase the intrinsic capacitance of the filter, resulting in a reduction in bandwidth. Although silicon technology can improve performance, by comparing solutions 3 and 4, we find that the performance of ST's monolithic solution is better than the two-chip solution (solution 3).

In addition to comparing filtering performance, we also need to compare ESD protection performance. By measuring the clamping voltage values ​​of different solutions, designers can evaluate and find the most suitable technology and topology to protect the entire system to meet IEC safety requirements.

In order to better evaluate the performance of the LTTC filter with internal varistor and the performance of the LTTC filter with external Zener diodes on the printed circuit board, several ESD tests were performed.





The red measurement value represents the LTTC common mode filter with external Zener diodes. After applying an 8kV contact ESD pulse, the clamping voltage rises to 250V, which is almost 5 times that of the silicon filter solution with built-in TVS protection diodes.

The blue measurement results represent the LTTC common mode filter with internal varistor. Because the parasitic capacitance value is reduced due to integration inside the filter, the clamping voltage of this filter is significantly improved compared to the red measurement value. However, the clamping voltage still rises to 150V, which is still three times higher than the monolithic common-mode filter with built-in TVS protection diode from STMicroelectronics.

The main concern of portable application designers is to reduce the number of components, optimize the space of the printed circuit board and the system cost. In this regard, integration technology brings multiple benefits to designers, prompting them to choose common-mode filters in their designs, thus promoting the development of new common-mode filters. As mentioned above, because integration technology suppresses parasitic inductance, data bandwidth and attenuation are improved, so by integrating multiple functions on a single chip, the ESD and filtering performance of common-mode filters are greatly improved. In fact, the protection diode is integrated into the filter structure, which helps to simplify the system design and reduce the connection lines on the printed circuit board; therefore, there is no parasitic inductance to affect the filtering suppression effect and protection performance.

From the perspective of optimizing the board space, integration technology is a must. Common-mode filters with built-in protection devices significantly reduce the space of the printed circuit board and reduce the number of components. The following figure compares a two-line LTTC common-mode filter with two external protection chips (TVS diodes in 0402 packages) and an ST monolithic ECMF filter that implements the same function. The

LTTC filter-based solution occupies about 4.5mm2 of space on the printed circuit board, while the ESD protection filter two-in-one chip CMF occupies only 2.8mm2, saving 40% of the printed circuit board space. Taking into account the spacing of components, the advantage of the ST solution is even more obvious, saving more than 50%.

In addition, this design example using ST's CMF filter can also save 70% of the number of components.

This new generation of common-mode filters with built-in protection chips brings multiple benefits to designers. In addition to being particularly suitable for filtering disturbances and suppressing noise in high-speed differential signaling applications, the new products are also particularly suitable for enhancing EMC interference resistance, significantly reducing the number of components on the printed circuit board, and ultimately reducing the area of ​​the printed circuit board.

STMicroelectronics' new generation of CMF filters with built-in ESD protection chips has two products: the two-line ECMF02-2AMX6 and the four-line ECMF04-4AMX12. Both products are packaged in plastic through-hole packages with package areas of 1.7mm x 1.6mm and 1.5mm x 3.3mm respectively. For designers of portable products that want to design smaller appearance, STMicroelectronics has also launched CMF filters in WCSP packages.