Determining the most appropriate filter for a given application.

By Olivier Meilhon, ON Semiconductor

The complexity of cell phones and other portable electronic equipment is continually increasing. As a result, handheld products have become far more susceptible to
 Figure 1. On Semiconductor’s NUF4001 is a low capacitance, 4-line EMI filter with ESD.
noise and data corruption. A quick and easy way to prevent such issues is through the implementation of EMI filters. Because a filter for one application is not always applicable for another, it is important to choose carefully. Following nine simple steps can simplify the selection of the most appropriate filter for a given application.

Step 1: Determine the number of lines needed

The first parameter to consider in selecting an EMI filter is the number of lines to be filtered. The number of lines to be filtered typically comes in multiples of two. Most integrated filter devices provide filtering for 4, 6, or 8 channels.

So, in applications requiring more than 8 lines of filtering, a combination of the above filters will be needed; for example, an LCD display connector typically carries 16 to 20 lines of data. To protect 16 lines against EMI and ESD, one can use combinations of 2 × 8ch or 4 × 4ch.

Step 2: Determine the speed of data

The second parameter to consider is the speed of the data (data rate) to be transmitted across the data lines. Data rate is usually expressed in megabit per second or Mb/s. The data rate will determine the bandwidth of the filter (f3dB), which is expressed in mega hertz, or MHz. To get a nice clean signal across, one need to pass a minimum of 3 to five harmonics. Using this "Rule of Thumb", the resulting bandwidth of the filter can be calculated as follow: f3dB = (data rate) × 5 / 2

Since we pass 5 harmonics at 2 bits per cycle (or 1.0 Hz).

To meet a data rate of 12 Mb/s, a filter with a minimum bandwidth of 30 MHz will be needed.

Step 3: Determine how much noise immunity is needed (attenuation above the data rate bandwidth)

Noise immunity really translates into how much attenuation of EMI/RFI (Electromagnetic Interference/Radio Frequency Interference) is desired. Typical cell phones transmit data to the carrier at 850, 900 or 1900 MHz. This RF signal often creates noise on the internal data lines of the phone and can corrupt the data. Also noise from the unfiltered data lines can interfere with the RF signal. For this reason, the attenuation performance of a filter is looked at 800 MHz and above. A good starting point is to select a filter that can provide an excess of 㪱 dB of attenuation at frequencies of 800 MHz and higher.

Step 4: Determine how much insertion loss is acceptable (attenuation below the data rate bandwidth).

Insertion loss is an important parameter because it determines the amplitude of the signal that will be passed through the filter. High insertion loss will results in an overly attenuate signal that will be
 Figure 2. The NUF8001MU is a low capacitance, 8-line EMI filter with ESD protection in a UDFN package.
difficult to decode at the other end if its amplitude is too low. It can be compensated for by over driving the input signal but this can be draining on the battery. For that reason, insertion loss should be minimized. A good figure of merit is an insertion loss below 2 dB.

Step 5: Determine if ESD protection is required

Often filters are places around I/O connectors where they are susceptible to ESD events. By selecting a filter which integrates ESD diodes, one can not only ensure a clean EMI free signal, but also an ESD protected system downstream, without the need for a second ESD specific component. Typically one should look for an IEC-6100-4-2 (level 4) compliant part.

Step 6: Determine the maximum acceptable leakage current

Leakage current is the enemy of all portable devices. A leaky unit will cause constant drain on the battery and decrease talk time. Typically select a filter with a leakage current no greater than a few micro am/s.

Step 7: Define the size and package constraints

Low profile and small footprint are common requirements. Most filters are less than 1.0 mm in height, with many now even as thin as 0.55 mm. This is critical when placed in location such as the edge of the board of slim phones where headroom is limited.

Next, is BGA or DFN package preferred? Filters by their nature are commonly found at the edge of the PCB since they often need to be place near a connector. BGA packages are made of glass and tend to be more fragile and are know to pop off the board during drop test. DFN are typically more robust and preferred for these locations.

Step 8: Determine the required clamping voltage

Clamping voltage is the maximum voltage surge the subsystem can tolerate before electrical failure. This determines the maximum breakdown voltage. Amplitude of the transmitted signal will determine the minimum breakdown voltage. Breakdown level should remain above the amplitude of the transmitted signal to avoid clamping. An EMI filter with a clamping voltage of 14 V would be of little good for a data line with a peak voltage of 1.5 V.

Step 9. Determine the need for a bidirectional or unidirectional device

The final parameter to be considered is whether the transmitted signal is referenced to a circuit ground or a virtual ground, that is, it is DC offset. Most data signals are referenced to circuit ground, while audio signals can be either. Typically if a class D audio amplifier is driving the audio signal, the signal is reference to zero. If a class AB audio amplifier is driving, the audio signal is offset. Signals referenced to circuit ground use a unidirectional filter. Signals referenced to a virtual ground use a bidirectional filter. About the Author

Olivier Meilhon is a strategic marketing engineer at ON Semiconductor in the company’s portable market segment. Previously he worked for six years in the industry as an applications engineer. Olivier has a BSEE and MBA in technology management. He can be contacted at olivier.meilhon@onsemi.com