EMI7204MU, EMI7206MU, EMI7208MU

http://onsemi.com

3

Theory of Operation

The EMI720X combines ESD protection and EMI

filtering conveniently into a small package for today’s size

constrained applications. The capacitance inherent to a

typical protection diode is utilized to provide the

capacitance value necessary to create the desired frequency

response based upon the series resistance in the filter. By

combining this functionality into one device, a large number

of discrete components are integrated into one small

package saving valuable board space and reducing BOM

count and cost in the application.

Application Example

The accepted practice for specifying bandwidth in a filter

is to use the 3 dB cutoff frequency. Utilizing points such as

the 6 dB or 9 dB cutoff frequencies results in signal

degradation in an application. This can be illustrated in an

application example. A typical application would include

EMI filtering of data lines in a camera or display interface.

In such an example it is important to first understand the

signal and its spectral content. By understanding these

things, an appropriate filter can be selected for the desired

application. A typical data signal is pattern of 1’s and 0’s

transmitted over a line in a form similar to a square wave.

The maximum frequency of such a signal would be the

pattern 1−0−1−0 such that for a signal with a data rate of

100 Mbps, the maximum frequency component would be

50 MHz. The next item to consider is the spectral content of

the signal, which can be understood with the Fourier series

approximation of a square wave, shown below in

Equations 1 and 2 in the Fourier series approximation.

From this it can be seen that a square wave consists of odd

order harmonics and to fully construct a square wave n must

go to infinity. However, to retain an acceptable portion of the

waveform, the first two terms are generally sufficient. These

two terms contain about 85% of the signal amplitude and

allow a reasonable square wave to be reconstructed.

Therefore, to reasonably pass a square wave of frequency x

the minimum filter bandwidth necessary is 3x. All

ON Semiconductor EMI filters are rated according to this

principle. Attempting to violate this principle will result in

significant rounding of the waveform and cause problems in

transmitting the correct data. For example, take the filter

with the response shown in Figure 4 and apply three

different data waveforms. To calculate these three different

frequencies, the 3 dB, 6 dB, and 9 dB bandwidths will be

used.

Equation 1:

x(t) +

1

2

)

2

p

a

S

n + 1

1

2n * 1

(eq. 1)

Equation 2 (Simplified form of Equation 1):

x(t) +

1

2

)

(eq. 2)

2

p

1

) p20

3

) p20

5

)AAA

FREQUENCY

(Hz)

100k

1M

100M

1G

10G

10M

−3 dB

−6 dB

−9 dB

Figure 4. Filter Bandwidth

From the above paragraphs it is shown that the maximum

supported frequency of a waveform that can be passed

through the filter can be found by dividing the bandwidth by

a factor of three (to obtain the corresponding data rate

multiply the result by two). The following table gives the

bandwidth values and the corresponding maximum

supported frequencies and the third harmonic frequencies.