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SNUF6401 Datasheet(PDF) 4 Page - ON Semiconductor |
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SNUF6401 Datasheet(HTML) 4 Page - ON Semiconductor |
4 / 6 page NUF6401, SNUF6401 http://onsemi.com 4 Theory of Operation The NUF6401MN 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 8 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) + 12 ) 2 p a S n + 1 1 2n * 1 sin((2n * 1)w0t) (eq. 1) Equation 2 (simplified form of Equation 1): x(t) + 12 ) 2 p sin(w0t) 1 ) sin(3w0t) 3 ) sin(5w0t) 5 ) AAA (eq. 2) Frequency (Hz) 100k 1M 100M 1G 10G 10M Figure 8. Filter Bandwidth −3 dB −6 dB −9 dB f1 f2 f3 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. |
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