AN4511 STMICROELECTRONICS | Alldatasheet
Document overview
- Manufacturer or author: Provided By ALLDATASHEET.COM(FREE DATASHEET DOWNLOAD SITE)
- PDF pages: 21
Technical content
Datasheet sections
- 1 Single-ended and differential links
- 2 Risks of EMI on differential links
- 3 Common mode filters
- 4 The main parameters
- 5 Benefits of the common mode f ilter
- 5.1 Effect on skew
- 5.2 Effect on the radiated noise
- 6 Specific benefits of ECMF series
- 7 Conclusion
- 8 Revision history
In our connected world, data exchange very fast and their content is shown in a large screen with high-definition capabilities. These features include high-speed serial links, working in different modes and increasing data rate. However, fast transition times and long lines may induce radiations due to common mode noise. In order to avoid these issues, the most efficient device to use is a common mode filter. In this article, the basic parameters of this device and its benefits are described.
1 Single-ended and differential links
current to ground is near zero and there isn’t any ground bounce issue (Figure 1). Figure 1. Differential signaling
2 Risks of EMI on differential links
Figure 2. Example of skew between interior and exterior traces on a flex Figure 3. Radiations induced by skew on a differential link called: common mode filter (Figure 4).
Figure 4. Common mode filter on a differential link
3 Common mode filters
The common mode filter is based on two coupled inductors (Figure 5). Figure 5. Symbol of the common mode filter
AN4511 Common mode filters so Equation 5 in differential mode, I2 = -I1 Equation 6 and Equation 7 The filter presents a low resistive impedance equal to DC resistance of inductances therefore a low attenuation of the high-speed differential signal (see Figure 6). In common mode I2 = I1, therefore Equation 8 and Equation 9 The filter presents a high inductive impedance, therefore a high rejection of the common mode noise (see Figure 6). ML= Z1 R1 jω L1 M–() R1≈()+= Z2 R2 jω L2 M–() R2≈()+= Z1 R1 jω L1 M+() R1 jω 2L×+=+= Z2 R2 jω L2 M+() R2 jω 2L×+=+=
Figure 6. Common mode and differential mode in CMF
4 The main parameters
from Agilent technology, working up to 3 GHz. Figure 7. Differential and common mode impedance measurement frequency represents the sum of inductance resistances of the filter. Figure 8. Typical common mode and differential mode impedance vs. frequency
- SCC21 common mode rejection, the filter efficiency cuts the unwanted noise in a specific frequency range
- SDD21 defines the differential bandwidth of the filter, the filter ability to drive the main signal without distortion
5 Benefits of the common mode filter
- The skew brought by two different trace lengths
- Radiated noise on a USB-MHL cable
Another example is described in AN4356.
5.1 Effect on skew
Figure 15 shows the used simulation schematic. MHL link is built at 2.25 Gbps. Figure 15. Skew simulation schematic is not equal to zero (Figure 16). Figure 16. Common mode noise with and without skew the more polluting frequencies. In this case filtering is necessary.
Figure 19. Improvement on the noise level due to ECMF02-3HSM6
5.2 Effect on the radiated noise
To establish CMF effect of the radiated noise, a phone is placed into an anechoic chamber. To avoid measuring unwanted frequencies, the phone is in a shielded box. placed 3 m far from the system (Figure 20 and Figure 21). Figure 20. Radiated noise test setting (anechoic chamber view)
6 Specific benefits of ECMF series
thanks to sensitive digital circuits (Figure 23), improving system quality and reliability. Figure 23. ECMF with integrated ESD protection value is also measured on the application environment, see Figure 25. Figure 24. ECMF02-2HSLMX6 TLP characteristic
7 Conclusion
The common mode filter is the best solution to avoid issues like common mode noise induced by radiated or conducted RF or skew between tracks. It allows a system to improve its EMI robustness. This application note helps the user to choose the filter with more accuracy, fitting it to his needs, the equipment and the standards used. The wide range of filters offered by STMicroelectronics covers many applications (MIPI, HDMI, MHL, USB.) and includes internal ESD protection allowing the equipment to be more robust against external disturbances.
8 Revision history
Table 1. Document revision history 29-Jul-2014 1 Initial release.