How to achieve integrated ESD/EMI protection with dual audio filters

As portable and wireless devices become more complex, they are increasingly vulnerable to electrostatic discharge ( ESD ) and electromagnetic interference ( EMI ). Especially in electronic devices such as stereo headphones, mobile phones, portable multimedia players, PDAs or notebook computers, EMI needs to be reduced to ensure high audio quality of the electronic devices.

Figure 1. The CA04F2FT5AUD010G dual audio filter combination occupies an area of ​​1.37 x 0.92 mm2.

Typically, electromagnetic interference ( EMI ) is generated by radio frequency and affects audio lines. At the same time, audio input and output can cause distortion in radio frequency lines. To address this problem, EPCOS has developed a new combined dual audio filter (CA04F2FT5AUD010G) that can effectively reduce parasitic noise and improve audio quality. The filter is placed in a 0405 dual-channel package (see Figure 1) and is characterized by low series resistance and the integration of EMI filtering and ESD protection.

Figure 2 shows an example of an integrated ESD/EMI protection application. Designing this new audio filter requires integrating the functions of ten discrete components on the same chip (Figure 2). With its 20 MHz cutoff frequency and low series resistance of 0.2Ω, this filter provides a good solution for applications requiring low passband attenuation. For all four-band GSM frequency formats (850/900/1800/1900 MHz), UMTS format (2.1 GHz), and GPRS/WLAN Bluetooth frequency (2.4 GHz), its attenuation is higher than -20 dB in the frequency range from 200 MHz to 4 GHz. At 900 MHz, its attenuation value is very high, exceeding -60dB (Figure 3).

Figure 3. Insertion loss of the CA04F2FT5AUD010G audio filter

Filters using TVS and Zener diodes have very obvious nonlinear characteristics in the audio signal path. Therefore, they can greatly increase the total harmonic distortion (THD). In comparison, the dual ESD/ EMI audio filter produced by EPCOS has low THD characteristics and a noise value of less than -100dB at 1kHz, so it has a strong ability to handle ultra-clear sound.

Figure 4: The dual clamp structure improves ESD pulse protection capability. The first varistor clamp reduces the ESD pulse from 8kV to 200V, and the second varistor clamp reduces the ESD pulse to 70V.

The audio filter is also designed to provide excellent ESD protection for sensitive ICs (IEC61000-4-2, Level 4 standard). It uses a dual clamp structure to reduce ESD pulses from 8 kV to 70 V (Figure 4). In addition, the audio filter has a very stable ceramic structure. Even after 1000 ESD pulses, the dual ESD/ EMI audio filter can still maintain its original high filtering performance.