Modern materials and technology produce electro-static discharge (ESD) and electro-magnetic interference (EMI) as ever-present hazards. What we wear and what we touch create potential for ESD. Digital technology has added to the EMI hazard always present in any electrical device. ESD can destroy electronic components in mobile phones. The mobile phone can be easily replaced but the disruption to the user can be significant. Circuit designers need to ensure that they employ measures to suppress the potentially damaging effect of ESD.
EMI presents itself in audio circuits as hisses, crackles, buzzes and generally poor sound quality. Users will not tolerate such disturbances in what they expect to be a quality communications device. So, EMI disturbances must be filtered out of the audio circuits.
ESD–Causes, Consequences and Suppression
Almost everyone has experienced the effect of static electricity. Since we were cave dwellers, we've seen it in lightning. And, in miniature, it is still a significant threat today. It is everywhere. Comb your hair with a plastic comb and check the static charge produced. Put your arm near the screen of a television and see the hairs on your arm stand up. This is the effect of static charge.
Get out of your car and touch the door. You may feel a shock from the discharge of static electricity. With more and more electrical devices in the home and workplace, static electricity is a constant hazard. People who build or repair electronic devices protect themselves, and the devices they are working on, by electrically connecting themselves to the device. This is how they avoid the damage to electronic devices that electrostatic discharge (ESD) can inflict.
We can see the damage lightning can inflict on buildings and trees. But even a tiny discharge, beyond human detection, can damage sensitive electronic circuits if ESD protection is not optimized. Mobile phones have some protection from ESD. External connections to the audio circuits are among the most common sources of ESD. Simply plugging in headphones and a microphone may mean subjecting your mobile phone to an ESD event.
Figure 1 shows what can happen to an electronic component when subjected to an ESD event. The minute hole in the gate oxide cripples the component.
Like all consumer goods, mobile phones have to be ESD certified according to the
IEC 61000-4-2 recommendations. This requires the device to be able to sustain, without damage, a ±15 kV air discharge (through 330 Ω/150 pF) that corresponds to more than a 45 A strike(1) in 1 ns. This is a high energy pulse compared to the ESD human body model test applied to integrated circuits. To protect the main ICs, additional ESD protection has to be implemented at each potential ESD entry point. Typically, ESD suppression devices produce a controlled output called 'clamping voltage.'
Figure 2 shows the output from an ESD protection device (clamping voltage) during an ESD event.
EMI–Causes, Consequences and Filters
An electrical current flowing in a conductor creates a magnetic field near that conductor. Change the current, and the magnetic field will change. So simply switching on or off an electrical current will change the magnetic field. The change in the magnetic field can induce signals in other conductors near the current carrier. This is the basic principle of the generation of electricity.
Domestic and industrial electrical supplies have an alternating current of 50 or 60 Hz. This is in the audible frequency range. This constant change in the current generates signals in nearby conductors at the same frequency. For those of you old enough to have used a Hi-Fi with separate player and amplifier, this is the buzz you heard when the chassis of the separate units were not electrically connected.
Consider now the digital world of today where signals are constantly changing between on and off states:
- Radiated and conducted EMI can be generated at the audio input/output and then transmitted to higher frequency RF lines causing signal distortion.
- The RF signal emitted by the phone antenna (TDMA burst) can be received by the long wires of the headset acting as an antenna and induce EMI noise in the audio signal path.
A given mobile phone will emit only during its own time slot. The fundamental frequency of the envelope signal is 1/4.615 ms = 217 Hz. The harmonic frequencies are 434 Hz, 651 Hz, etc. Such frequencies are audible frequencies. Figure 4 shows the envelope signal of a mobile telephone.
When the mobile phone is in contact with a base station or when two mobile phones are close to each other, the bursts emitted are conducted into the audio paths through microphones, speakers, audio connectors, or headset wires. See Figure 5.
The consequence is a significant degradation of audio quality.
By implementing EMI filters as close as possible to the entry point of the EMI disturbance, it is possible to minimize the audio quality degradation. See Figure 6.
The choice of the filter should be driven by its bandwidth, its cut-off frequency, and its stop-band rejection characteristics. Another factor in creating a good user experience in sound quality is total harmonic distortion (THD). A poor level of THD can ruin the sound quality of an otherwise excellent audio system. Ideally, the THD value of the EMI filter should be at least a few decibels better than the weakest link in the signal chain.
Typically the required characteristics are:
- Stop band attenuation of at least -25 dB for the frequency bands of
- Stop band attenuation of at least -20dB for frequency bands of 10–800 MHz
- Less than -70 dB (A) THD+N (0.03%) on MIC lines to provide high quality audio capabilities
Circuit Board Space Considerations
Mobile phones are integrating more and more multimedia functions such as GPS, MP3, FM, BlueTooth, and DVB-H. These features require additional board space while keeping the same form factor for the phone. Designers must look for the lowest possible board space consumption for ESD and EMI solutions.
Three Solutions Compared
Several solutions are available on the market, but not all offer a complete solution.
Figure 7 shows three possible solutions.
This solution uses 24 discrete components to provide ESD suppression and EMI filtering. This solution is not optimized in terms of ESD suppression and EMI filtering effectiveness due to the parasitic inductances of the PCB tracks–induced over voltages (Li/dt) and antennae-like behavior. It works, but the cost and reliability of mounting 24 discrete components calls into question the effectiveness of this solution.
Low Temperature Co-fixed Ceramic (LTCC) and Varistor Solution
LTCC EMI filters can be fine tuned to match filtering requirements. But, varistors have high clamping voltages (most likely VCL > 100 V) and thus are not optimized to offer quality ESD protection of sensitive sub-micron ICs.
Integrated Passive and Active Devices
This technology combines protection diodes and passive elements such as resistors and high-density capacitors on a monolithic silicon chip. Compared with the two solutions above, the IPAD solution:
- Fulfills all the ESD suppression and EMI filter requirements.
- Provides considerable board space saving (up to 78%).
- Offers significantly greater reliability and lower implementation costs due to the monolithic nature of the devices.
This article has presented the causes and potential consequences of ESD and EMI hazards on audio interfaces in mobile phones and outlined the requirements for ESD suppression and EMI filtering.
The comparison of the available solutions shows that an integrated ESD protection and EMI filter, provide the best ESD protection level (lowest VCL ) and the best stop band attenuation, as well as offering other advantages such as intrinsically better reliability and lower implementation costs.
1 When the ESD gun is shorted to the device under test with a 1 meter cable.
About the Author
Richard Renard is a Product Marketing Engineer for STMicroelectronics' ASD & IPAD Division in Tours, France. He has a Masters degree in Electronic Engineering and twelve years of experience in the semiconductor industry. He can be reached at: firstname.lastname@example.org