Have you wonder why plugging a charging able to your phone during in car navigation throws off the navigation heading by large degrees. Similarly, have you wonder why wonder your camera capture fringe or periodic noise during wireless charging, but noise disappears when wireless charger is removed.
There is any underlying interference within the electrical system inside the product that give rise to these problems. In industry, we label these issues system coexistence.
In order for ensure coexistence/interoperability of all main functional subsystem to function in conjunction without any user noticeable effects, a quality product needs to go through system coexistence test.
System coexistence is generally break into two portions digital/baseband and RF system. The latter is generally referred to RF desense. Please see RF desense for detailed analysis. In this article, we will focus on digital subsystem coexistence.
Note: Not all subsystem are digital, some are mix signal circuits such as ADC/DACfor or power circuits such as inductive charging.
Co-existence in system design (i.e PCB level design) refers to the ability of each subsystem to operate in conjunction with each other within a complex design (i.e consisting of different subsystems) evaluated at desired modes of operation.
Steps devise a coexistence test
Coexistence Matrix consists of following:
Column: Aggressors: Any subsystem that operates with high current and high frequency switching is a strong source for induced electromagnetic interference.
- USB charging circuit, wireless charging circuit, audio amplifiers, DDR, processor, etc.
Row: Victim: sensitive circuits such as ADC inout, analog in and output, analog voltage rails for powering sensors conversion stage, etc.
Cell: Acceptance criteria: for audio output speakers, we want to any acoustic noise to be under for instance 30 dBa. for ADC, we want the noise floor not to be increased by maximum allowed threshold, for analog signals, we want the SNR to main a minimal dB level, and for sensors, we want the reading error to be under maximum allowable deviation from the ground truth.
- Note: These acceptance criteria is very product depended and a good engineer need to work with product manager and technology specialist to come up with an acceptable criteria of each subsystem
Example 1: Charger affect Compass reading
Battery charging causes 20 degrees error in magnetometer(electronic compass) reading. The aggressor is the charging circuit and the victim is the magnetometer in this case.
What causes magnetometer to report erroneous reading?
magnetometer is designed to detect weak earth magnetic and is highly sensitive to any induced magnetic field in its vicinity. However, induced magnetic field strength is proportional to current strength and current loop size. Charging current and its high loop path exactly fits the description.
Charging current travels from USB connector to the battery and back to the USB forming a current loop and creating a magnetic field. This magnetic field is picked by magnetometer, hence throwing off its reading.
To mitigate this specific case, we want to make sure all charging current returns directly in ground plane underneath the charging power plane by route power plane directly above the ground plane or have a ground pour underneath the power trace. Depending on PCB design, there could additional connection from the main board to the USB connector. one needs to ensure that this additional connection (i.e a cable or flex) carries low current. One thing that could be done is adding a resistor (10 to 100 ohms) to the ground connection between cable of flex to the USB connector to force a high ground impedance to limit any DC current flow, reducing magnetic field of this secondary current loop.
Example 2: Wireless Charging affect Camera Image quality
Modern camera module uses a CMOS imager sensor (CIS) to transfer raw image data via a flexible printed circuit board to interface with the application processor on the main PCB board. The imager contains a CMOS pixel array, analog processing (e.g. AMP and filter), and an ADC which digitized pixel value into 8 or 10 bits. This ADC and pixel array is powered by analog power rail which requires has a very clean source with low voltage ripple in mV range.
The wireless inductive coil resonant at frequency of 100 of KHz induced a voltage ripple on the long power trace for the analog power rails. This induced voltage ripple noises is modulated on top of the analog cell voltage read by the ADC which results in periodic noise seen during camera capture.
- Use digital filter in frequency domain to filter suppress noise such as band reject filter
- Shield the camera module flexible printed circuit (FPC) with EMI materials.
- Note: it's always a good practice to shield PFC with EMI film laters but in this specific case, the charging magnetic frequency is too low for EMI filter to be effecting.
- Shield the wireless charging coil with ferrite sheet
- Turn of wireless charging whencamera is on.
- Note: this depends on product requirement intended for user usage and experience.
Example 3: Buzzing sound heard on headphone when charging
Earbuds are drive by a stereo DAC from a CODEC chip on the any consumer device with audio jack. The DAC line out is entirely a analog signal.
Charging circuit presents a high current and generate voltage noise due to charger circuit. This noise can propagate through the system power plane, which also used to power the DAC chip.
- Conducted charger circuit noise propagates through system power plane to DAC power circuitry. This noise gets modulated on the analog line out to the headphone. Then the voltage noise is heard as buzz noise by headphone.
Mitigation: Add ferrite bead and decoupling capacitors at the DAC power pins to shunt the noise before noise gets into internal DAC circuitry/or add ferrite bead filter at charger circuit to stop noise propagation at the source..
Most of coexistence issue can be solved by improving power layout such as using star connection to isolate power coupling among each shared power subsystem, and in addition using single point grounding for all return currents to reduce ground current loops
- Note: ground loops generates common impedance ground noises for sensitive circuits requiring a good clean reference ground for data conversion such as ADC.
- Add aluminum shield for all ICs reduces radiated noise interference.
- Add ferrite sheet to shield magnetic field generated by wireless charger coil.
- Add EMI film on the outer layers on flexible printed circuit boards.
- Note: EMI film in this case can provide up to 60 to 80 dB isolation for high frequency noises. It is also effective to reduce noise emission from the high speed interface on the module, so it does not impact RF receivers of the radio.
- Add ferrite beads for noise power traces at its source (i.e place ferrite bead right at output of a noise power regulator (SMPS)
- Increase line to line separation from high speed digital interfaces to reduce crosstalk.
- In software, we can turn off the aggressor such as a NFC polling when an known victim such as a Camera is on.
Summary & Conclusion
- Defined coexistence as a the interoperability of each subsystem to work on conjunction with out any user impact.
- Defined coexistence maxrix with three components: aggressor, victim, and acceptance criteria in order to verify and evaluate the level of system coexistence.
- Provided examples of coexistence problems with its root cause and possible mitigation solution.
- Provided mitigation in 3 categories: layout, filtering, and software optimization.
Coexistence is an important validation and design step for electronics products to ensure its performance. As products miniaturizes with ever more functionalities, system coexistence is the key to a quality product. A good designer needs to root cause and provide solution to all possible coexistence issues via thorough and repeated testing.