System Coexistence


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.

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. 

Example Analysis

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.

Interference Path: 

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. 

Interference Path: 

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.


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.

Interference path:

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..

Coexistence Mitigation


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


Interleave operation

Summary & Conclusion

 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.