While observing the audio output of my smartphone on the oscilloscope, I noticed a signal being generated when I touch the touchscreen. This signal is around 500Hz and appears in groups of 3 peaks, with intensity proportional to the skin/touchscreen contact area:

The output is from an AUX cable connected only to the probe.

At a lower level, a similar phenomenon occurs when I bring the palm of my hand close to the touch screen at a few millimeters away, somewhat like a theremin. However, I have never heard anything when I connect the smartphone to headphones, the car amplifier, or the guitar amplifier.

On the other hand, if I connect the audio output to the input of my battery-powered opamp RC4580 (no connection to the AC power grid), the undesired signal is perfectly audible with each touch of the screen. But if I touch GND with my finger, the signal disappears completely:

Output from RC4580 with a bit of ripple at 50Hz plus the aforementioned signal.

The circuit is a very simple inverting summing amplifier:


I have tried with a different model of smartphone, and the same problem occurs, but the buzzing has a different tone.
What could be the cause of this issue, and how could I solve it so that it is not amplified by the opamp? Thank you.

Audio recording of the interference

 

Try turning off all sound and Haptic Feedback on press options to see if that helps.

 

It occurs both with audio output enabled and without any output, as seen in the first image. What do you mean by "turning off Haptic Feedback"?

 

https://en.wikipedia.org/wiki/Haptic_technology

Haptic feedback
Haptic feedback (often shortened to just haptics) is controlled vibrations at set frequencies and intervals to provide a sensation representative of an in-game action; this includes 'bumps', 'knocks', and 'tap' of one's hand or fingers.

The majority of electronics offering haptic feedback use vibrations, and most use a type of eccentric rotating mass (ERM) actuator, consisting of an unbalanced weight attached to a motor shaft. As the shaft rotates, the spinning of this irregular mass causes the actuator and the attached device to shake. Piezoelectric actuators are also employed to produce vibrations, and offer even more precise motion than LRAs, with less noise and in a smaller platform, but require higher voltages than do ERMs and LRAs.[20]

 

Ok, it disabled, also tested with airplane mode and also tested with an ipad.

 

Ok, it disabled, also tested with airplane mode and also tested with an ipad.

If it's not that, then maybe it's from the touch capsense circuits that generate pulsed EM fields to sense changes in touch sensor capacitance. Some of that energy could be following the audio path in some conditions of surrounding objects.

 

It definitely depends on the touchscreen reading system. I was able to verify that by connecting only the GND of the phone to the GND of the board, the noise is still heard. Furthermore, I tried with a TDA2030 module, one of the many available online, and the issue also occurs with this board. Therefore, it is not a problem exclusive to my board, strange that no one has ever encountered this issue.