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Topic: SSD1351 Color Oled screw up audio signal (Read 702 times) previous topic - next topic

AloyseTech

Hi,

I have a custom board with an SSD1351 based Oled (the same circuit used for Adafruit Oled), a SAMD21 (arduino zero), and a LM386 amplifier connected to the DAC. Whenever the display is on, the sound is messed up, even if the dac setpoint is steady (with analogWrite(A0, 512)). When the display has no pixel on, the sound is good. As soon as a pixel is turned on, the sound is very noisy!

The Oled use a step up / boost converter to get 13V, could it be the problem? the switching frequency is around 1.5MHz, so it shouldn't...

Do you have any ida how and where to filter the noise?

Thanks!

pokos

It may be because of two reasons:
- the OLED and/or its power supply are too close to audio signal wires. This way you have an EMI issue;
- the device has a wide ground loop, for example, amplifier ground is connected to the system ground twice: to the power supply and to the audio source ground while audio source ground is connected to the power ground with a separate wire. Better way is to connect all the grounds to the power supply in "star"manner.

And any way add a ceramic or a polymer between OLED's VCC and GND pins.   

Good luck!

AloyseTech

Every device is decoupled with at least a 100nF ceramic cap. In fact, the DAC trace is running through all the PCB. It's also possible that there is multiple path to ground, because I added ground plane with via in each big "empty" polygon of the PCB...

Is there a quick-win method? like a RCR filter or something like this?

pokos

I guess RCR or LCL would not help much. Could you please provide a photo of the assembly?
The ground plane usually solves the ground-loop issue if every module is connected to the plane at single point. OLED may have two points: power cord and signal return, this way it has a loop again.
A picture would be appreciated here.

AloyseTech

The device is a two pcbs stack with dark soldermask, I can't take a good picture. ANyway, I have checked my design and I think I have a ground loop due a to much via. I will ensure only one ground way for every audio devices.
If I place the audio amp right next to the dac output, I should have less noise, right?

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