Teensy 4.1 Embedded Audio Device Help

[acarson]

Hi everyone Im in the process of building an embedded audio fx processor/midi controller with teensy 4.1 as the main brain and im having trouble working with the SGTL5000 and digital noise from the board. Im still relatively new to a lot of this so bear with me. Ive made multiple revisions to the board with different circuits for the SGTL including giving it its own separate AGND ground pour, having LDO Isolation for the sgtl power etc. but nothing seems to eliminate the digital noise I keep hearing from the audio output. The board itself has 9 RGB LEDS and a 4 digit display which I can definitely through the audio output. So on this next board revision Im thinking of just using the Teensy Audio Module Rev D2 schematic for audio output.

So here are my questions to you:
1. Given where the components are placed on my pcb (see image below) where would be the best place to put the SGTL circuit? On my last revision (see other image) I had it in the bottom right corner of the board close to the audio input and output jacks.

2. Are there any specific types of RGB LEDs I could use instead that don't produce so much noise? The current ones im using are https://www.digikey.com/en/products/detail/feztek/FZ2812-0807/28312656 mainly using these because of their low power consumption.

3 What are the best practices when It comes to using LEDS on imbedded audio boards?

4. Since it looks like the Teensy Audio Module Rev D2 uses a single ground plane, is it even necessary to have a dedicated AGND and GND pour on the board at all? Or would just a single GND pour work fine?


Any and all guidance would be extremely helpful here! Thanks

 

[MarkT]

1) LEDs will produce heavy switching noise as they have logic-fast drive and take a lot more current than a logic signal (think ~100A/µs current slew rates!) - keep separate from analog stuff by distance, avoid large loop area in the current carrying conductors too. Chose a PWM frequency above 20kHz can help, not using PWM helps a lot more. Decouple LED supply well (100nF, 1µF, 330µF).
2) The SGTL5000 is fairly low-fi for a I2S DAC/ADC as its designed to go in a mobile phone at low cost. I know for instance the microphone input channel has obvious spurs in its spectrum. Separate high-quality ADC and DAC will get far better performance.
3) Separate analog and digital power/ground is definitely the way to go for minimum digital hash getting into an audio signal. Think about loop area with any power/ground wiring (i.e. use ground plane if at all possible).
4) Condition the audio signals on input and output with opamps and RF-rejection circuitry. Don't use type-II ceramic caps in the audio signal path, they distort heavily.