ADC catch 1kHz noise caused by ESP32

[lockfield]

Hey guys,

I'm trying to use the ADC on ESP32. The ADC is directly connected to a LPV321n amplifier on the same PCB with ESP32. The amplifier gives a bias for input signal. Only a LDO circuit, an op amp circuit and an ESP32-WROVER-E are on the PCB.

When ESP32 works, the ADC catch a 1kHz noise as the figure attached (which also appears on an oscilloscope). When I disconnect the ESP32 from the PCB and use an oscilloscope to check the op amp output, there is no such noise. So the noise comes from ESP32 obviously.

1kHz_noise.png (65.68 KiB) Viewed 3632 times

The noise is about 20mV, and about 1kHz. I think it should match some clock, but I can't figure it out. No bluetooth or WIFI is working when ADC samples (In fact, I have observed a larger noise about 150mV at first, which is cause by BLE).

What is the source of the 1kHz noise? Does anyone have ideas?

[adrien]

I also noticed a noise at 1kHz albeit on Vcc and I think it was because of the OS ticks. You could check that in the configuration (default is 100Hz iirc).

In my setup I had noise on a cheap power amplifier and could both hear it and see it on a scope; when I changed the tick configuration, the noise sounded different but unfortunately I didn't have the scope setup ready anymore and couldn't check.

Unfortunately for me, changing the configuration didn't prevent the noise from being amplified (I could maybe try 10Hz and maybe the audio streaming through I2S+DAC will still be without gaps) and the (small) dips on Vcc got larger, probably due to more work being done by the ESP32.

I'm surprised the dips on Vcc were constant even when as long as 16ms and even with decoupling caps, I'm not that good at electronics and I lacked time to investigate.