Electrical – eliminating audio hum


I'm working on a circuit that feeds a microphone into the ADC of the STM32F4 Discovery and then outputs that exact signal out of the microcontroller's DAC and then to a headphone jack.

I am able to hear everything that the microphone picks up, but I am getting a humming noise when listening to the circuit. The humming noise is present no matter if the microphone is attached or not. I've done the following to try and isolate the problem:

  • Connected the ADC pin to the DAC pin (still hums)
  • Disconnected the ADC pin (still hums)
  • Disconnected the DAC pin (no hum)
  • Grounded ADC pin (no hum)

I'm a little confused as to the what the problem could be. Judging by the tests I would think the problem would be in the Sallen-Key Filters after the DAC, but I have no idea how to alleviate it.


simulate this circuit – Schematic created using CircuitLab

Are there any glaring problems with the circuit above that would cause the hum?

EDIT: Sorry the 100 Ohm Resistor R5 is supposed to be 100k Ohms

Best Answer

The power supply rejection is effectively -6dB because of R3/R4. Place a capacitor across R4 to filter the AC voltage from the divider.