AC Signal Input

Apparently there was a mix-up between the ESP32-C3 and the STM32.

Being ESP32, it's another forum: https://www.esp32.com/

None of the STM32 microcontrollers support symmetrical ADC inputs, i.e. true differential.
You require a negative supply voltage for that. By the way, this applies to almost all MCUs of other vendors, too.

You can either amplify/attenuate and shift the signal to the range acceptable for the onboard ADC in an appropriate preamplifier, or use an external ADC converter.
Though both variants would require symmetrical power supply in one way or another.

If you need just the zero crossing events, there are simpler methods and circuits.