Espressif Systems ESP Faq page 80

Chapter 4. Software framework
If I float the ADC pin and print out VDD3P3 value (65535), then the voltage of VDD3P3 should be 65535/1024
≈ 63 V. Why this is not the correct voltage value?
The input of ADC should be in the range of 0 V to 3.3 V (the upper limit varies in different chips). The
floating measurement is an undefined state.
What is the input resistance of ESP32 ADC?
ADC is capacitive and can be considered as a large resistance.
When using ESP32's ADC to detect the power supply voltage, is it necessary to divide the voltage?
The ADC reference voltage of ESP32 is 1100 mV, but the ADC measurable range can be increased
by configuring the internal attenuation. For more information on the measurable range, please refer to
ADC Section in the chip datasheet. If the measurable range cannot satisfy your requirement, please add
an external voltage division circuit.
What is ESP32's highest sampling rate in ADC DMA mode?
ESP32 supports up to 2 MHz of sampling rate theoretically.
When an ESP32 calling adc2_get_raw() between esp_wifi_start() and esp_wifi_stop(), the
read operation fails. What is the reason?
Since Wi-Fi also uses ADC2, and the Wi-Fi driver has a higher priority, the application may fail to read
using adc2_get_raw() during the operation period of Wi-Fi. It is recommended to check the return
value of this function and re-measure it after failure.
Does ESP32 support using ADC2 and Bluetooth simultaneously?
Yes.
What is the sampling rate range supported by the ADC DMA mode of the ESP32-S2 chip?
Frequency limit : 611 Hz ~ 83333 Hz.
Does the ADC of ESP32 support simultaneous sampling of multiple channels?
No, If you are using ADC to do multi-channel sampling, please implement it via ADC polling scheme.
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