The sampling rate of the ESP8266 or ESP32 Analog-to-Digital Converter is 2 Mega-samples per second (MSPS) maximum. Although the manufacturer Espressif recommends using a lower clock rate [1]
In order to support this rate, the ADC has to be operating in DMA (Direct Memory Access) mode and Wi-Fi has to be disabled. DMA enables the collection of data from the ADC and storage to a large buffer in memory.
When the Wi-Fi is operational, the sampling rate is much lower – at 1 kilosample per second (ksps).
The block diagram is shown below.
It is a 10-bit ADC
The sampling rate of the ESP8266 ADC is the same as that of the ESP32 that replaced it [2].
References
[2] ESP8266 on Wikipedia
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