Maximum sampling frequency of Arduino (internal vs external ADC)
The first thing to determine the data transfer speed is to find out what digital interface your external ADC actually uses. This is most often SPI or parallel interface.
For SPI, the speed is determined by the speed of the SPI interface on your microcontroller and ADC, whichever is slower.
For parallel, it is determined by the GPIO speed (or parallel interface if you have one) on your microcontroller.
Here's a tip: Do NOT use external ADCs until you are more familiar with the microcontroller and ADCs in general. External ADCs are are often a LOT of work which is why so almost every microcontroller comes with an integrated ADC and why many people use the &quot;crappy&quot; integrated ADC on the microcontroller. You can't achieve proper resolution anyways at 16-bits without some serious custom PCB design; Too much noise, inaccurate voltage references, etc.