They all have an ADC, it's part of the chip. It's just not broken out on all of them. It's also not that great, 10bit resolution is so-so and it only works for

They all have an ADC, it's part of the chip. It's just not broken out on all of them. It's also not that great, 10bit resolution is so-so and it only works for signals between 0 and 1V. This means you can't easily use for instance the awesome TMP36 temperature sensors with it. I mean you can, but you need something like a voltage divider (which adds error) and you need to recalculate the voltage to temperature conversion.