fat16lib could you just explain how it works, the general concept without coding, the flow of the timer and the adc interrupt ?
The ADC conversion for the first pin is started by timer 1, not by an interrupt. This allows the jitter in the first conversion to be less than one CPU cycle no matter what else is happening. The ADC conversions for the remaining pins are started in the ADC done ISR.
See the AVR datasheet to see how timer 1 can trigger an ADC conversion.