Hello;
There are two main ways of doing that. You can have your external signal routed to the StartScan pin, and that will control the start of a new scan. When a new scan is started, the internal counter that generates the Convert* signal is triggered. That method will take at least 10n seconds delay after you have the StartScan signal coming in, so the ADC will do one convertion.
The other way is to have the external signal tyed straigh to the Convert* pin. With that method, as soon as the external signal comes is, the ADC do one convertion, without any delay.
Hope this helps.
Filipe