A circuit or an i.c that will give a digital output high or low based on whether the input signal which is analog and is around 3.3V
Use two comparators like so:





I am not sure how your 8kHz signal comes into it, you haven't made it clear what you're talking about! Also, you have not said what voltage your second supply is at, or how well it is regulated etc. Lastly, you also have not said what voltages you want the output digital high/low to be, or how much current it must be able to supply. But the above should be a good starting point for you. If it is not sufficient, let me know why and we can work on it.