I´m not really sure if the clock is very solid...
It is as solid and jitter-free as the Arduino clock.
if I take your advice. What do you think?
I was not necessarily offering advice. (I'll save that for the end of this post.) You asked for a way to get a 2 MHz clock. I showed how you can generate a 2 MHZ clock using an Arduino timer.
The other possibility would be, that I create the 2MHz frequency with FlipFlops (ICs). A friend told me, that that would be better.
Define "better." Has your friend ever used this ADC? Where did he/she get the clock? Has your friend ever created a clock using an Arduino timer for any purpose? Any purpose at all?
I would be interested in knowing in what way a "clock created with flip-flops" is "better." I mean, I can see where a clock generated by a properly specified oscillator module rather than one derived from the Arduino clock might have better frequency stability and accuracy and better jitter characteristics, but would this necessarily make it "better" for your application than a clock generated with an Arduino timer???
Where can I get an 16MHz frequency?
Ask your friend to show you what he/she has in mind and to cobble one up for you
. Has the Arduino Uno a clock output, or how can I create 16MHz at a pin?
If you are going to use the Arduino clock as a source for your derived 2 MHz output, what's the point? If you don't need Timer 2 for other things, it costs nothing and requires no external circuitry to create a 2 MHz clock on pin 11.
If you need pin 11 for something else, you can consult the ATmega data sheet to find what other pin can be used with Timer 2.
If you need Timer 2 for something else, you can, maybe, use Timer 1 with either of its Output Compare pins.
If you need both Timer 1 and Timer 2 for other things and/or you need the associated Output Compare pins for something else, or if the Arduino-generated clock is not, for any reason, suitable for you application, then my advice is to buy a 2 MHz oscillator to connect to your ADC. See Footnote.
Bottom line: Good advice from a good friend may be more valuable than advice from a stranger on the Internet. My advice is free and is freely given. It is worth exactly as much as you want it to be worth.
davekw7x: "Over and out."