How to decide which key was pressed on a calculator?
This will not answer your question but will give you some background information why your project will be difficult.

Figure 1. Keyboard scanning matrix. Image source: The Cuckoo's Nest.

The animation is showing that (for this example) the rows are switched on in sequence 0, 1, 2, 0, 1, 2, ....
If a key is pressed then that COLumn will turn high.
To know which key was pressed you have to know:

Which ROW output is on.
Which COLumn input is on.



So for you to do this you need to monitor each row and column and you have to be faster than the calculator so analog might not be a good idea. The problem is more difficult because the calculator is probably not the same voltage as your micro-controller.
This might not be worth the trouble!