Here are two ways to read the voltage to determine which button was pressed.
On the top, current takes the shortest path so only one button will be registered by design. It is also cheaper to use many resistors of the same value.
On the bottom,it is possible to detect combination presses:
Button Press - Voltage / 10-Bit ADC (0-1023) / 12-Bit ADC (0-4095)
----------
Button 25K - 4V / 818 / 3276
Button 50K - 3.33V / 681 / 2727
Button 100K - 2.5V / 511 / 2047
Button 200K - 1.67V / 341 / 1367
Button 400K - 1V / 204 / 819
Button 25/50K - 4.29V / 877 / 3513
Button 25/50/100K - 4.38V / 896 / 3587
Button 25/50/100/200K - 4.41V / 902 / 3611
Button 25/50/100/200/400K - 4.43V / 906 / 3628
Button 50/100K - 3.75V / 767 / 3071
Button 50/100/200K - 3.89V / 795 / 3185
Button 50/100/200/400K - 3.95V / 808 / 3235
Button 100/200K - 3V / 613 / 2457
Button 100/200/400K - 3.18V / 650 / 2604
Button 200/400K - 2.14V / 437 / 1752
Arranged from low to high:
Button 400K - 1V / 204 / 819
Button 200K - 1.67V / 341 / 1367
Button 200/400K - 2.14V / 437 / 1752
Button 100K - 2.5V / 511 / 2047
Button 100/200K - 3V / 613 / 2457
Button 100/200/400K - 3.18V / 650 / 2604
Button 50K - 3.33V / 681 / 2727
Button 50/100K - 3.75V / 767 / 3071
Button 50/100/200K - 3.89V / 795 / 3185
Button 50/100/200/400K - 3.95V / 808 / 3235
Button 25K - 4V / 818 / 3276
Button 25/50K - 4.29V / 877 / 3513
Button 25/50/100K - 4.38V / 896 / 3587
Button 25/50/100/200K - 4.41V / 902 / 3611
Button 25/50/100/200/400K - 4.43V / 906 / 3628
With the exception of the last three of the 10-bits ADC result, the other values are 10 values or more apart.
