Don't get the right Voltage of my DAC pin

[skipzens]

Hello,

I have a problem with my Project...
My Project is about to use a Keypad with my Arduino Due to select any Voltage between 2V - 4V, to get this Voltage I use my DAC0 PIN and connect it with an OP at the end.
After I finished my Code I realized that no matter which Voltage I enter it is everytime 0,01V less than it should be..
on the Internet I found a formula to calculate the right decimal number for the DAC0:

[color=#404040][b][i]V[/i]out = [i]V[/i]refX(input/2[i]N[/i]-1)+ [i]V[/i]offset[/b][/color]

I changed it to get what I need

 int  input2 = (((voltage/2.0)-0.56)/2.2)*4095;

If you are wondering why in the first place stands /2.0 it is for the OP at the end to get my full voltage.
I tried to expanded the formula but nothing helped...

I hope I have explained it good enough that everyone can understand it.
I'm looking forward to get some solutions to fix it
Thank you!

[ard_newbie]

The maximum output current for both DACs is 3mA -----> Be sure to have at least 1.5 KOhms resistors in serie to avoid burning your DACs !

SAM3X-Arduino Pin Mapping | Arduino Documentation

Moreover, DAC output is within 1/6 * 3.3V and 5/6 * 3.3V:

How to modify analog output range of Arduino Due

[skipzens]

ard_newbie:
The maximum output current for both DACs is 3mA -----> Be sure to have at least 1.5 KOhms resistors in serie to avoid burning your DACs !

In no toturial someone said I need a resistor for my DAC0... :o

This is how I connected the DAC0 with the OP. So do I need a Resistor before I connect it to 1IN+?

Moreover, DAC output is within 1/6  * 3.3V  and 5/6  * 3.3V:

I know this and that is the Problem.. I checked on my Arduino which numbers it exactly is and it was possible to what I need. The thing is that the Output is smaller than I expected. I wanted to have for example 0,556V but instead I get 0,546V.