user10700: I am using a sensor which delivers a 4 to 20 mA output.
Converting this with a 250 Ohm resistor to ground I convert this to a 1 - 5 V output.
My question how can I change the 0V reference, so I can use the full spectrum
I found a way to change the 5V reference (analogReference()).
Is it possible to change the 0V reference of a input to 1V?
Any other solution to my problem is welcome as well! (As long as it doesn't involve I2C or anything which lowers my looptime drastically)

user6834: The ADC is ground-referenced, and there's no way to change that. The question is if that's necessary. The ADC has a 10-bit resolution, that's good for 1024 different levels between 0 and 5 V. If you convert the 4-20mA with a 250 Ω resistor to a 1-5 V range you still can discriminate between more than 800 different values. 
If that's not enough you can use a differential amplifier to subtract 1 V:

By choosing R3 = 1.25 R1 you convert the 0-4 V range to a 0-5 V range.

10700: <p>I am using a sensor which delivers a 4 to 20 mA output.
Converting this with a 250 Ohm resistor to ground I convert this to a 1 - 5 V output.</p>

<p>My question how can I change the 0V reference, so I can use the full spectrum
I found a way to change the 5V reference (analogReference()).
Is it possible to change the 0V reference of a input to 1V?</p>

<p>Any other solution to my problem is welcome as well! (As long as it doesn't involve I2C or anything which lowers my looptime drastically)</p>
 6834: <p>The ADC is ground-referenced, and there's no way to change that. The question is if that's necessary. The ADC has a 10-bit resolution, that's good for 1024 different levels between 0 and 5 V. If you convert the 4-20mA with a 250 &Omega; resistor to a 1-5 V range you still can discriminate between more than 800 different values. </p>

<p>If that's not enough you can use a differential amplifier to subtract 1 V:</p>

<p><img src="https://i.stack.imgur.com/wV1zG.gif" alt="enter image description here"></p>

<p>By choosing R3 = 1.25 R1 you convert the 0-4 V range to a 0-5 V range.</p>
