ADC PCF8591 - change range for measuring
In theory, you can set a VREF of 20mV. To get this, you can use a voltage divider:







simulate this circuit &ndash; Schematic created using CircuitLab

However, from the datasheet you gave us, page 16, you can find that the minimum VREF is VSS+1.6. Since 2.5 &lt; VDD &lt; 6.0, and the maximum VREF is VDD, this gives you a range for VREF of 1.6 to 6.0 volts. With this chip, you're not going to measure 0 to 20mV with the significance you want.

You can add other elements, like an op-amp, to amplify the input signal. You would get a higher input voltage, so that the actual 20mV gets into the range you can use.