Using input impedance to measure current, see the detailed blog post here from Sigzig:
Input impedance on measurement equipment is usually kept as high as it can in an effort to prevent loading down the signals the equipment is measuring. For example most general purpose digital multimeters aim for an input impedance of at least 1M ohm. The Zig-4 has a lower input impedance of 330k ohms but this is still high enough for many/most situations. That being said it is still important to understand input impedance and its affect on your measurements.
In this post I thought it would be fun to actually take advantage of the 330k input impedance of the Zig-4 as a sort of low current sensing.
The goal: To see if we can use the Zig-4 to detect a change in current in the very low nA range.
Read more Here