The MLT1122 functions both as an electrometer and a pre-amplifier. The electrometer, by way of its extremely high impedance, effectively converts the low current produced by the probe to a voltage only signal that can be read by a PowerLab.
Note that the MLT1122 has 2 CR2032 batteries that convert the current to a voltage and provide the polarization voltage to power the electrodes to drive the reaction. As a result, the MLT1120 O2 probe needs to be isolated from other measurements (e.g., pH and redox) to avoid ground loops.
One of the main reasons for using micro probes such as the MLT1122 is that the consume very little oxygen and can be used to measure oxygen in stagnant or other non-stirred solutions.