A micro-watt power subthreshold current modulator suitable for low-power and low-noise sensor interfaces is presented. The prototype design is based on Subthreshold Source Coupled Logic (STSCL) cells and implemented in a 0.18 μm standard CMOS technology. The modulator operates with the supply voltage of 0.8 V which is significantly lower than the nominal supply of the technology node (1.8 V). It consumes 5.43 μW of power at the maximum bandwidth of 20 kHz. The obtainable current-sensing resolution ranges from ENOB=7.1 bits at a 5 kHz bandwidth to ENOB=6.5 bits at a 20 kHz bandwidth. The obtained power-efficiency of 1.5 pJ per conversion outperforms any existing current-mode analog- to-digital converter (ADC) design and is comparable to conven- tional voltage-mode Continuous-Time (CT) Σ∆ modulators. The modulator generates very low levels of switching noise thanks to the continuous-time operation and subthreshold current-mode circuits that draw a constant subthreshold current from the voltage supply. The modulator is used as an interface for sensors with current-mode output in ultra low-power conditions and can also be used for power monitoring circuits.