This paper presents a low-power voltage monitor for small-form-factor, low-sample-rate biomedical Internet of Things (IoT) applications aiming to convert the analog output voltage 0-0.2 V of a biosensor into digital code. This design consists of 3 oscillators and a periphery logic circuit. A reference oscillator that can provide stable output frequency even supply voltage floats range of 0.35-0.45 V. Two sensitive oscillators which the output frequency will change rapidly due to the output voltage from biosensors. A resistive input stage is used in each sensitive oscillator to improve the linearity of the output. The SPICE simulation performed in 65-nm CMOS technology shows that the nonlinearity error is within -1.56 mV/+1.19 mV when the input voltage is 0-0.2 V, much lower than previous works. The power consumption of this design is 49.6 nW when the voltage supply is 0.4 V.