A 50 Hz SC notch filter for IoT applications

This paper presents a SC 50 Hz notch filter for an analog to digital acquisition channel in an IoT water management sensor node. The notch filter is used to attenuate the unwanted 50 Hz interference, allowing to reduce the required effective resolution of the ADC, which is a ΣΔ modulator with a sampling frequency of 2 MHz. Due to the large ratio between the sampling frequency and the pole frequency, which increases the capacitor values dispersion, charge division branches are used to allow the delivery of a small charge using a larger capacitor, reducing the capacitor spread. The factor by which the value of the capacitors can be increased is controlled by the number of set/reset cycles that are used in the charge division branches. The maximum number of cycles is dependent on the time constants of each node and the gain value of the amplifiers. The notch filter has a total power dissipation below 300 μW and electrical simulations show an attenuation of approximately 30 dB in the unwanted 50 Hz signal.