# A Survey of NFC Sensors Based on Energy Harvesting for IoT Applications
## 2 NFC Energy Harvesting
### Tag ICs with Energy Harvesting  
Table 2 lists several NFC IC representatives with energy-harvesting capabilities. Some of these support ISO14443-3 or ISO15693. The table includes the maximum sink current (usable for external electronic devices such a microcontroller or sensors) and the nominal voltage (for low current consumption). The level of energy-harvesting voltage at the output is generated by the rectification of an RF signal in a non-regulated DC voltage that is only limited by the RF input clamping circuit. The maximum sink current is a function of the magnetic field present at the input. This value is obtained for the highest magnetic field; however, in most ICs, the voltage at this point decreases. Typically, currents around 5 mA for output voltages between 2 and 3 V and magnetic fields of the order of 3.5-5 A/m are obtained (e.g., 6 mA current and 1.7 V at 3.5 A/m is obtained for the M24 LR-E-R or 4 mA and 3 V at 5 A/m for the ST25 DV). Silicon Craft recently reported an IC with up to 10 mA (for the maximum field of 7.5 A/m) integrating an analog-to-digital converter (ADC) oriented to chemical sensors. Each series has a different memory size from 4 to 64 kbit and can be connected to other devices or microcontrollers using the I\({}^{2}\)C bus or serial-to-parallel interface (SPI). Although most of them are designed to be connected to a microcontroller, the MLX90129 from Melexis, the SL13 from AMS, and the SIC43x from Silicon Craft integrate an analog/digital (A/D) interface for autonomous sensor acquisition. Another special case is model RF430 FRL152H from TI, which integrates a low-power microcontroller MSP430 and a 14-bit digital signal A/D interface. For a well-designed batteryless tag, the main restriction in the read range is the power-up condition (Equation (1)) compared with the load modulation sideband amplitude (Equation (4)). Equation (1) can be expressed in terms of the magnetic field. For correct RF to DC conversion, the average magnetic field (\(H_{av}\)) (Equation (9)) received by the NFC IC must be above a threshold H-field (\(H_{min}\)). If the magnetic field is above that threshold, the harvesting voltage output can be below the desired value for the required current load. \(H_{av}\) depends on both the reader and tag antennas and the coupling, and therefore, on the distance between the reader and the tag. \(H_{av}\) is measured by the  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  
magnetic antenna factor, \(AF\) (Equation (10)). A procedure for calibrating the antenna factor of the tag antenna is described in Reference [44]. \[H_{av}(A_{RMS}/m)=V_{RMS}\cdot AF, \tag{9}\] \[AF=\frac{Z_{0}+Z_{in}}{j\omega\mu_{0}Z_{0}A\cdot N}, \tag{10}\]  
where \(A\) is the loop area, \(N\) is the number of loops, \(Z_{0}\), is the reference impedance (50 \(\Omega\)), and \(Z_{in}\) is the input impedance of the antenna measured with the VNA. The root-mean-square voltage (\(V_{RMS}\)) is obtained from the power \(P\) measured with a spectrum analyzer (\(V_{RMS}=(Z_{0}\cdot P)^{1/2}\)). The minimum \(H\)-field as a function of tag resonance frequency can be described using Equation (11) [43]. \[H_{min}\approx\frac{\sqrt{\left[1-\left(\frac{f}{f_{r}}\right)^{2}\right]^{2}+ \frac{1}{Q_{T}^{2}}}}{2\pi f\mu_{0}A\cdot N}\cdot U_{min}, \tag{11}\]  
where \(f_{r}\) is the resonance frequency of the tag, \(Q_{T}\) is the total quality factor of the tag given by Equation (6), and \(U_{min}\) is the minimum voltage required for the tag operation, which depends on the chip IC design and technology used. Equation (11) shows the importance of the tag resonance frequency being tuned to the operation frequency (13.56 MHz) and the highest tag quality factor for achieving a larger read range. In energy-harvesting tags, the maximum sink current for the sensor depends on the magnetic field. In order to increase this value, the tag is located at a lower distance compared to a conventional NFC tag, because an extra input power for the external devices is required.