# Fully Printed Flexible Single-Chip RFID Tag with Light Detection Capabilities
## 2 Materials and Methods  
### Tag Design  
Figure 1 illustrates the layout of the fabricated RFID tag, including the footprints of the different elements included. This passive tag working at the UHF band includes the RFID chip compatible with Electronic Product Code (EPC) Generation 2 RFID standard [19]. This RFID chip was selected because it integrates a sensor front-end (SFE) with different sensor conditioning stages and a 10-bit Analog to Digital Converter (ADC). In the presented work, no extra circuitry is required to interface the sensor. Furthermore, this RFID chip has an in-built temperature sensor. Advanced Design System 2013 (Keysight Technologies Inc., Santa Rosa, CA, USA) was used to design the printed antenna and the matching network. The antenna impedance must be a complex conjugate of the chip impedance to maximize the power transfer between the antenna and the chip [20]. The RFID chip input impedance was taken from our previous work [11] and its value is \((31-j286)\Omega\). A dipole antenna resonating at the European UHF RFID band defines the radiofrequency interface. First, we designed the antenna resonating at 868 MHz with its real part matching the real part of the RFID chip impedance. Finally, as a matching network, one RF Surface Mount Device (SMD) inductor in series on one of the dipole arms was needed to provide the necessary large inductive part [20]. This SMD inductor could be avoided by changing the antenna layout (i.e., bow-tie antenna [11]), but we decided to include it in order to reduced area and materials. The tag has been designed to occupy a total area of 5 \(\times\) 5 cm\({}^{2}\). An external capacitor of 2.2 \(\upmu\)F (C\({}_{\text{Rectf}}\) in Figure 1) has been placed between the RF rectifier output (V\({}_{\text{Rectf}}\)) and the RFID chip ground terminal. The purpose of this component is to smooth the signal coming from the RF rectifier and, in this way, extend the maximum read range, as indicated by the manufacturer [19], but it is not mandatory. As already mentioned, a temperature sensor is integrated into the chip. Two internal voltage references set the lower and upper limits of this converter, and, therefore, a concrete resolution and range are selected. The minimum resolution of this sensor is 0.18 \({}^{\circ}\)C in a range from \(-\)89.3 \({}^{\circ}\)C to 84.6 \({}^{\circ}\)C, and 0.23 \({}^{\circ}\)C in the widest range of temperature (from \(-\)89.3 \({}^{\circ}\)C to 147.9 \({}^{\circ}\)C). A fully printed organic photodiode (OPD) is integrated into this tag. The photodetector is directly connected to the SFE of the RFID chip. To read out a current value, it is not required to have any external component. This RFID chip has been previously used with a commercial photodetector [11]. Figure 2 illustrates the sensor configuration mode selected for the read-out of the OPD, including the conditioning stage in the RFID chip. Figure 1: Layout of the radiofrequency identification (RFID) tag. The folded dipole encases a square of 5 \(\times\) 5 cm\({}^{2}\), and the width of the arms is 5 mm. The active area of the Organic Photodiode is given by the overlapping of a bottom Ag line and a top conductive polymer line, both with a width of 3 mm, with a total surface of 9 mm\({}^{2}\). Figure 2: Schematics of the Sensor Front-End (SFE) configuration. This configuration is suitable for measuring the photocurrent generated by photodiodes. An operational amplifier with negative feedback is combined with a selectable feedback resistor (\(R_{\text{fb}}\)) (185, 400, 875, 1875 or 3875 k\(\Omega\)) (see Figure 2). The non-inverting input is fixed to 135 mV, so the input voltage in the 10 bits ADC is related to \(I_{Sens}\) value by:  
\[V_{ADC}=135 mV+I_{Sens}\cdot R_{fb}. \tag{1}\]  
Two internal voltage references, Vref1 and Vref2 individually selectable in steps of 50 mV between 160 and 610 mV, set the lower and upper limits of the ADC. These limits are defined as \(2\times\text{V}_{\text{refl}}-\text{V}_{\text{refl}}\) and Vref1.