# Passive UHF RFID Tag for Multispectral Assessment
## 2 Materials and Methods  
### Sensor Tag Architecture  
Figure 1 shows a schematic diagram of the RFID tag architecture. This architecture was presented in rigid FR4 substrate in a previous conference paper [47]. The tag comprises an SL900A RFID chip (AMS AG, Unterpremstaetten, Austria) compatible with the EPC Gen 2 RFID standard [48], a microcontroller unit (MCU) PIC16 LF1703 (Microchip Technology Inc., Chandler, AZ, USA), an RGB colour sensor KPS-5130 PD7C (Kingbright Electronic Co., Issum, Germany) with three silicon photodiodes (one each for red, green, and blue), a Silicon PIN IR photodiode TEMD7100X01 (Vishay Intertechnology, Inc., Malvern, PA, USA), a UV Schottky-type photodiode GUVA-S12 SD (Roithner LaserTechnik GmbH, Vienna, Austria), and extra circuitry to interface the optical sensors. Moreover, the RFID chip has a fully integrated temperature sensor and two inputs for external sensors. We used this RFID chip in a previous work where two sensors were handled with no MCU [24]. Figure 1: Schematic diagram showing the optical sensor mode of the RFID chip and its connection with the five photodiodes and the microcontroller unit. Now, we have included a MCU to expand the possibilities of sensor reading with a novel strategy based on controlling a low power array of analog switches as it will be explained below. The tag has a passive architecture, in which the antenna harvests the energy necessary for powering up the system from the reader's EM field. The antenna was designed to resonate at 868 MHz, corresponding to the UHF European RFID band. The radio frequency interface includes an RF Surface Mount Device (SMD) inductor to match the chip input impedance [48]. The RFID chip is directly connected to the microcontroller, which was selected for its low power consumption (nanoWatt XLP technology). The microcontroller communicates with the RFID chip by means of a Serial Peripheral Interface (SPI) bus. The RGB colour sensor and the IR and UV photodiodes are connected to the Sensor Front End (SFE) of the RFID chip, which comprises different sensor conditioning stages and a 10-bit Analog to Digital Converter (ADC). The voltage input range of the integrated ADC is set with two voltage references (V\({}_{01}\) and V\({}_{\mathrm{ref}}\)) selectable in 50 mV steps from 160 mV to 610 mV. In addition, the V\({}_{01}\) voltage reference can be directly tied to ground. The V\({}_{\mathrm{ref}}\) voltage defines the lower voltage limit, while the upper voltage limit is defined by 2V\({}_{\mathrm{ref}}-V_{01}\). These voltage references allow the selection of a particular resolution and range. As shown in Figure 1, the sensor conditioning stage internally consists of an operational amplifier with negative feedback, combined with a selectable feedback resistor R\({}_{\mathrm{f}}\) (185, 400, 875, 1875, or 3875 k\(\Omega\)). The non-inverting input is fixed at 135 mV. Therefore, this configuration is a current-to-voltage conversion stage with selectable gain by choosing the value of the feedback resistor. It allows measurements with an optical sensor based on a reverse-bias diode current, such as a photodiode. The input voltage in the ADC V\({}_{\mathrm{ad}}\) is related to the reverse diode current:  
\[\mathrm{I}_{\mathrm{D}}\,=\,(\mathrm{V}_{\mathrm{ad}}-135\,\mathrm{mV})/ \mathrm{R}_{\mathrm{f}} \tag{1}\]  
Our RGB colour sensor is composed of a three-channel Si photodiode sensitive to red (peak sensitivity wavelength \(\mathrm{\lambda}_{\mathrm{P}}\) = 620 nm), green (\(\mathrm{\lambda}_{\mathrm{P}}\) = 550 nm), and blue (\(\mathrm{\lambda}_{\mathrm{P}}\) = 470 nm) spectral regions. The TEMDY100X01 infrared photodiode has a peak sensitivity at 950 nm and the GUVA-S12 SD is sensitive to ultraviolet radiation (\(\mathrm{\lambda}_{\mathrm{P}}\) = 350 nm). Each photodiode channel generates a current proportional to the incident radiation. Figure 2 depicts the spectral sensitivity of each photodiode according to the manufacturer's data [49; 50; 51].