# Passive UHF RFID Tag for Multispectral Assessment
## 2 Materials and Methods
### Sensor Tag Architecture
In theory, the chip does not require any external components to read out a voltage value. However, since we wanted to sequentially measure the output of each photodiode in the same access, we included a switch based on five discrete MOS transistors combined with the microcontroller unit. The transistors are BSS138 LT1 NMOS (Semiconductor Components Industries LLC, Phoenix, AZ, USA) with a resistance value in on-region \(\mathrm{R}_{\mathrm{DS(ON)}}\) = 3.5 \(\Omega\). Each transistor is connected to the anode of each  
Figure 2: Spectral sensitivities of each photodiode according to manufacturer’s data. Vertical lines at 350 nm and 650 nm delimit the available manufacturer’s data for the RGB colour sensor. photodiode (Figure 1). The sensors have a common cathode connected to the inverting input of the operational amplifier (EXT2 pin in the RFID chip). The gates of the five transistors are connected to different microcontroller outputs. The microcontroller generates the switch control signals, sequentially activating each output at a time. Therefore, one photodiode (ultraviolet, blue, green, red, or infrared) is read at each access. Every time an output is active, the microcontroller sends an SPI command to the RFID chip to measure the corresponding photodiode current. Then, the microcontroller saves the sensor readings using SPI commands on a specific location of the RFID chip's non-volatile memory so it can later be accessed by the RFID reader. At this point, we must state that the proposed switch array to interface more than one sensor can be scaled up not only to five sensors, as presented here, but also to a higher amount of them taking into account the availability of MCU I/O ports that can be used to drive the switches. Therefore, we have designed a generalizable electronic architecture as interface between multiple sensors and a single RFID chip for the development of passive sensing tags. In order to measure each photodiode output current, it is necessary to properly select the feedback resistor value and the ADC input voltage range. In the case of the feedback resistor, the RFID chip includes an autorange function that automatically selects a proper \(\mathrm{R_{f}}\) value to provide unsaturated measurements within the ADC input voltage range. As stated above, the limits of the ADC input voltage are \(\mathrm{V_{ref}}\) and \(\mathrm{2V_{ref}}-\mathrm{V_{o1}}\). By default, these reference voltages are \(\mathrm{V_{o1}}\) = 0 V and \(\mathrm{V_{ref}}\) = 310 mV, so the ADC input voltage ranges from 310 mV to 620 mV, leading to a detection range of reverse diode current from 45.24 nA (with \(\mathrm{R_{f}}\) = 3875 k\(\Omega\)) to 2.62 uA (with \(\mathrm{R_{f}}\) = 185 k\(\Omega\)). Other detection ranges can be easily configured by choosing different values of \(\mathrm{V_{o1}}\) and \(\mathrm{V_{ref}}\). This is done through SPI commands sent from the microcontroller unit to the RFID chip. The minimum detectable current is 32.3 nA and the maximum one is 5.86 uA. Figure 3 shows the fabricated RFID tag with its main components labelled. A commercial RFID reader compatible with EPC Gen2, DK-UHF RFID HP2 (IDS Microchip AG, Wollerau, Switzerland) is used to power up and read the tag. The latter is done by sending EPC Gen 2 Read commands to access the RFID chip EEPROM memory locations where the measurements have been saved through SPI commands. The RFID reader antenna consists of a circular polarized patch antenna A0025 (Poynting antenna, Samrad, South Africa).