# A Sub-50 nW Implantable Body Temperature Sensor and Its Application to a Rat
## 3 Characterization of Fabricated Temperature Measurement Channel (CH_T)  
Figure 4a shows a photograph of the chip fabricated using a TSMC 180 nm process. The size of the CH_T region is 610 \(\upmu\)m \(\times\) 185 \(\upmu\)m, and the regions of the main blocks BRG, SMP, VCO, and TDC are marked. Figure 4b shows the evaluation board for testing the function and performance of the fabricated chip (NA10). The evaluation board consists of a daughter board that mounts the fabricated chip, a mother board that supplies power to the daughter board and has monitoring terminals, and an MCU board that has an MCU for digital signal interface with the PC. The daughterboard is always newly manufactured for the chip, because the chip is mounted in the form of chip on board (COB), and the motherboard and MCU board can be commonly reused regardless of the ASIC. The daughterboard and motherboard are stacked and combined using a 2.54 mm pitch header pin array, and the motherboard and MCU board are connected with connectors. Figure 4c shows the measurement setup to verify the temperature sensing performance of CH_T of NA10 chip. The output value of CH_T was measured while changing the temperature of T-chamber (PKK-50, Kambic(tm), Semic, Slovenian) from 34 \({}^{\circ}\)C to 43 \({}^{\circ}\)C in 1 \({}^{\circ}\)C intervals. For CH_T evaluation, a thermistor with a B25/50 coefficient of 4250 K and a resistance of 57.6 k\(\Omega\) at 37 \({}^{\circ}\)C was mounted on the FPCB (FPCB_ELEC10). To create an environment similar to in vivo measurement, the FPCB thermistor and reference thermometer were immersed in a phosphate-buffered saline (PBS) solution. The resistance value of the thermistor mounted on the FPCB varies depending on the temperature of the T-chamber, and the coefficient value output from CH_T of the chip NA10 also varies accordingly. This is stored in the notebook via the daughterboard, motherboard, and MCU board via SPI communication. The actual temperature of the T-chamber was measured using a reference thermometer system calibrated by a nationally recognized calibration institute. The reference thermometer system consists of a transducer (PT-100, RTD-2-1 PT100 KN2528-36-T, DWYEROMEGA(tm), Michigan City, USA) whose resistance varies depending on the temperature and a reference thermometer reader (UT-ONE B03B, Batemika(tm), Vipava, Slovenian) that reads the change in transducer resistance. The values of the reference thermometer reader are synchronized with the data of the evaluation board and measured on the notebook. To check whether CH_T is working properly and to directly measure the VCO output values (\(CLK_{\text{P}}\), \(CLK_{\text{N}}\)), the corresponding signal was output and observed with an oscilloscope (MSOX4104A, KEYSIGHT, Santa Rosa, CA, USA). Figure 5 shows the temperature sensing characteristics of the measured CH_T. The temperature sensor calibration was performed in a temperature chamber (PKK-50, Kambic, Semic, Slovenian) at 1 \({}^{\circ}\)C intervals between 34 \({}^{\circ}\)C and 43 \({}^{\circ}\)C. The self-made temperature sensor and the standard platinum sensor (UT-ONE B03B, Batemika, Vipava, Slovenian) calibrated by a nationally accredited institution were placed together in the temperature chamber. After the temperature of the chamber stabilized, the values of the temperature sensor and the platinum sensor were read, and the platinum sensor measurement value was treated as the true temperature. In Figure 5a, the \(x\)-axis is the value measured by the reference thermometer when the T-chamber was set at 1 \({}^{\circ}\)C intervals in the range of 34 \({}^{\circ}\)C to 43 \({}^{\circ}\)C, and the \(y\)-axis is the TDC output value of CH_T. Since 37 \({}^{\circ}\)C is the normal body temperature of a rat, the trimming resistor setting of the BRG in CH_T was adjusted so  
that the TDC output was near 0 at 37 \({}^{\circ}\)C. Figure 5b shows the calibration results using the measured reference temperature and the measured TDC value.