## 3 Measurement Results of Proposed Wheatstone Bridge Readout IC  
The proposed Wheatstone bridge readout IC is manufactured using a 0.18 \(\upmu\)m one polysilicon and six metal layers (1P6M) complementary metal-oxide-semiconductor (CMOS) process. A chip microphotograph of the proposed Wheatstone bridge readout IC is shown in Figure 8. The fabricated chopper-stabilized multipath operational amplifier has an area of 1.18 mm\({}^{2}\), while the area of the Wheatstone bridge readout IC with three-operational amplifier IA, LPF, buffer, and 12-bit SAR ADC is 8.6 mm\({}^{2}\). Figure 6: Schematic of the HFP. Figure 7: Schematic of the current buffer. Figure 9 shows the transfer functions of the three-operational amplifier IA using various 5-bit programmable resistor settings; a gain adjustment of approximately 38-70 dB is possible. The gain of the three-operational amplifier IA is adjusted in inverse proportion to R\({}_{1}\) and R\({}_{3}\) according to Equation (3). The resistances of R\({}_{1}\) and R\({}_{3}\) are at their maximum (835 k\(\Omega\)) when the 5-bit register bits are all set to high, and are at their minimum (26 k\(\Omega\)) when all register bits are set to low. As a result, the three-operational amplifier IA has a gain of 38 dB when the registers controlling R\({}_{1}\) and R\({}_{3}\) are high, and 70 dB when these are set to L, L, L, H, H. For each register setting, we can compare the gain calculated by Equation (3) with the measured gain; these results are shown in Table 2. As a result of the comparison, there are some errors of gain. These errors are expected to be caused by the layout stage and mismatch. These 5-bit programmable resistors must be properly adjusted for the range of the input voltages. The input range for this Wheatstone bridge readout IC is \(\pm\)20 mV. Figure 8: Microphotograph of the proposed Wheatstone bridge readout IC. Figure 9: Measured transfer functions of the Wheatstone bridge readout IC. Figure 10 shows the output voltage and digital code output of the Wheatstone bridge readout IC based on input resistance. The input resistance range is 53.5-58.5 k\(\Omega\), and the coefficient for the determination of the output voltage trend line is 0.9995 according to Figure 10a. Finally, the coefficient for the determination of the output digital code trend line is 0.9993 according to Figure 10b. In addition, the R\({}_{\rm o}\) resistance of the Wheatstone bridge is 56 k\(\Omega\), and the input resistance range is a \(\pm\)20 mV based on input voltage range. Since the minimum gain of the proposed Wheatstone bridge readout IC is 37.53 dB (75.25 times) and the supply voltage is 1.8 V, the maximum input voltage range can be calculated as approximately 24 mV, which is the value divided by 1.8 V by 75.25. However, in this research, the maximum input voltage range was set to 20 mV because the proposed Wheatstone bridge readout IC may not operate normally near the minimum and maximum supply voltages. The input resistance range varies with the R\({}_{\rm o}\) resistance of the Wheatstone bridge, and the voltage difference at the differential output of the Wheatstone bridge cannot exceed the input voltage range. This input voltage range changes with the gain adjustment of the Wheatstone bridge readout IC, and the resolution of the output signal is maintained. Since the input voltage range is adjustable, it can be applied to sensors with more various specifications. Measurement results show that the differential output voltage range of the Wheatstone bridge readout IC is \(-\)1.36 V to 1.63 V at a 1.8 V supply voltage. Figure 11 shows the conversion error of the measured output voltage. The conversion error is a comparison of the ideal output voltage value calculated as a primary function and the measured output voltage value. If the conversion error is large, the linearity of the Wheatstone bridge readout IC is reduced.