## 3 Measurement Results  
### Measurement Results Using the Implemented Sensor on a Printed Circuit Board (PCB)  
A photograph of the fabricated die is shown in Figure 7. The proposed readout circuit was fabricated using 0.18-\(\upmu\)m complementary metal-oxide semiconductor (CMOS) technology. The active area of the fabricated die was 9.61 mm\({}^{2}\). The measurement setup is shown in Figure 8. For measurement devices, Keysight's 35670A was used for spectrum analysis, Rohde & Schwars's RTE 1032 was used for signal acquisition, and Keysight's 33500B was used for waveform generation. Arduino's Arduino Due microcontroller was mounted on a PCB for the digital processing of analog output signals and was directly connected to the digital output of the ADC. In the voltage-sensing mode, the gain  
Figure 6: Fully differential amplifier with rail to rail input stage in the PGA. CMFB: common mode feedback, INN: negative input, INP: positive input, OUTN: negative output, OUTP: positive output. could be adjusted from 26 dB to 55 dB by fixing the PGA gain to 13.9 dB. The gain of the PGA was also adjustable from 13.9 dB to 44 dB. The output voltage in the voltage-sensing mode is shown in Figure 9a. The transfer function in the voltage-sensing mode is shown in Figure 9b. A DC gain of 37.4 dB was measured, and the cut-off frequency was 182 Hz. The measured input referred noise in the voltage-sensing mode is shown in Figure 10. The measured input referred noise at 1 Hz was 2.09 \(\upmu\)V/\(\upmu\)Hz. The RMS input referred noise was calculated to be 5.25 \(\upmu\)V\({}_{\text{rms}}\) over the bandwidth from 1 Hz to 200 Hz. Figure 8: Measurement setup for the proposed readout circuit. Figure 7: Photograph of the fabricated die, SAR: successive approximation register. Figure 9: (**a**) Output in the voltage-sensing mode. (**b**) Transfer function in the voltage-sensing mode. In the resistive-sensing mode, the gain, which is the ratio of the output voltage to the input voltage, equals the gain of the voltage-sensing mode because the resistive-sensing mode shares the configuration of the voltage-sensing mode. The linearity expressed by the ratio of the digital output code to the input resistance is shown in Figure 11a. The coefficient of determination, R\({}^{2}\), was calculated to be 0.9991. The resistance noise floor is shown in Figure 11b. The resistance noise floor was calculated by dividing the input referred noise by 180 mV/k\(\Omega\), which was the ratio of the input voltage to the input resistance. This ratio was calculated using a bridge sensor consisting of four 10 k\(\Omega\) resistors. The resistance noise floor at 1 Hz was 1.16 m\(\Omega\)/\(\surd\)Hz. The RMS resistance noise floor was calculated to be 2.9 m\(\Omega_{\mathrm{rms}}\) over the bandwidth from 1 Hz to 200 Hz. In the current-sensing mode, the gain, which is the ratio of output voltage to the input current, could be adjusted from 0.544 M\(\Omega\) to 11.5 M\(\Omega\) by fixing the PGA gain to 13.9 dB. The output in the current-sensing mode is shown in Figure 12a. The transfer function of the current-sensing mode is shown in Figure 12b. The DC gain was measured to be 1.24 M\(\Omega\) and the cut-off frequency was 244 Hz. The measured input referred current noise is shown in Figure 13. The measured input referred current noise at 1 Hz was 25.5 pA/Hz. The RMS input current noise was 742 pArms over the bandwidth from 1 Hz to 200 Hz. In the capacitive-sensing mode, the linearity expressed by the ratio of the digital output code to the input capacitance, is shown in Figure 14a. The coefficient of determination, R\({}^{2}\), was calculated to be 0.9949. The measured capacitance noise floor is shown in Figure 14b. The capacitance noise floor was measured by dividing the gain by the output noise density, which is the ratio of output  
Figure 11: (**a**) Linearity expressed as the ratio of the digital output code to the input resistance. (**b**) Resistance noise floor. Figure 10: Measured input referred noise in the voltage-sensing mode.