## 6 Designed Architecture of the Smart Sensory Glove Based on AlN-Based Sensors
### Development and Testing of the Conditioning Section for the AlN-Based Flexible Sensors  
Figure 23 shows the block diagram of the analog front-end developed for the AlN-based piezoelectric sensors. This architecture comprises the following main sections:  
* Preamplifier, which transforms the input signal provided at the sensor's high-impedance output into a low-impedance signal source. * Amplifying-filtering block, which filters and amplifies the voltage signal provided in output from the preamplification. * Analog to digital Converter, which digitalizes the signal provided by the transducer. * Power supply. A charge amplifier represents the preamplification stage, a circuital configuration wherein output voltage is proportional to the total injected electrical charge or the input current's integral [130; 131; 132; 133]. Figure 24 depicts the circuital configuration of the charge amplifier implemented in the designed conditioning section based on the AD8542 ARMZ opamp. The sensor has been represented with the equivalent Thevenin circuit, i.e., as a voltage generator Vq in series with Rp and Cp in parallel. Cc represents the cable's capacitance, Ri  
Figure 24: Schematic of the designed charge amplifier. The piezoelectric transducer is represented with its Thevenin equivalent circuit. Figure 23: Block diagram of the front-end dedicated to the AlN-based piezoelectric sensors [129]. is used to prevent electrostatic discharges (ESD), while Rf and Cf are the feedback resistance and capacitance, respectively. A first voltage buffer stage was equipped with a Vcc voltage divider that provided a Vcc2 on the non-inverting terminal, using two resistors R1 and R2, with the same value, enabling the stage to be supplied with a single power supply. Indeed, the buffer output was connected to the non-inverting terminal of the charge amplifier and the sensor's ground. In this way, the DC contribution (\(\frac{\mathrm{Vcc}}{2}\)) on the output is not amplified. Indeed, replacing the capacitors with an open circuit and applying the superposition principle on the two amplifier's terminals results in a DC output contribution of  
\[\mathrm{Vo}= \frac{\mathrm{Vcc}}{2}\left(-\frac{\mathrm{Rf}}{\mathrm{Ri}+ \mathrm{Rp}}\right)+\frac{\mathrm{Vcc}}{2}\left(1+\frac{\mathrm{Rf}}{\mathrm{ Ri}+\mathrm{Rp}}\right)\Rightarrow\mathrm{Vo}= \frac{\mathrm{Vcc}}{2} \tag{6}\]  
A common-mode voltage equal to \(\frac{Vcc}{2}\) is present on the charge amplifier's output by disconnecting the sensor from the circuit. The transfer function at the output of the charge amplifier is given by  
\[\frac{\mathrm{Vo}}{\mathrm{Vq}}=-\frac{\mathrm{Rf}}{\mathrm{Ri}+\mathrm{Rp}} \;\frac{\left(1+\mathrm{sRpCp}\right)}{\left(1+\mathrm{sRfCf}\right)\left(1+ \mathrm{sRfCp}+\mathrm{Cc}\right)\right)} \tag{7}\]  
The function has a zero at the frequency \(\mathrm{f}_{\mathrm{Z}}=\frac{1}{2\pi\mathrm{RpCp}}\), determined by the sensor impedance \(\mathrm{Rp}\) and \(\mathrm{Cp}\), and two poles: the first, which depends on Rf and Cf on the feedback loop, determines the lower cut-off frequency \(\mathrm{f}_{\mathrm{L}}=\frac{1}{2\pi\mathrm{RfCf}}\) ; the second, depending on \(\mathrm{Ri}||\mathrm{Rp}\) and \(\mathrm{Cp}||\mathrm{Cc}\), determines the upper cut-off frequency \(\mathrm{f}_{\mathrm{H}}=\frac{1}{2\pi\frac{\mathrm{RfB}}{\mathrm{Ri}+\mathrm{Rp }}\left(\mathrm{Cp}+\mathrm{Cc}\right)}\). For \(\mathrm{f}=0\), the output signal has a static gain: \(\mathrm{G}=\frac{\mathrm{Rf}}{\mathrm{Ri}+\mathrm{Rp}}\). The mid-band gain (the frequency range between \(\mathrm{f}_{\mathrm{L}}\) and \(\mathrm{f}_{\mathrm{H}}\)) is given by \(\mathrm{G}=\frac{\mathrm{Rp}\cdot\mathrm{Cp}}{\left(\mathrm{Ri}+\mathrm{Rp} \right)\cdot\mathrm{Cf}}\). Beyond the high cut-off frequency \(\mathrm{f}_{\mathrm{H}}\), the gain amplitude drops by \(-20\) dB/deca. The components' values are shown in Figure 24, where sensor resistance and capacitance were assumed equal to \(\mathrm{Rp}=1\) M\(\Omega\) and \(\mathrm{Cp}=1\) nF, respectively.