# Enhanced Linearity in Intracranial Pressure Monitoring System Through Sample Isolation Bridge ROIC
## 3 ROIC Implementation Details
### Cross-Connection Instrumentation Amplifier Scheme  
The current feedback instrumentation amplifiers (CFIAs), three-opamp configuration, and capacitive coupling topology are commonly employed in the implementation of instrumentation amplifiers (IAs) [33; 34; 35]. Illustrated in Figure 4 is the structure of an IA. To effectively transmit the input voltage, the IA must possess a high input impedance. While the three-opamp topology offers excellent linearity and high input impedance, it suffers from low power efficiency due to the requirement for three low-noise amplifiers. The CFIA, on the other hand, achieves a high input impedance; however, its gain accuracy is restricted by the mismatch in transconductances between the input and feedback elements, and its power efficiency is affected by the two transconductors involved [36; 37; 38]. In capacitively coupled topology, low-frequency signals cannot be transmitted due to the capacitor's DC-isolating effect, resulting in a limited low-frequency response of CCIAs. Signals may be partially or completely lost in DC or low-frequency signal measurements. The input impedance of a capacitively coupled system varies with frequency, which tends to  
Figure 4: Instrumentation Amplifier (**a**) three-opamp topology, (**b**) CFIA, and (**c**) capacitively coupled topology. introduce loop instability. The frequency-dependent impedance of the capacitive coupling network further limits the available signal bandwidth of CCIA, as demonstrated by the 200 kHz range implemented in this work, which is more advantageous than the signal bandwidth of CCIA architecture. Capacitively coupled circuits are susceptible to noise at both the high- and low-frequency ends. The capacitive network at the input may generate noise and distortion. The amplifier has a limited ability to handle the exact swing of the input signal. Capacitors take time to charge, and the charging time of the capacitors may cause signal delay. The charging effect of the capacitors may significantly affect the output signal when the input signal changes rapidly or when the signal changes frequently. The transient response of the signal may become slower and is not suitable for highly dynamic signal processing. Additional compensation circuitry may be required, increasing the complexity of the design. The capacitance value of the coupling capacitor and the performance of the amplifier can vary with temperature [39,40]. This is a disadvantage in some applications where high stability is required. DC drift and signal instability may lead to measurement errors. Additional measures are required to compensate for the effects caused by temperature and drift. The input network of a capacitively coupled amplifier may lead to non-linear signal distortion, especially at large input signal levels. It may also result in degradation of the linearity and accuracy of the output signal, so it is not suitable for applications requiring precise measurements and linear output. In recent years, research has actively focused on IA structures that exhibit strong power efficiency to address the aforementioned issues. The traditional three-opamp IA has several advantages, such as common mode signal rejection, high input impedance, and precise gain. However, it is inadequate when a fully differential output signal is necessary. As advances in accuracy propel the performance of components within fully differential signal chains, one of the primary advantages is improved noise rejection, which can be influenced by signal routing. When this noise is picked up at the output, it is likely to cause common errors, which will then be further diminished within the signal chain. Moreover, it is possible to convert differential signals into single-ended signals while using signal ranges that share the same power supply. Consequently, the signal-to-noise ratio (SNR) is improved with fully differential signaling.