# Enhanced Linearity in Intracranial Pressure Monitoring System Through Sample Isolation Bridge ROIC
## 3 ROIC Implementation Details
### Cross-Connection Instrumentation Amplifier Scheme
Various methods have been explored to implement fully differential amplifiers using standard components, although each approach presents its own set of challenges [41,42], as analyzed in the following section. One method involves employing an operational amplifier to power the reference pin while configuring the input in positive mode and positioning the negative input at the midpoint between two identical resistors that link the outputs. This setup utilizes the internal amplifier's output as the positive signal and the output of the operational amplifier as the negative signal. Given that the outputs originate from different amplifiers, any discrepancies in dynamic performance between these devices can significantly impact the overall functionality of the circuit. Furthermore, the equality of the two resistors leads to a shift in the output common mode (CM) in tandem with the output signal, which can cause distortion. When designing this circuit, it is crucial to account for stability in selecting amplifiers, and it may be necessary to include a feedback capacitor in the operational amplifier, which restricts the circuit's overall bandwidth. Ultimately, the gain range of the circuit relies on the capabilities of the internal amplifier, rendering a gain of less than 1 unachievable. One alternative method involves linking two amplifiers in parallel. This setup offers an improved matching of the drive circuit and enhanced frequency response compared to earlier configurations. Nevertheless, it cannot achieve a gain lower than 2. Additionally, the circuit necessitates gain resistors that are precisely matched to accommodate a purely differential signal. Any disparity among these resistors leads to a variation in the output common-mode level, similar to the previous arrangement. Both approaches have their limitations concerning the potential gain and the need for matching components. We have developed a fully differential output that ensures accurate gain or attenuation through a single gain resistor by cross-connecting the internal amplifiers, as illustrated in Figure 5. By linking the two reference pins, users can modify the output-sharing configuration according to their requirements. A schematic of the designed IA is shown in Figure 5, and a schematic of the IA opamp containing the transconductance block is shown in Figure 6. The gain of the IA is defined as follows:  
\[\frac{V_{OUT}}{V_{IN}}=2\times\left(\frac{R_{1}}{R_{2}}-\frac{R_{1}}{R_{3}} \right)\times\frac{R_{4}}{R} \tag{1}\]  
In contrast to NMOS, PMOS technology exhibits reduced flicker noise due to its significantly lower likelihood of capturing and releasing carriers. Consequently, we have selected PMOS as the input device for the amplifier, necessitating an increase in the dimensions of the buffer's input components to minimize noise levels. To mitigate noise, the PMOS input transistor in Gm1 is designed with a substantial size of 256 \(\upmu\)m/220 nm [43]. The input-referred noise power spectral density (PSD) of the readout, not accounting for the noise from the Wheatstone bridge, can be expressed by Equation (3). \[V_{inoise}=\sqrt{8 kT\bigg{(}\frac{\gamma}{4g_{m1}}+\frac{R^{2}}{R_{4}}\bigg{)} }\approx\sqrt{\frac{2 kT\gamma}{g_{m1}}} \tag{2}\]  
In 0.18-\(\upmu\)m CMOS technology, transistors operating at 1.8 V have a \(\gamma\) value of approximately 0.5. Given that \(\gamma\) divided by 4 times \(g_{m1}\) is much greater than the ratio of \(R_{2}\) to \(R_{4}\), the noise impact from \(R_{4}\) is minimal in this readout circuit. As illustrated in Figure 6, six amplifiers of cross connection IA are implemented using a basic telescopic cascade topology to minimize overall power consumption. It operates with a current of 5 \(\upmu\)A and delivers an open loop gain exceeding 60 dB, featuring an output swing of \(\pm\)600 mV while powered by a 1.8 V supply. This proposed cross connection IA scheme offers a high degree of design flexibility, enabling customizable configuration of gain at each amplifier stage to meet the specific needs of the application.