# Enhanced Linearity in Intracranial Pressure Monitoring System Through Sample Isolation Bridge ROIC
## 2 System Design of the ROIC  
The ROIC comprises an IA and a SAR ADC, where signal distortion frequently arises at the interface between these two circuits. Figure 3a illustrates the ROIC utilizing a conventional SAR ADC. The sampling frequency, denoted as \(Clk_{s}\), of the SAR ADC is configured to be either two or five times the analog input frequency, \(f_{IN}\). Typically, IAs are not engineered to drive ADC inputs, primarily due to their insufficient linearity, which is essential for high-resolution ADC performance. Linearity, or total harmonic distortion (THD), is often the most significant limiting factor, rendering IAs incapable of directly driving the ADC. When complex waveforms are digitized, any distortion introduced can obscure the original signals, leading to corrupted data. Additionally, significant peaks are generated at the _Va_ node [26; 27; 28]. Alternatively, these large residual peaks can be eliminated by using an active low-pass filter, which also helps to reduce out-of-band noise and improve signal-to-noise ratio (SNR), although this is not its primary function. However, this approach also adds complexity to the circuit design, requiring a trade-off between the greater isolation caused by this current injection and the degradation in build-up time performance caused by a low-pass filter formed in this way [29; 30]. Another strategy involves incorporating a single-pole RC filter between the IA and the SAR ADC. This configuration allows the RC network to restrict the bandwidth of the incoming signal, thereby minimizing the noise transmitted from the amplifier and the preceding circuitry to the ADC. Nonetheless, excessive bandwidth limitation can lead to increased settling times and signal distortion. Consequently, the output voltage from the preamplifier retains high quality while also achieving lower power consumption in comparison to a scenario in which the IA is directly interfaced with the SAR ADC. The RC filter effectively reduces the amount of out-of-band noise that flows into the ADC input and aids in dampening transients from the switched capacitors at the ADC input. Therefore, the integration of an RC filter with the IA-SAR ADC interface necessitates a careful balance between noise suppression, settling time constraints, and power efficiency, as demonstrated in the subsequent analysis of component trade-offs [31; 32]. Figure 3b shows the ROIC using a conventional SAR ADC with an RC filter. Capacitor \(C_{FILT}\) helps to mitigate the step change in amplifier output voltage caused by this current step, but the amplifier is still subject to its interference and needs to be built up in time before the end of the acquisition cycle. Resistor \(R_{FILT}\) isolates the driver from \(C_{FILT}\) and reduces its effect on stability when driving large capacitors. Some factors need to be considered regarding the choice of \(R_{FILT}\) and \(C_{FILT}\) values. \(C_{FILT}\) acts as a charge bucket for the input charge backlash, minimizing the voltage step and thus improving the build-up time. Too large may affect amplifier stability and may drop the LPF roll-off frequency too low to allow signals to pass through, and too small results in too much charge backlash at the ADC input to build up in time. \(R_{FILT}\) provides isolation between the amplifier output and \(C_{FILT}\) to ensure stability. Too large may make the build-up time constant too long. It may also result in an elevated THD when accounting for the ADC input nonlinear impedance. The \(C_{FILT}\) is too small, and the amplifier may become unstable, or its forward path buildup may be compromised. We propose an application of an ROIC based on a sampling isolation technique as shown in Figure 3c. The input signal shown is connected to the bootstrapped switch in the SAR ADC through the sample isolation module, and the SAR ADC is connected to the output of the IA through the sample isolation module. The sample isolation technique effectively isolates DC and low-frequency noise between the instrumentation amplifier and the ADC.