# Current-Feedback Instrumentation Amplifier Using Dual-Chopper Fill-In Technique
## 2 Proposed Chopper Fill-In Instrumentation Amplifier
### Configuration of the CFIA and RRL  
A chopper was used at the input and before the integrator. The chopper-induced ripple caused by the input offset is one of the major drawbacks of the chopping technique. As shown in Figure 6, an RRL was added in this study to reduce ripple. Figure 5: (a) The top architecture of the proposed CFIA and (b) fill-in technique. CH2 modulates the input offset of \(G_{m1}\) to the chopper frequency as the offset current changes. After passing through the integrator, the ripple is displayed as a voltage at the output of \(G_{m4}\). The capacitance (\(C_{m}\)s) detects the integrated ripple and converts it to current, followed by CH3 demodulation of the current offset. After passing through the current buffer, \(C_{int}\) converts the offset current to voltage, and \(G_{m3}\) adds the opposite current to the offset current before sending it to the main amplifier. Figure 7 depicts a schematic of the proposed CFIA at the transistor level. It includes a PMOS input stage, a folded cascode, and a Monticelli class-AB output stage. In addition, the \(R_{CMS}\) and \(C_{CMS}\) detect the output common-mode voltage in the common-mode feedback (CMFB) circuit. The current-mode error amplifier generates the control voltage (\(V_{CMFB}\)), which adjusts the common-mode output voltage by biasing the gates of \(M_{P10}\) and \(M_{P11}\). A high open-loop gain of more than 100 dB reduces the finite gain error. In addition, a nested Miller capacitor was added to the CMFB stage and primary simplifier to ensure frequency stability with a 50\({}^{\circ}\) phase margin. Tables 1 and 2 list the values of the transistors and the passive elements of the CFIA. Figure 6: CFIA and RRL configuration in one channel. Figure 7: Transistor-level schematic of the proposed CFIA. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Transistor** & **Size (W/L) (\(\mu\)m)** & **Transistor** & **Size (W/L) (\(\mu\)m)** \\ \hline \(M_{P1}\), \(M_{P2}\) & 18/3.2 & \(M_{N1}\), \(M_{N2}\) & 13.6/10 \\ \hline \(M_{P3}\) & 6/3.2 & \(M_{N3}\), \(M_{N4}\) & 5.6/4 \\ \hline \(M_{P4}\)-\(M_{P9}\) & 80/0.5 & \(M_{N5}\), \(M_{N6}\) & 2/14 \\ \hline \(M_{P10}\), \(M_{P11}\) & 26/3 & \(M_{N7}\), \(M_{N8}\) & 2/1 \\ \hline \(M_{P12}\), \(M_{P13}\) & 10/1 & \(M_{N89}\), \(M_{N10}\) & 2/5 \\ \hline \(M_{P14}\), \(M_{P15}\) & 4/9 & \(N_{N11}\)–\(M_{N14}\) & 12/2 \\ \hline \(M_{P16}\), \(M_{P17}\) & 8/1 & & \\ \hline \(M_{P18}\), \(M_{P19}\) & 20/2 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Values of transistors in the proposed CFIA. \begin{table}
\begin{tabular}{c c} \hline \hline
**Component** & **Value** \\ \hline \(C_{C}\) & 412 ff \\ \hline \(R_{C}\) & 134 k\(\Omega\) \\ \hline \(R_{CM}\) & 2.3 M\(\Omega\) \\ \hline \(C_{CM}\) & 206 ff \\ \hline \end{tabular}
\end{table} 
Table 2: Values of the passive components in the proposed CFIA. Figure 8: Schematic of the proposed current buffer in the RRL. Table 5 is about the noise contribution of the proposed circuit. Most noise source is the thermal noise of the transistor and also has a flicker noise component in the feedback resistor of the CFIA. Figure 9 shows the glitch reduction simulation results. Figure 9a shows that the fill-in differential output reduced chopper glitches in the time domain. The simulation results for glitch reduction in the frequency domain are displayed in Figure 9b. The fill-in differential output reduced chopper glitch by 78 dB compared with the differential output of each channel when using an 18 kHz chopper frequency with 10 mV and 1 kHz sine input. \begin{table}
\begin{tabular}{c c} \hline \hline
**Component** & **Value** \\ \hline \(C_{C}\) & 1.2 pF \\ \hline \(R_{C}\) & 11.2 k\(\Omega\) \\ \hline \(R_{CM}\) & 224 k\(\Omega\) \\ \hline \(C_{CM}\) & 1.6 pF \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Values of the passive components in the current buffer. Figure 9: Glitch reduction simulation result for the proposed CFIA: (**a**) transient simulation result and (**b**) amplitude spectrum.