A 1.8–GΩ Input-Impedance 0.15–μV Input-Referred–Ripple Chopper Amplifier With Local Positive Feedback and SAR-Assisted Ripple Reduction

This article presents a chopper-stabilized multipath current-feedback instrumentation amplifier (CFIA) that can also be reconfigured to a general-purpose operational amplifier (OPA). By utilizing a local positive feedback loop (LPFL), both the CFIA and the OPA benefit from gain-independent impedance boosting while retaining the merits of chopping, i.e., low noise and low offset. In addition, the chopping-induced ripple, one of the main issues of conventional chopper amplifiers, is suppressed by a passive high-pass filter with the SAR-assisted offset calibration. The prototype IC is implemented in a 0.18- $\mu \text{m}$ CMOS process and draws 1.07-mA current from a 5-V supply. The chopper CFIA exhibits the minimal input impedance of 1.8 $\text{G}\Omega $ and the maximum input referred ripple of 0.15 $\mu \text{V}$ (ten samples for both tests). These correspond to at least 1.13 $\times $ and 1.3 $\times $ improvements on the state-of-the-art IAs, respectively. The CFIA and the OPA also achieve excellent power efficiency (the highest GBW/ $\text{I}_{\mathrm {SUPPLY}}$ ), competitive input offset (4 and 2 $\mu \text{V}$ ), and input noise density (39 and 20 nV/ $\surd $ Hz).