A 12-μV Offset Low-Input-Bias-Current High-Precision CMOS Operational Amplifier with Automatic Offset Calibration

This brief presents a low-input-bias-current ($I_{B}$), high-precision CMOS operational amplifier utilizing an automatic offset calibration (AOC) technique. The proposed AOC incorporates a BOXCAR switched-capacitor (SC) integrator to improve calibration accuracy without degrading $I_{B}$ and input impedance. The prototype has been implemented in a standard $0.18-\mu \mathrm{m}$ complementary metal-oxide-semiconductor (CMOS) process. Simulation results show that an intrinsic input offset voltage ($V_{O S}$) ranging up to $\pm \mathbf{2. 6 ~ m V}$ can be calibrated within $500 \mu ~\mathrm{s}$, achieving a residual $V_{o s}$ of $12 \mu ~\mathrm{V}$. Benefiting from the reduced $V_{o s}$, the common-mode rejection ratio (CMRR) is improved to 108.7 dB. An extremely low typical $I_{B}$ of $18 f ~\mathrm{A}$ makes the prototype well suited for high-precision and highimpedance interfaces that require wide bandwidth.