This brief presents a CMOS analog frontend (AFE) IC with high input impedance low input-referred noise, which for wearable electrocardiogram monitoring. The IC has three main parts, pseudo-differential chopper-stabilized instrumentation amplifier, switched capacitor filter, and low-power continuous–time <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${{\Sigma}} {{\Delta }}$ </tex-math></inline-formula> modulator. The pseudo-differential structure with chopper stabilization improves the input impedance, input–referred noise and chip area. Notably, we propose a new type of negative capacitor circuit to further boost the input impedance. Furthermore, the auto-zero technique is applied to the ripple reduction loop for suppressing the second harmonic ripple and maintain operation throughout the loop cycle. A dc servo loop suppresses the electrode dc offset. In <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.18~\mu \text{m}$ </tex-math></inline-formula> CMOS process, the proposed AFE IC occupies an active area of 0.99 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , consumes 5.8/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$46.6~\mu \text{A}$ </tex-math></inline-formula> from a 1.8 V supply, and achieves an input impedance above 2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{G}{{\Omega }}$ </tex-math></inline-formula> , a CMRR of 102 dB and a PSRR of 72 dB.