A 13.9-nA ECG Amplifier Achieving 0.86/0.99 NEF/PEF Using AC-Coupled OTA-Stacking

An ultra-low power electrocardiogram (ECG) recording front-end intended for implantable sensors is presented. The noise-limited, high power first stage of a two-stage amplifier utilizes stacking of operational transconductance amplifiers (OTAs) for noise and power efficiency improvements. The proposed technique involves upmodulated/chopped signals being applied to ac-coupled, stacked inverter-based OTAs that inherently sum the individual transconductances while reusing the same current, thereby enhancing the noise efficiency. Two prototype designs were fabricated in a 180-nm CMOS process. The three-stack version consumes 13.2 nW and occupies 0.18 mm
²
, whereas the five-stack implementation consumes 18.7 nW and occupies 0.24 mm
²
. State-of-the-art NEF and PEF metrics of less than unity, 0.86 and 0.99, respectively, are reported for the five-stack version. These correspond to ~3× improvement in terms of energy efficiency compared to prior ultra-low power, sub-100-nW amplifiers.