Key result
A five-stack AC-coupled OTA-stacking ECG amplifier achieved state-of-the-art NEF and PEF metrics of 0.86 and 0.99, respectively, representing a ~3× improvement in energy efficiency over prior designs.
The proposed AC-coupled OTA-stacking technique significantly improves the energy efficiency of ultra-low power ECG amplifiers for implantable sensors.
May enable longer battery life for implantable ECG sensors; extends prior amplifier designs but leaves clinical translation open.
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 mm2, whereas the five-stack implementation consumes 18.7 nW and occupies 0.24 mm2. 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.
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Mondal et al. (2019) studied this question. AC-Coupled OTA-Stacking ECG Amplifier vs. prior ultra-low power, sub-100-nW amplifiers was evaluated on Noise Efficiency Factor (NEF) and Power Efficiency Factor (PEF). A five-stack AC-coupled OTA-stacking ECG amplifier achieved state-of-the-art NEF and PEF metrics of 0.86 and 0.99, respectively, representing a ~3× improvement in energy efficiency over prior designs.
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