Design of OTAs for ultra-low-power sigma-delta ADCs in medical applications

High-resolution sigma-delta ADCs are gaining significant interest in ultra-low-power medical applications, where accurate measurement of low-frequency and weak electrophysiological signals is required. Operational transconductance amplifiers (OTA) are the key analog component and the most power-hungry part of the sigma-delta (ΣA) modulators. This paper presents a study of OTAs for ultra-low-power operation, including design and a comparative analysis of four OTA architectures implemented in 65nm CMOS Technology. The requirements for OTA gain and GBW are driven in terms of ΣA ADC specifications. The OTAs´ impact on modulator SNR has been investigated by simulation. The results show that a two-stage OTA with load compensation yields highest SNR and lowest power dissipation amongst the four OTAs in this study.