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.