A Continuous-Time Delta-Sigma Modulator Using a Modified Instrumentation Amplifier and Current Reuse DAC for Neural Recording

Abstract

The use of continuous-time delta-sigma modulators (CTDSM) as a replacement of traditional analog sensor front ends has become an increasingly popular choice. The absence of traditional sensor interface circuits, such as instrumentation amplifiers (IAs), reduces area and power consumption at the cost of robustness offered by these circuits. Selection of the integrator type in the CTDSM is a selection of the ADC properties. This article presents a current feedback CTDSM with high input impedance, wide linear operating range, and large-signal common-mode rejection as a neural recorder front end. We propose the modification of an IA for operation as an integrator in a delta-sigma modulator and the current reuse of the feedback DAC current to bias this integrator stage. Deploying the current reuse technique, the total power consumption of the modulator is reduced by 22%. The ASIC is developed in a 180-nm high-voltage CMOS technology and the analog front end achieves a dynamic range of 90 dB and a total harmonic distortion (THD) of 0.005% when configured for a full-scale input signal of 208 mVpp. A common-mode rejection ratio (CMRR) of 90 dB is measured for a common-mode disturbance of 325 mVpp while the analog power consumption per channel is 23 μW.