A Low Distortion Continuous Time Sigma Delta Modulator using a High Input Impedance Instrumentation Amplifier for Neural Recording

In this paper an architecture for a continuous time sigma delta modulator used in a neural recording channel is proposed. The instrumentation amplifier based input structure of the converter exhibits a high input impedance and linear behavior. Properties making its use interesting in analog frontends for direct application on sensor elements. Circuit linearity is achieved solely by negative feedback. A prototype was developed as a 2nd order single bit modulator in a cascade of integrators feedforward form and was taped-out in a 180nm technology. Simulations show an input impedance of >1GΩ and a THD of 0.012%. The input referred noise is 2.2µV at an average current consumption of 14µA occupying an area of 0.07mm2.