A current controlled oscillator based readout front-end for neurochemical sensing in 65nm CMOS technology

Abstract

This paper presents the design of an integrated current-controlled oscillator (CCO) based readout front-end for neurochemical sensing applications. The readout front-end chip is implemented in 65 nm CMOS technology and occupies an area of 0.059 mm 2 . The proposed design supports an input current range of 1.2 µA (±600 nA) and can also be configured to support wider current range. The CCO-based structure utilized in this design results in noise averaging of the detected neurochemical input signal due to its inherent ∆Σ first-order noise shaping and anti-alias filtering characteristics. Thus, the prototype chip achieves a current resolution of 100 pA and can detect dopamine concentrations as small as 10 µMol based on measured data from novel diamond-like carbon electrodes. In addition, the digital codes obtained from the readout front-end attain a signal-to-noise (SNR) of 82 dB and linearity limited effective-number-of-bits (ENOB) of 8 at full current range input, without employing any calibration or linearization techniques. The proposed read-out front-end consumes 33.7 µW of power in continuous operation.

A Sensor Interface for Neurochemical Signal Acquisition

“…The circuit requirements are defined based on the need to provide a wide cell voltage range (V cell ) of 1.5V between WE and RE, while sensing a wide range of the electrochemically induced current (I cell ) that is flowing between CE and WE. In addition, the acquired current is digitized with an oscillatorbased structure, which does not require analog sampling and benefits from technology scaling [8]. Finally, the digital output (D out ) from the sensor interface circuit can be utilized by closed loop brain machine interfaces for neurostimulation or prosthesis control.…”

“…The current acquistion (IA) stage is designed to provide stable V cell between −0.7V to 0.8V potential window based on cyclic voltammetry sweep of the input voltage (V in ). The oxidation (I ox ) and reduction (I red ) current peaks from the neurochemical can be detected during the forward and reverse sweep of the input voltage as described in [8]. The potential window of 1.5V defines the input common-mode range (ICMR) and determines the minimum supply voltage of the operational amplifiers (opamp).…”

“…Similar RC-filter is applied to the current acquisition and oscillator reference current paths (E I ref , O I ref ). The opamp OP 1 together with OP 2 control V cell by setting the voltage at the reference and working electrodes respectively as [8],…”

“…Hence, the acquired cell current is defined by the ratio of the applied V cell to the total impedance of the cell (Z cell ) as [8]:…”

“…The controllable bias current I ctrl that flows into the CCO decreases or increases from the defined IA reference current E I ref , based on the acquired redox current I cell from the neurochemical as [8]:…”