A 0.0308mm2 4.15pJ/conv VCO-Based Current Sensing Front-End with 2nd-Order Δ2 ΔΣ Modulation

A photocurrent sensor with a wide dynamic range (DR) and high resolution is crucial in many biosensing applications [1] –[4]. In particular, the implantable recording device for monitoring neural activities in vivo through fluorescence [2] has strict power and area constraints. A conventional current sensing front-end adopts $\text{a}\Delta \Sigma$ modulator with various quantizers to achieve high-resolution [1], [3] –[6]. However, an analog loop filter and multi-bit DAC consume high power and occupy a large active area. Recently, the $\Delta - \Delta \Sigma$ structure has been reported to relax the requirements of the $\Delta \Sigma$ modulator by reducing input amplitude [7]. Still, it suffers from static power and degraded resolution by DAC noise. $\text{A}\Delta \Sigma$ modulator with a VCO-based quantizer (VCOQ) reduces static power consumption and realizes an area-efficient structure, but it does not address the noise of the DAC [8]. This paper presents $\text{a}2 ^{nd} -$order $\Delta^{2}- \Delta \Sigma$ modulator based on a VCOQ with a PWM I-DAC for the precise acquisition of incoming current in an area- and energy-efficient form factor. The proposed $\Delta^{2}$ signal produced by $\text{a}2 ^{nd} -$order IIR-filter substantially attenuates the magnitude of input signals, enhancing the linearity and DR. A differentiator followed by the VCOQ with a passive integrator configures an additional $\Delta \Sigma$ modulator, increasing noise shaping order without a feedback loop. In addition, the PWM I-DAC substituting the multi-bit I-DAC is devised to mitigate noise further, realizing the high resolution of 1 pA with 500-Hz bandwidth.