A VCO-Based 2nd-Order ?²-?S Modulator for Small-Size High Energy-Efficient Current Sensing Front-End

In this letter, a 2nd-order ?(2)-?S modulator consisting of a voltage-controlled-oscillator-based quantizer (VCOQ) and a current digital-to-analog converter (DAC) with a pulse width modulator (PWM) is presented for the precise acquisition of a wide-range photocurrent in an area-and energy-efficient form factor. The proposed ?(2)-modulation realized by the 2nd-order infinite impulse response (IIR) filter on the feedback significantly attenuates the magnitude of input signals, enhancing the DR and linearity. Moreover, an additional differentiator followed by the VCOQ features the negative feedback loop in the 2nd-order AE modulator, improving noise shaping with no additional current DAC noise. In addition, a 1-bit PWM current DAC substituting the multibit current DAC is devised to mitigate the noise from the current DAC, realizing the high resolution of 1 pA with 500-Hz bandwidth. The prototype chip fabricated in a 110-nm CMOS occupies 0.0308 mm(2) and achieves Walden FoM of 4.15 pJ/conv.