A Fully Integrated Current-Mode Continuous-Time Delta-Sigma Modulator for Biological Nanopore Read Out

This paper presents a fully integrated low noise current-to-digital converter based on current-mode continuous-time delta-sigma modulators (CT DSMs). The circuit was realized in standard 0.35 mu m complimentary metal-oxide-semiconductor process and achieves a noise floor as low as 200 fA(rms) within a bandwidth of 10 kHz. Thanks to the continuous-time architecture, the oversampling together with the implicit anti-aliasing behavior of the current-mode CT DSM prevents the back folding of any out-of-band signals and noises. Running at a sampling frequency of 1.6 MHz, the circuit achieves a measured worst case anti-aliasing filtering of -110 dB for an out-of-band signal at 1.59 MHz. This characteristic eliminates the need of an anti-aliasing filter, which reduces the complexity as well as the area and power consumption. Two feedback versions, a transistor based as well as a capacitor-based current division have been used to reduce the reference current noise. The transistor-based current division feedback modulator consumes a power of 6.73 mW and occupies an area of 0.63 mm(2), whereas the capacitor based consumes a power of 4.6 mW and occupies an area of 0.32 mm(2). Both of them operate at a supply voltage of +/- 1.5 V.