A Preset-Free Energy-Efficient Bidirectional Zero-Crossing-Based Integrator

The zero-crossing-based integrator (ZCBI) offers an energy-efficient alternative to traditional operational transconductance amplifier (OTA)-based switched-capacitor (SC) integrators in analog-to-digital converters (ADCs) such as Delta-Sigma (ΔΣ) and pipeline ADCs, particularly in low supply voltage scenarios. To achieve zero-crossing detection, the ZCBI requires a preset operation to set the output to supply voltage at the start of the integration phase. This preset operation consumes extra dynamic power in each integration phase, and limits the speed and dynamic range of the integrator. This paper introduces a new ZCBI that can determine the direction of zero crossing by a pre-judgement circuit, hence circumventing the need for the preset phase and supporting continuous two-way integration. As a proof of concept, a self-timed incremental zoom ADC using the proposed ZCBI has been fabricated in a standard 180-nm CMOS technology. This ADC consists of a coarse 5-bit SAR ADC and a fine second-order single-bit ΔΣ modulator. The ADC consumes 150 μW from a 1.8-V supply. An SNDR of 77.4 dB is achieved in a 6.25 kHz bandwidth, leading to a Schreier FoM of 154 dB. It is foreseen that implementations in a more advanced technology will further enhance the energy efficiency of the proposed design.