A passive second‐order noise‐shaping SAR ADC architecture with increased freedom in NTF synthesis and relaxed clock‐jitter issue

Noise‐shaping (NS) successive approximation register (SAR) analogue‐to‐digital converters (ADCs) are an attractive architecture for power and area efficiency in moderate resolution and bandwidth applications. NS SAR ADCs employing a passive integrator are good candidates for their omission of power‐hungry and scaling‐unfriendly amplifiers. However, existing passive NS SAR ADCs have a very limited degree of freedom in realising their noise transfer function (NTF), resulting in poor in‐band noise suppression. In this letter, a passive switched‐capacitor 2nd‐order NS SAR ADC that is the first of its kind to offer controllable complex poles to optimise the NTF is proposed. This is achieved by eliminating a capacitor charge clearing operation in a conventional 1st‐order NS SAR. The proposed NS SAR ADC also offers simple circuits insensitive to process–voltage–temperature (PVT) variations, and small area with only two extra clock phases. Transistor‐level circuit simulation of a 7‐bit, 8‐MHz bandwidth, 128 MS/s prototype in a 65‐nm CMOS shows a 70 dB signal to noise and distortion ratio (SNDR), an improvement of 19 dB compared to that of a regular 7‐bit SAR ADC.