A Mode-Reconfigurable Second-Order NS-SAR ADC With NTF Synchronous Optimization
Yuhua Liang, Shida Song, Nuo Xu, Yuke Shen, Zhangming Zhu
Abstract
This paper presents a mode-reconfigurable 2nd-order noise-shaping successive-approximation-register (NS-SAR) analog-to-digital converter (ADC) featured with the ability of optimizing the noise transfer function (NTF) synchronously. Three operation modes, the energy-saving (ES) mode, the normal (NM) mode, and the high-resolution (HR) mode, can be supported by reconfiguring the resolution of the internal SAR ADC (N<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${}_{\mathrm {SAR}}$</tex-math> </inline-formula>) and the oversampling rate (OSR), while the NTF optimization is realized by modulating locations of zeros and poles in the context of a specific mode to enhance the NS effect. The prototype ADC is fabricated in a 180-nm CMOS and operates under a 1.8-V supply. Operating at 5MS/s sampling rate, the ADC’s OSR can be configured as 8 or 16 in different operation modes, so its bandwidth can be configured as 312.5kHz or 156.25kHz. With N<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SAR</sub> and OSR reconfigured to be 8-bit and 8 for the ES mode, it achieves a signal-to-noise and distortion ratio (SNDR) of 80.3dB and consumes <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$201\mu $</tex-math> </inline-formula>W. Attributing to the NTF optimization, 27% power saving can be reached in the ES mode with respect to that when the optimization were disabled for the same SNDR. In the HR mode, N<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SAR</sub> and OSR are set to be 9-bit and 16, respectively. And a SNDR of 93.3dB is obtained at the expense of consuming <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$280\mu $</tex-math> </inline-formula>W. It is evidenced that the SNDR can be boosted by 6.4dB owe to the NTF optimization, resulting in an optimal SNDR-based Schreier figure-of-merit (FoM<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${}_{\mathrm {S}}$</tex-math> </inline-formula>) of 180.8dB for the ADC. The values for N<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SAR</sub> and OSR are determined to be 9-bit and 8 for the NM mode, and the achievable SNDR is 88.2dB. The occupied core area is 0.537 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>.