An ultra-low power (0.7 mW), high-speed (50 MSamples/s) analog to digital converter (ADC), presented by IMEC at this year's ISSCC conference, achieves a figure of merit of 65 fJ per conversion step.
This is 2.5 times better than the best ADC of this kind ever reported in research papers and an order of magnitude better than the best commercially available ADC IP blocks in 90nm CMOS. IMEC's successive approximation (SAR) ADC design is especially suited for nomadic applications in the IT realm. Its power scales linearly with the clock rate over a wide range which makes it ideal for software-defined radio applications. It is implemented in pure digital CMOS technology, making it suitable for scaling to the 45 nm CMOS node and below. The design is available as 'white box IP' for transfer to the industry.
The SAR ADC design is related to the what bis claimed to be the world's first true software-defined radio front-end IC also presented by IMEC at ISSCC 2007. This front-end chip is widely programmable for all present and future standards between 174 MHz and 6 GHz.
Instead of the active charge redistribution in the capacitor arrays of a conventional SAR architecture, the low-power architecture of this SAR ADC uses a passive charge-sharing concept to sample the input signal and to perform the successive-approximation cycling. As a consequence, the SAR operation is no longer based on voltage comparisons. It operates completely in the charge domain, which yields the record performance of the new design.
This way, the fundamental power limits of the original SAR architecture are overcome — by doing all the charge redistribution passively. The only active elements in the IMEC SAR ADC are the comparator and digital controller, so enabling ultra-low power consumption. Since the comparator doesn't consume any power during inactive mode, the power consumption of the ADC scales linearly with the sampling frequency.
The fully digital implementation of the ADC requires only MOS switches and metal-oxide-metal capacitors, making the ADC scalable towards the 45 nm node and beyond. This latest SAR ADC from IMEC outperforms all state-of-the art commercial ADCs by a factor of 10. It is 3.7 times better than ADCs in the same process generation (90 nm) and it is by a factor 2.5 better than any other high-speed research ADC.