At today's International Solid State Circuit Conference, imec
and Holst Centre report an ultra-low power 8 bit analog to digital convertor
(ADC) consuming only 30fJ energy per conversion step. This world-class figure
of merit ADC is especially suited for upcoming low energy radios in the ISM
(industrial, scientific and medical) radio bands such as low-energy Bluetooth
or IEEE 802.15.6 for body-area networks.
Imec and Holst Centre realized this ultra-low power ADC with record performance
by using a unique concept that combines a successive approximation (SAR) architecture
working completely in the charge domain with an asynchronous controller. By
doing all the charge redistribution passively, the power consumption of the
SAR ADC is already drastically reduced compared to conventional SAR ADCs. An
asynchronous controller is implemented to further minimize the power consumption
and to allow operation on a single external sampling clock. This asynchronous
implementation thus has no clock-driven precharge phase but instead self-synchronizes
the various building blocks to maximize the speed of operation and to minimize
the power consumption.
The chip was implemented in a 90nm digital CMOS technology. Measurements on
silicon show a power consumption of only 69µW at a sampling rate of 10Msamples/s
and a standby power of only 17nW. Since none of the ADC building blocks consumes
any static power, the power consumption of the ADC scales linearly with the
sampling frequency. Thus, the figure of merit of 30fJ/conversion step is maintained
from 10kSamples/s to 10MSamples/s making it the widest power-efficient range
published amongst comparable state-of-the-art designs.
“This result proves that imec and Holst Centre have built up extensive
know-how in ultra-low power design within their program on ultra-low power radios
for body-area networks. This extreme low-power ADC is applicable in ultra-low
power radios usable in a wide range of applications from healthcare to industrial;”
said Bert Gyselinckx, general manager imec the Netherlands at Holst Centre.