Ultra-Low Voltage Subthreshold Binary Adder Architectures for IoT Applications: Ripple Carry Adder or Kogge Stone Adder

In this study, 8, 16 and 32 bits ultra-low power, robust, Kogge Stone (KSA) and Ripple Carry (RCA) adders using a commercially available 130 nm bulk CMOS technology have been designed and analyzed at subthreshold supply voltages ranging from 140–160 mV and temperature range of 27–50 °C at 5 kHz frequency for implantable biomedical devices. Simulation results based on netlists extracted from layout confirm that with a marginal increase in the supply voltage of the RCA compared to that of the KSA adder at the same speed, the power consumption and energy per operation, as well as the area of the RCA is far less than KSA. For example, when increasing the supply voltage of the 8 bit RCA by 44 mV compared to that of the KSA adder, the energy per operation for the KSA is about 3.5 times higher than that of the RCA. We have investigated different RCA topologies and considered the minimum energy point varies with different topologies. In addition, in the case of low throughput applications, using the stacked inverters for the full adder will reduce the leakage current and the total energy per cycle of the circuit. For the Minority-3 based 32 bits RCA with stacked inverters, the energy per cycle improves 15 percent compared to that of Minority-3 based 32 bits RCA at Vdd = 150 mV.