A 3-Bit Flash Spin-Orbit Torque (SOT)-Analog-to-Digital Converter (ADC)

In this article, a 3-bit Flash spin-orbit torque analog-to-digital converter (SOT-ADC) is presented, which works based on switching of a perpendicular-anisotropy magnetic tunnel junction (p-MTJ) by the spin Hall effect (SHE) assisted by spin-transfer torque (STT). To quantize the input signal into eight states, a heavymetal (HM) with different cross-sectional areas that are shared with seven magnetic tunnel junctions (MTJs) is utilized. To enable the deterministic switching, STT currents are employed. However, such currentsmake challenges during conversion and sensing phases, which are addressed in thiswork. The SOT-ADC eliminates the (2(n) - 1)-time duplication of the input current (I-in) in conventionaln- bit current-mode complementary metal-oxide-semiconductor (CMOS) Flash analog-to-digital converters (ADCs). Each MTJ acts as a comparator that compares the input signal (current) with its own critical current (I-C) as a reference current (I-ref). Therefore, the power-hungry comparators in CMOS Flash ADCs can be replaced by simple latch-based comparators for sensing the states of MTJs. Moreover, instead of using different sizes of transistors to create various values of I-ref, i.e., I-ref1, 2I(ref1), . . ., fixed currents as sensing currents are used, which leads to reducing mismatch issue and chip area. Because of the influence of the resistance of HM, each MTJ has an exclusive reference voltage (V-ref) that is created by a dummy 3-bit spin-orbit torque (SOT)-quantizer. According to the simulation results, power consumption and maximum sampling rate (including all conversion, sensing, and reset phases) of the ADC are 416 mu W and 102 MS/s in 180-nm CMOS technology, respectively. In addition, the differential nonlinearity (DNL) and integral nonlinearity (INL) are -0.258 and -0.275 least significant bit (LSB), respectively.

Automated summary

This article presents a 3-bit Flash spin-orbit torque analog-to-digital converter (SOT-ADC) that utilizes the spin Hall effect assisted by spin-transfer torque to switch a perpendicular-anisotropy magnetic tunnel junction. By utilizing heavy metal and fixed currents for sensing, the ADC eliminates the need for power-hungry comparators and reduces mismatch issues and chip area.