A study of the NMOS flip-flop sensor and a comparison with the bipolar technique

Abstract

This paper presents a family of simple, microprocessor-compatible, compact digital-output sensors. These sensors operate by bringing a flip-flop into the unstable state over a large number of cycles and observing the resulting number of ‘ones’ or ‘zeros’. The ratio of ‘ones’ to ‘zeros’ is a measure of any imbalance in the system introduced by, for example, the introduction of some external signal. Due to the nature of the unstable state, this system is highly sensitive to small pertubations. By inserting a sensing element into the flip-flop structure, a family of compact and sensitive digital-output sensors can be produced. This paper examines the technique using standard NMOS technology and a comparison is made with similar devices produced in bipolar technology. Although high sensitivity is achieved, it is found to be lower than that of bipolar devices. However, the development of these NMOS devices yields a valuable extension to the flip-flop family of sensors in terms of new compact structures and compatibility with signal-processing circuitry.