The requirement to control each possible degree of freedom of digital circuits becomes a necessity in deep submicron technologies. This requires getting a set of monitors to measure each one of the parameters of interest. This paper describes a monitor fabricated in a 90nm CMOS technology which is able to estimate the circuit activity. The output of such monitor can be used as a tool to decide how to adjust the circuit working conditions to get the best power/performance circuit response. The paper presents the implementation and experimental results of a test chip including such monitor.