Methods and apparatus for a diagnostic ring oscillator (RO) circuit for DC and transient characterization. The RO circuit includes a plurality of symmetrical stages coupled via a feedback signal line and forming an inverter chain, where each stage includes a CMOS inverter comprising a pair of pMOS and nMOS transistors coupled between power-gating transistors respectively coupled to a positive voltage source and ground. An output of a CMOS inverter for the stage is coupled to an input for the CMOS inverter of a next stage. The first stage is an enable stage configured to set the inverter chain into a defined logic state, followed by multiple pre-stageDUT stages. The output of the last stage is feed back to the input of the enable stage to form a feedback signal. The RO circuit can operate in multiple modes including an AC mode, a DC mode, and a hybrid mode.

A ring oscillator (RO) circuit for capturing one or more characteristics relating to aging of CMOS circuitry in a CMOS device has been described. The RO circuit includes a plurality of stages coupled via an RO feedback signal line and forming an inverter chain. The plurality of stages include, for each stage, a respective CMOS inverter comprising a pair of pMOS and nMOS transistors followed by a pass gate, wherein an output of a pass gate for a stage is coupled to an input for the respective CMOS inverter of a next stage. The plurality of stages include an enable stage to enable the inverter chain to be put into a free oscillating mode or another mode in which the RO circuit does not freely oscillate. The plurality of stages include a Device Under Test (DUT) stage preceded by a pre-stage where respective supply rails of the DUT stage and pre-stage are isolated from one another.