The modern integrated circuit (IC) manufacturing process has exposed the fabless semiconductor industry to hardware Trojans that threaten circuits bound for critical applications. This paper investigates an on-chip sensor's effectiveness for Trojan detection in an application specific integrated circuit (ASIC) and proposes new techniques to improve the sensor's sensitivity to Trojan switching activity. The sensors serve as power supply monitors by detecting fluctuations in their characteristic frequencies due to malicious inclusions (i.e. hardware Trojans) in the circuit under authentication. Our proposed on-chip structure was implemented and fabricated on an ASIC test chip using IBM 90nm technology with controlled hardware Trojans. This work analyzes the impact of both sequential and combinational Trojans with varied partial activity, area, and location on the proposed on-chip structure and demonstrates that stealthy Trojans can be effectively detected with this technique, even when obfuscated by circuit switching activity and process and environmental variations.