Approximate computing is a promising computing paradigm that trades off power consumption and performance for error-tolerant applications. Approximate computing has been widely studied, such as for arithmetic circuits and accelerators. However, recent research has shown security vulnerabilities in approximate circuits. Hardware Trojans are one of the major threats to hardware circuits and have not been fully studied for approximate computing. Majority voting (MV) based on vendor diversities has been proposed as an effective technique to mask and/or detect hardware Trojans when assembling trusted chips with untrustworthy components. However, the randomness and diversity of inherent errors in approximate circuits can invalidate the MV technique. In this paper, for the first time, the authors present the challenges to approximate circuits when multiple vendors are considered to provide IPs for approximate circuits (IPac). Experiments demonstrate that the MV strategy is not applicable when trusted chips are assembled with IPacs. A comparison-based technique is proposed to thwart hardware Trojan attacks on approximate circuits. The experimental results show a high effectiveness of the proposed method to detect hardware Trojans in approximate circuits