Common-frequency full-duplex systems send and receive signals simultaneously in the same frequency. Therefore, self-interference will exist in the local receiver and interference cancellation is necessary. Digital interference cancellation is applied after Analog-to-Digital Converter (ADC) sampling. ADC bit number, Interference-to-Signal Ratio (ISR), and quantizing judgment scheme affect both interference cancellation and Symbol Error Ratio (SER). In this paper, the essential relationship among ADC bit number, ISR, and SER of Quadrature Amplitude Modulation (QAM) is analyzed; A closed-form expression of SER is developed; And simulation results are presented to support the SER expression. The simulation results show that: when ISR decreases or ADC bit number increases, SER of this kind of system decreases in large scale, and fluctuates in small scale; some fluctuations of SER are dramatic, which can approach to the SER of the system without quantization error.% 同频全双工由于在同时工作的收发通道上使用相同的频率资源，因此本地接收机需要进行自干扰消除。数字域干扰消除方法在模数转换器(ADC)器件采样后进行，ADC 位数、干信比、量化判决准则直接影响干扰消除效果和系统误码性能。该文分析了 ADC 位数、干信比、QAM 调制误码性能三者的内在关系；推导了误码率的闭合表达式；仿真验证了数学推导的正确性和有效性。仿真结果表明，随着干信比的减小和 ADC 位数的增加，误码率性能呈宏观改善趋势，但从特定的微观片段来看，会出现性能波动，甚至会接近无量化误差的误码率性能。
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