Abstract: To address data flicker and sampling failures of the acquisition card used with electronic current transformers (ECTs) in smart substations during disconnector operations, the coupling mechanism of mixed electromagnetic interference induced by very fast transient overvoltage (VFTO) superimposed with a damped oscillatory magnetic field is analyzed. An ADC coupling model considering the combined effects of conducted common-mode current and radiated differential-mode voltage is established, and an FPGA-based nanosecond-level synchronized mixed-interference test platform is developed to achieve time-domain alignment between IEC 61000-4-4 (EFT/B) and IEC 61000-4-10 damped oscillatory magnetic-field sources. Experimental results show that under synchronized mixed interference, both ENOB and SINAD exhibit pronounced nonlinear degradation, and the degradation exceeds the linear superposition of single-interference effects. Mitigation measures based on ground-plane partitioning and permalloy composite shielding are proposed, and retest verification confirms effective suppression of mixed interference and improved data acquisition integrity。