Weak-signal acquisition across a large common-mode voltage difference requires the isolation interface to provide both high-voltage withstand capability and low noise coupling.This paper investigates the degradation in measurement precision introduced by the magnetic-isolation link in a sigma-delta (ΣΔ) analog-to-digital converter (ADC) with an effective resolution of approximately 18 bits.A magnetically isolated measurement system based on silicon-based on-chip microtransformers is implemented, with the ADC, signal isolator, and isolated power supply integrated within the millimeter-scale cavity of a six-die hybrid package.Controlled comparisons are carried out on devices from the same fabrication batch under a non-isolated reference configuration and a fully magnetically isolated configuration.Measurements of isolated-power-supply ripple are combined with magnetic leakage-field estimates and steady-state thermal simulations of the package to analyze the principal noise-coupling paths.The fully magnetically isolated configuration reduces the signal-to-noise ratio (SNR) by 9.5 dB and raises the average in-band noise floor by 11.4 dB over 200–1200 Hz, without introducing additional prominent discrete spurs.Further analysis indicates that the rectifier ripple, with a peak-to-peak amplitude of 14 mV at a repetition frequency of 600 kHz, can couple through interlayer parasitic capacitance and the package parasitic network to generate a common-mode disturbance.Nonideal differential sampling and aliasing then convert this disturbance into in-band broadband noise, making it the main factor limiting system precision. These results provide experimental evidence for package-layout optimization and noise suppression in high-precision magnetically isolated measurement systems.