Certain magnetometers produce at their output signals that exhibit double-sideband amplitude modulation with carrier signal (AM) with an ultra-low modulation index (m ≪0.01). This low m poses challenges for AM-based readout electronics, particularly in terms of sampling rate, and bit resolution of the Analog-to-Digital Converter (ADC) - since the information resides in the envelope of the AM signal. This work proposes a residue-based readout combining a two-steps subranging ADC and an amplitude estimator to adjust the carrier amplitude to enhance the dynamic range and precision of the envelope signal capture. This coarse ADC integrates a 6.5-bit Flash ADC and a 6-bit switch-based Digital-to-Analog Converter (DAC), implemented in 180nm CMOS process. A key innovation is using a shared resistor ladder between them, enabling midrise quantization and bit-level redundancy. This configuration reduces hardware complexity by eliminating the need for separate reference ladders, achieving 386fJ/conv at 100MSps.