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(a) Three-dimensional representation of bead above a Hall sensor/microcoil pair. (b) Micrograph top view of two adjacent Hall sensor/microcoil pairs, implemented in a loose array.
The common-mode applied fields from the coils are rejected, while the differential induced field from the bead, detected by Hall sensor 1, is amplified. For further attenuation of the common-mode applied fields, a calibration feedback loop sets the current through coil 2 such that the output of the OCA is zeroed out.
Spectrum of the output of the ADC measured with a noise bandwidth (a) directly after autozeroing (the fundamental is used as the calibration tone) and (b) after application of the bead. The square wave tones resulting from the bead’s induced magnetization field are clearly visible. A bead magnetization field of is recorded.
Calculated, simulated, and measured component of the applied coil field and bead magnetization field, observed from the plane of the Hall sensor.
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