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2-Axis Wireless Phone Compass Circuit

• This circuit is optimized for voltage operation and using the HMC1052 AMR sensor in the 1.8 to 3.6 volt battery supply. To conserve battery life, you can power off the sensor and amplifier supplies (VDDSW) when not measuring the magnetic axis'. The set/reset MOSFETs can remain powered and will not draw current unless the gate logic toggles.
• The op-amp and MOSFET types are not specified to allow sourcing from multiple vendors. MOSFETs should have Rds(on) ratings of 0.4 ohms or lower per FET. All parts should be chosen for low voltage compatibility.
• Capacitors C3 through C5 should be chosen for low ESR performance. Stacked ceramic film types preferred for 0.5 ohm ESR nominal.
• Placement of the HMC1052 must permit a standoff from other SMT parts and ferrous metals for best performance. Compass accuracy is set primarily by processor ADC resolution and firmware to compute heading. Typically +/- 3 degree accuracy or better can be obtained.