I plan to use a MAG3110 and a GPS for my application device. Does the antenna of the GPS generate too strong hard-iron interference to MAG3110 so that I can't put the two in a single PCB?
That's a good question. Depends if your antenna is magnetic or not... By the way, hard-iron interference is not critical if it does not exceed the MAG3110 magnetic range (+-1000uT). In any case you will have Hard-iron (the antenna will probably increase it). I understand you plan to make the MAG3110 an electronic compass right? If it is the case, to calculate an azimut /angle you need to calibrate the sensor to remove hard-iron bias.
Your antenna may add soft-iron disruption and could bring important heading error.
I suggest you to check that by taking measurements with the antenna fixed near the magnetometer. You can use a Freedom board + Sensor shield to do these measurements.
If the antenna is aluminium then you should be OK.
If the antenna has any iron or nickel component and is un-magnetized then the distortion will be "soft iron" in which the geomagnetic field is steered through the antenna to some extent.
And if the antenna has iron or nickel and is magnetized then the interference is "hard iron".
The Freescale magnetic calibration algorithms at www.freescale.com/sensorfusion should be able to correct both hard and soft iron effects. We did some successful work with an Android tablet where a ferromagnetic film behind the LCD gave a 10:1 soft iron distortion.
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