Gain equalization for multiple axis magnetic field sensing

1. A magnetic field sensor comprising:

a plurality of magnetic field sensing elements;

a coil driver configured to generate a coil drive signal;

a coil structure coupled to the coil driver and configured to generate a reference magnetic field on each of the plurality of magnetic field sensing elements in response to the coil drive signal, wherein each of the plurality of magnetic field sensing elements is configured to generate an output signal responsive to the respective reference magnetic field and to external magnetic fields;

a memory configured to store reference gains associated with the plurality of magnetic field sensing elements, wherein the reference gains include a predetermined ratio of gains between two or more of the magnetic field sensing elements; and

a gain equalization circuit having inputs coupled to outputs of the plurality of magnetic field sensing elements and a plurality of outputs, the gain equalization circuit configured to:

extract a reference signal and an external signal from each of the plurality of magnetic field sensing element output signals, to measure a gain of each of the plurality of reference signals,

select a plurality of gain adjustment values such that a ratio of the measured gains multiplied by the corresponding ones of the gain adjustment values equals the predetermined ratio of gains between the two or more of the magnetic field sensing elements, and

apply the plurality of gain adjustment values to scale corresponding ones of the external signals.

2. The magnetic field sensor of claim 1 wherein the coil driver is configured to generate a coil drive signal as an alternating current (AC) signal having a frequency substantially different than a frequency associated with the external magnetic fields, wherein the gain equalization circuit comprises:

an external signal filter to extract the external signals from the plurality of magnetic field sensing element output signals; and

a reference signal filter to extract the reference signals from the plurality of magnetic field sensing element output signals.

3. The magnetic field sensor of claim 1 wherein the plurality of magnetic field sensing elements comprises a plurality of differential magnetic field sensing elements, wherein the coil structure is configured to generate a differential reference magnetic field on each of the plurality of magnetic field sensing elements, the magnetic field sensor further comprising:

a plurality of differential output switches each configured to couple outputs of a respective one of the differential magnetic field sensing elements in multiple different configurations.

4. The magnetic field sensor of claim 3 wherein at least one of the differential output switches is configured to couple the outputs of the respective differential magnetic field sensing element in a first configuration and in a second configuration, wherein in the first configuration, the differential magnetic field sensing element outputs are responsive to the external magnetic fields but not the respective differential reference magnetic field, and wherein in the second configuration, the differential magnetic field sensing element outputs are responsive to the respective differential reference magnetic field but not the external magnetic fields.

5. The magnetic field sensor of claim 4 further comprising a timing circuit to repeatedly alternate between the first configuration and the second configuration.

6. The magnetic field sensor of claim 1 wherein the coil structure comprises multiple coils coupled to the coil driver to receive matched currents.

7. The magnetic field sensor of claim 1 wherein the reference gains include predetermined absolute gains of the plurality of magnetic field sensing elements.

8. The magnetic field sensor of claim 7 wherein the gain equalization circuit is configured to adjust the gain of the external signals based on comparing the measured gains to the predetermined absolute gains.