What is the proper way to set the offset and sensitivity parameters for Allegro A1362 so MSP430 properly digitizes them?

Question

What is the proper way to set the offset and sensitivity parameters for Allegro A1362 Programmable Linear Hall Effect Sensor so that TI MSP430 5528 properly digitizes them?

Using observed values of MSP430's digitization, how do I select a sensitivity for the A1362? I am expecting the sensitivity to be around 5-6 mV/G and according to the data sheet the guaranteed range is 4.5-16mV/G

How do I set the steady state value of the A1362 to be around 1.25Volts? The MSP430 digitizes values from 0-2.5Volts, but the A1362 Hall-Effect sensor is bipolar, meaning the voltage increases and decreases. I would need the A1362's steady state value (with no magnet) to be 1.25V to allow for both north and south pole variations.

I know both questions above will require some experimentation with the A1362 Hall-Effect Sensor and magnet in order to determine the exact offset and slope, but I was hoping someone in the community could either point me to a some kind of a tutorial (youtube or guidelines) or give me a general idea of the procedure to do what is described above.

Will the required procedure look anything like steps outlined on pg. 54 here?

My understanding is I will be applying voltage pulses using a standard power supply.

I would greatly appreciate all / any advise as this is my first time doing something like this. Video tutorials would be much appreciated.

Answer 1

The preferred way to set up the Hall Sensor for initial experimentation would be to acquire the evaluation board, and then follow the instructions to use its software for calibration and configuration.

If that is not an option, similar results can be achieved manually, albeit with a bit more effort and some guesswork / empirical data. This is how it goes:

Required parameters:

• Expected magnetic field strength - active range expected?
  • Intrinsic, earth's magnetic field at surface = 300 to 600 milliGauss
  • Small toy magnet or fridge magnet = 30 to 150 Gauss
• Expected rate of change of field: How many cycles per second?
• V_cc of MSP430?
  • MSP430 digitizes from 0 to V_cc, and can tolerate up to V_cc+0.30 Volts
  • Permissible V_cc is up to 3.6 Volts
• Expected / desired full-scale range from Hall Sensor (assuming ADC10 of MSP430 is being used)?
  • Both upper and lower references can be changed from default GND and V_cc, to external reference voltages, 0 to 1.2 Volts as lower reference, and 1.4 to V_cc as higher
  • Internal voltages references of 1.5 Volts and 2.5 Volts are also available, to use as the full-scale limits.

Once the above parameters are determined, a suggested way to proceed would be:

1. Program quiescent voltage to around midway between the MSP430 ADC10 reference values you select
2. Determine actual sensitivity of your sensor using the magnet of choice, and a good voltmeter: The manufacturing tolerance range is pretty high. For instance, if using strong fridge magnets, the highest sensitivity range, 4.5 - 16 mV/G, could give a maximum of 4.8 Volts full-scale North-to-South, way more than the MSP430 can handle.
3. Program sensitivity based on above tests, don't use the maximum sensitivity mode of the part.
4. Don't count on built-in clamping of the device, use an external clamping arrangement if at risk of exceeding MSP430's V_cc+0.30 Volts.
5. Opt for ADC reference voltages in between the rail voltages instead of full 0 to V_cc, that way there is margin for error in case the sensor overshoots expected full-scale voltage, e.g. if a mobile phone rings nearby.
6. Use a capacitor on the bandwidth FILTER pin such that your bandwidth is limited to perhaps 10 times the expected cycles per second mentioned above - otherwise the output gets too jittery, even if a person with coins in their pockets walks by.

That's what I can recall off the top of my head, but if additional details are required, please edit the question accordingly.