TMAG5170-Q1: Reaction distance and accuracy

TMAG5170-Q1 is a linear Hall Effect Sensor with digital outputs. The A1 version which is currently available for early evaluation has three sensitivity ranges: +/- 25 mT, +/- 50 mT, and +/- 100 mT.  For example, we can consider the 25 mT case with no averaging enabled.  Here we have 12 bit resolution (increases to 16 bit with averaging enabled).  So for this case the device will convert an input ranging from -25 mT to + 25mT with an LSB size of about 12 uT.  

A Hall Sensing Element measures a voltage across a Hall Element which has a small applied current.  When a magnetic field orthogonal to the current is applied it forces the path of the current to shift which results in a measurable voltage across the Hall Element.  

A more detailed explanation of the effect can be found here:

training.ti.com/ti-precision-labs-magnetic-sensors

The result however is not that the total magnetic flux density is measured, but rather that we only detect the field along a single axis.  By integrating three sensors each oriented along a different axis, we can measure the complete field. This offers a lot of flexibility regarding placement and signal detection.  Additionally, with rotating disc/ring magnets, we are able to calculate the angular position of the magnet.  This is due to a 90 phase difference that is observed when monitoring two magnetic field components.

There are a few factors that will impact the accuracy of the measurement:

Input referred magnetic noise: With averaging off the RMS value is about 140 uT for the X or Y sensors, and about 64 uT in the Z direction.  Enabling averaging (up to 32x) can reduce this to as little as 25 uT (X,Y) or 11 uT (Z)

Conversions take real time, so there is some delay in fast moving systems.  Typically, however, if the device is operating at full speed a single axis conversion only takes about 25 us.  This will compound as various axes are enabled and averaging is turned on.

Sensitivity Error will impact the effective total range and accuracy.  This is very much like gain error in a typical amplifier.  We will also see accompanying linearity errors of the output, and mismatch from channel to channel.   TMAG5170-Q1 offers a typical sensitivity accuracy of +/-0.1%.  Channel to channel sensitivity matching is typically about 0.15%. For highly sensitive applications, it would be best to consider a calibration routine for optimal performance.  If precise matching of two components is needed, the device allows the user to apply attenuation to any one component.  This is particularly useful when performing off axis angle measurements where the field components are usually unequal in amplitude.

Offset may also be present.  This will appear as a fixed shift for the output, and is typically within +/- 0.15 mT.

Magnetic fields exceeding the programmed input range are not harmful to the device.  They will result in saturation of the ouptut, however.

Trigger Mode offers three settings.  Using the SPI command bits, CS Pin Pulse, or Alert Pin Pulse.  In terms of hysteresis for the trigger voltage, it is necessary to satisfy VIH or VIL to guarantee the device properly observes transitions between low and high states.