MAGNETIC PROBE (SENSOR) INTERFACE

DRV401-EP

Dynamic error correction ensures high dc precision over temperature and long-term accuracy. The DRV401 uses

analog signal conditioning; the internal loop filter and integrator are switched capacitor-based circuits. Therefore,

the DRV401 allows combination with high-precision sensors for exceptional accuracy and resolution. The typical

characteristic curve, DRV401 and Sensor Linearity, shows an example of the linearity and temperature stability

achieved by the device.

A demagnetization cycle can be initiated on demand or on power-up. This cycle reduces offset and restores high

performance after a strong overload condition. An internal clock and counter logic generate the degauss function.

The same clock controls power-up, overload detection and recovery, error, and time-out conditions.

The DRV401 is built on a highly reliable CMOS process. Unique protection cells at critical connections enable the

design to handle inductive energy.

The magnetic field probe consists of an inductor wound on a soft magnetic core. The probe is connected

between pins IS1 and IS2 of the probe driver that applies approximately +5 V (the supply voltage) through

resistors across the probe coil (see Figure 2a).

The probe core reaches saturation at a current of typically 28 mA (see Figure 2a). The comparator is connected

polarity, causing the probe circuit to oscillate in a frequency range of 250 kHz to 550 kHz. The oscillating

frequency is a function of the magnetic properties of the probe core and its coil.

The current rise rate is a function of the coil inductance: dI = L × V × dT. However, the inductance of the field

probe is low while its core material is in saturation (the horizontal part of the hysteresis curve) and is high at the

vertical part of the hysteresis curve. The resulting inductance and the series resistance determine the output

voltage and current versus time performance characteristic.

Without external magnetic influence, the duty cycle is exactly 50% because of the inherent symmetry of the

magnetic hysteresis; the probe inductor is driven from –B saturation through the high inductance range to +B

saturation and back again in a time-symmetric manner (see Figure 2b).

If the core material is magnetized in one direction, a long and a short charge time result because the probe

current through the inductors generates a field that either subtracts or adds to the flux in the probe core, either

driving the probe core out of saturation or further into saturation (see Figure 2c). The current into the probe is

limited by the voltage drops across the probe driver resistors.

The DRV401 continuously monitors the logic magnetic flux polarity state. In the case of distortion noise and

excessive overload that could fully saturate the probe, the overload control circuit recovers the probe loop.

During an overload condition, the probe oscillation frequency increases to approximately 1.6 MHz until limited by

the internal timing control.

secondary winding, and the magnetic flux in the sensor main core becomes uncompensated.

14

Submit Documentation Feedback

Copyright © 2008, Texas Instruments Incorporated

Product Folder Link(s): DRV401-EP