Designing Embedded Hardware

Magnetic Field Sensor

The final sensor we’ll look at is the AD22151 magnetic field sensor by Analog Devices. Its primary use is for position and proximity sensing. A magnetic source is used as a reference point, and the sensor’s distance from that source may be easily determined by the measured field strength. The sensor has built-in temperature compensation and amplification.

The circuit for this sensor is shown in Figure 12-20. It’s a little bit more complicated than the other sensors we’ve looked at so far.

Figure 12-20. AD22151 magnetic field sensor circuit

The sensor operates off a 5V supply, decoupled to ground using a 100nF capacitor. There are four resistors required for correct operation. R1 is the temperature compensation resistor. R1 should be connected between pins 1 and 3 or pins 2 and 3, depending on the applied magnetic field. For large external fields, R1 connects pins 1 and 3, as shown in Figure 12-20. For smaller fields, connect R1 between pins 2 and 3. The AD22151 datasheet has plots of values for R1 versus required compensation levels. Check with the manufacturer of your magnetic source^[19] as to the required compensation value, and use this in conjunction with the datasheet to determine R1.

For magnet data, try http://www.magtech.com.hk, http://www.eastindustries.net, or http://www.millennium.com.hk as places to start.

R2 and R3 set the signal gain of the internal ...