HMC1001/1002/1021/1022

12

www.honeywell.com

360k-ohm feedback and reference resistors are chosen to provide a nominal 230mV/V/gauss gain characteristic or

1.15V/gauss gain with VDD at 5 volts. Other values than 360k-ohms may be chosen; with smaller resistances for larger

fields and larger resistances for lower field strengths. Be aware that sensor bridge offsets factor into the signal gain

selection as the offsets may be as large as the signal to be measured. See application note AN212 on methods to handle

bridge offset voltages.

As a magnetometer, the circuit outputs (Xout and Yout) should be measured against VREF and scaled for 1.15 volts per

gauss using a 5 volt sensor/amplifier power supply (VDD). Since the sensor’s bandwidth is 5 MHz, the sampling rate of

the outputs can be very fast, to the point were the filtering and speed of the amplifiers begins to effect the measurements.

Resolution will be mostly to the size of the Analog-to-Digital Converter (ADC), where a 10-bit ADC would spread its 1024

counts across the power supply or tighter.

As a compass, the two outputs constrain the earth’s magnetic field measurement to horizontal orientations with the Xout

and Yout feeding the heading equation of arctan (Yout/Xout) in degrees. The Xout direction of the HMC1002 should be

mounted to the forward direction of the product for proper orientation. If a tilt-compensated compass is desired, a third

axis could be made from the spare LMV324N amplifier and a HMC1001 sensor. Refer to the technical papers on

compassing from the website for more detail on compass implementation.

Field Detector or Current Sensor

A simple sensor implementation is shown in Figure 6 for a single axis sensor and signal conditioning circuitry for detecting

a magnetic disturbance, or as a current sensor when placed near a current carrying conductor. For more details on

current sensing, see application note AN209 on the website.

From Figure 6, the HMC1021 sensors are different from the HMC100x parts in that the bridge resistances increase to

1100 ohms and the set/reset strap resistance increases to 4.5 ohms. Because the minimum set/reset peak current is

down to 0.5 amperes, the set/reset drive circuit can now be run at common supply rails of 5 or 3 volts (VDD). Due to the

increased resistance of the set/reset strap, capacitor C3 can be reduced to about 0.22uf to maintain the desired 1 to 2

microsecond time constant. Capacitor C2 is typically chosen to be about ten times the series capacitor value, or 2.2uf.

The same pulse transition scheme in Figure 5 applies to Figure 6.

The sensor/amplifier circuit is likewise similar but the 1mV/V/gauss sensitivity requires a gain boost by increasing

feedback/reference resistors for sensing low fields like earth’s magnetic field. If a 2 or 3-axis compass is to be designed

with the HMC102x series sensor, parts like the HMC1022 plus the HMC1021Z can be used, with replication of the

difference amplifier stages for each axis. By choosing the 1 Meg-ohm and 4.99k-ohm resistors, the gain with a 5 volt

supply produces about a 1V/gauss transfer characteristic and centered at half supply (2.5 volts).

An instrumentation amplifier could be substituted for the operational amplifier to minimize external discrete components,

but the very low cost of op-amps like the LMV741/LMV358/LMV324 family is hard to beat if price is more important than

printed circuit board footprint. The signal output of the amplifier can be directly placed on the input of an ADC and further

processed in digital form. If the ADC range spans the power supply range, then a 10-bit ADC can have count 512 of 1024

used as the zero gauss point when the output rests at half-supply. If 3 volt operation is required, the designer can

substitute the IRF7507 part for the IRF7105 for 2.7 volt logic drive of the complementary MOSFET gates.