+

-

+

-

R/2

R/2

SIG +

SIG -

WITH 'R << R,

THEN RTH | R/2

THUS,

(b)

VTH± =

VEXC ± VSIG

2

R + 'R

R - 'R

SIG +

SIG -

R - 'R

R + 'R

VEXC

(a)

19

LMV641

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SNOSAW3D – SEPTEMBER 2007 – REVISED AUGUST 2016

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Typical Applications (continued)

8.2.2.2 Detailed Design Procedure

In this circuit, the use of a 9-V alkaline battery exploits the LMV641’s high voltage and low supply current for a

low-power, portable-current-sensing application. The sensor converts an incident magnetic field (through the

magnetic flux linkage) in the sensitive direction, to a balanced voltage output. The LMV641 can be used for

moderate to high current sensing applications (from a few milliamps and up to 20 A) using a nearby external

conductor providing the sensed magnetic field to the bridge. The circuit shows a Honeywell HMC1051Z used as

a current sensor. Note that the circuit must be calibrated based on the final displacement of the sensed

conductor relative to the measurement bridge. Typically, once the sensor has been oriented properly, with

respect to the conductor to be measured, the conductor can be placed about one centimeter away from the

bridge and have reasonable capability of measuring from tens of milliamperes to beyond 20 amperes.

In Figure 41, U1 is configured as a single differential input amplifier. Its input impedance is relatively low,

however, and requires that the source impedance of the sensor be considered in the gain calculations. Also, the

asymmetrical loading on the bridge will produce a small offset voltage that can be cancelled out with the offset

trim circuit shown in Figure 41.

Figure 42 shows a typical magnetoresistive Wheatstone bridge and the Thevenin equivalent of its resistive

elements. As we shall see, the Thevenin equivalent model of the sensor is useful in calculating the gain needed

in the differential amplifier.

Figure 42. Anisotropic Magnetoresistive Wheatstone Bridge Sensor, (a),

and Thevenin Equivalent Circuit, (b)