+

-

+

-

SIG +

SIG -

WITH 'R << R,

THUS,

(b)

2

R + 'R

R - 'R

SIG +

SIG -

R - 'R

R + 'R

(a)

LMV641

SNOSAW3C – SEPTEMBER 2007 – REVISED FEBRUARY 2013

www.ti.com

In this circuit, the use of a 9-volt 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 (via the magnetic

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

high current sensing applications (from a few milliamps and up to 20A) 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 43, 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 43.

Figure 44 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 44. Anisotropic Magnetoresistive Wheatstone Bridge Sensor, (a),

and Thevenin Equivalent Circuit, (b)

Using Thevenin’s Theorem, the bridge can be reduced to two voltage sources with series resistances.

ΔR is

normally very small in comparison to R, thus the Thevenin equivalent resistance, commonly called the source

magnetic field, all of the resistances are considered equal. The voltage at Sig+ and Sig

and Sig+ - Sig

− = 0. Bridges are designed such that, when immersed in a magnetic field, opposite resistances in

the bridge change by ±

ΔR with an amount proportional to the strength of the magnetic field. This causes the

active elements, the output voltage is:

14

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