LMV641 datasheet(13/18 Pages) NSC

13 / 18 page

In this circuit, the use of a 9-volt alkaline battery exploits the

LMV641’s high voltage and low supply current for a low pow-

er, 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 cali-

brated based on the final displacement of the sensed con-

ductor relative to the measurement bridge. Typically, once the

sensor has been oriented properly, with respect to the con-

ductor 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 4, U1 is configured as a single differential input am-

plifier. Its input impedance is relatively low, however, and

requires that the source impedance of the sensor be consid-

ered 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 4.

Figure 5 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 ampli-

fier.

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FIGURE 5. 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 re-

sistance, commonly called the source resistance, can be

taken to be R. When a bias voltage is applied between V

EXC

and ground, in the absence of a magnetic field, all of the re-

sistances are considered equal. The voltage at Sig+ and Sig

− is half V

EXC, or 4.5V, and Sig+ - Sig− = 0. Bridges are de-

signed such that, when immersed in a magnetic field, oppo-

site resistances in the bridge change by ±

ΔR with an amount

proportional to the strength of the magnetic field. This causes

the bridge's output differential voltage, to change from its half

EXC value. Thus Sig+ - Sig− = Vsig ≠ 0. With four active

elements, the output voltage is:

Since

ΔR is proportional to the field strength, B

S, the amount

of output voltage from the sensor is a function of sensor sen-

sitivity,

This

expression

can

rewritten

SIG = VEXC · S · BS, where

S = material constant (nominally 1 mV/V/gauss)

S = magnetic flux in gauss

A simplified schematic of a single op amp, differential ampli-

fier is shown in Figure 6. The Thevenin equivalent circuit of

the sensor can be used to calculate the gain of this amplifier.

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FIGURE 6. Differential Input Amplifier

The Honeywell HMC1051Z AMR sensor has nominal 1 k

Ω

elements and a sensitivity of 1 mV/V/gauss and is being used

with 9V of excitation with a full scale magnetic field range of

±6 gauss. At full-scale, the resistors will have

ΔR

≈ 12Ω and

108 mV will be seen from Sig− to Sig+ (refer to Figure 7).

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