Datasheet

+
-
+
-
R/
2
R/
2
SIG +
SIG -
WITH 'R << R,
THEN R
TH
|
R/
2
THUS,
(b)
V
TH±
=
V
EXC
± V
SIG
2
R + 'R R - 'R
SIG +
SIG -
R - 'R
R + 'R
V
EXC
(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
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 resistances are considered equal. The voltage at Sig+ and Sig− is half V
EXC
, or 4.5V,
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
bridge's output differential voltage, to change from its half V
EXC
value. Thus Sig+ - Sig− = Vsig ≠ 0. With four
active elements, the output voltage is:
14 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LMV641