1012:

988:

SIG + = 4.554V

988:

1012:

9V

SIG - = 4.446V

VSIG = 108 mV

-

+

SIG -

SIG +

VO = [(SIG + ) ± (SIG -)]

R4

R2

R4

R2

R1

R3

R1 = R2 = R3 = R4

VSIG = VEXC x

'R

R

20

LMV641

SNOSAW3D – SEPTEMBER 2007 – REVISED AUGUST 2016

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Copyright © 2007–2016, Texas Instruments Incorporated

Typical Applications (continued)

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 VEXC and ground, in the absence of a

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

− is half VEXC, or 4.5 V,

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

active elements, the output voltage is:

(2)

Because ΔR is proportional to the field strength, BS, the amount of output voltage from the sensor is a function of

sensor sensitivity, S. This expression can rewritten as , where

VSIG = VEXC · S · BS

where

•

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

•

BS = magnetic flux in gauss

(3)

A simplified schematic of a single op amp, differential amplifier is shown in Figure 43. The Thevenin equivalent

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

Figure 43. Differential Input Amplifier

The Honeywell HMC1051Z AMR sensor has nominal 1-kΩ elements and a sensitivity of 1 mV/V/gauss and is

being used with 9 V of excitation with a full scale magnetic field range of ±6 gauss. At full-scale, the resistors will

have ΔR ≈ 12 Ω and 108 mV will be seen from Sig− to Sig+ (see Figure 44).

Figure 44. Sensor Output with No Load