LMV641 datasheet(15/27 Pages) TI1

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AVCL =

RTHEV + R2

= 23.2

1012:

988:

SIG + = 4.554V

988:

1012:

SIG - = 4.446V

VSIG = 108 mV

SIG -

SIG +

VO = [(SIG + ) ± (SIG -)]

R1 = R2 = R3 = R4

VSIG = VEXC x

LMV641

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SNOSAW3C – SEPTEMBER 2007 – REVISED FEBRUARY 2013

(2)

Since

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 VSIG = VEXC · S · BS, where

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

BS = magnetic flux in gauss

A simplified schematic of a single op amp, differential amplifier is shown in Figure 45. The Thevenin equivalent

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

Figure 45. 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 46).

Figure 46. Sensor Output with No Load

Referring to the simplified diagram in Figure 45, and assuming that required full scale at the output of the

amplifier is 2.5V, a gain of 23.2 is needed for U1. It is clear from the Thevenin equivalent circuit in Figure 47 that

a sensor Thevenin equivalent source resistance, RTHEV, of 500Ω will be in series with both the inverting and non-

inverting inputs of the LMV641. Therefore, the required gain is:

(3)

Choosing R1 = R2 = 24.5 kΩ, then R4 will be approximately 580 kΩ. The actual values chosen will depend on the

full-scale needs of the succeeding circuitry as well as bandwidth requirements. The values shown here provide a

−3 dB bandwidth of approximately 431 kHz, and are found as follows.

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