Datasheet

A
VCL
=
R
4
R
THEV
+ R
2
= 23.2
1012: 988:
SIG + = 4.554V
988:
1012:
9V
SIG - = 4.446V
V
SIG
= 108 mV
-
+
SIG -
SIG +
V
O
= [(SIG + ) ± (SIG -)]
R
4
R
2
R
4
R
2
R
1
R
3
R
1
= R
2
= R
3
= R
4
V
SIG
= V
EXC
x
'R
R
LMV641
www.ti.com SNOSAW3C –SEPTEMBER 2007–REVISED FEBRUARY 2013
(2)
Since ΔR is proportional to the field strength, B
S
, the amount of output voltage from the sensor is a function of
sensor sensitivity, S. This expression can rewritten as V
SIG
= V
EXC
· S · B
S
, where
S = material constant (nominally 1 mV/V/gauss)
B
S
= 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 kΩ 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, R
THEV
, of 500Ω will be in series with both the inverting and non-
inverting inputs of the LMV641. Therefore, the required gain is:
(3)
Choosing R
1
= R
2
= 24.5 kΩ, then R
4
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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