OP1177/OP2177/OP4177
+15V
where δ = ΔR/R is the fractional deviation of the RTD resistance
with respect to the bridge resistance due to the change in temper-
ature at the RTD.
500Ω
200Ω
0.1µF
ADR421
4.37kΩ
For δ << 1, the preceding expression becomes
4.12kΩ
⎛
⎜
⎞
⎟
6
V
R2
R
δ
4.12kΩ
100Ω
OUT
⎛
⎜
⎝
⎞
⎟
⎠
1/2
OP2177
7
⎜
⎟
V ≅
V
=
O
REF
R1 R1
R
⎜
⎜
⎝
⎟
⎟
⎠
5
1+
+
R2
100Ω
20Ω
R2
R
R1
R2
R1
⎞
R2
⎠
⎡
⎤
⎛
⎜
⎝
⎞⎛
⎠⎝
⎞
⎠
⎛
⎜
⎝
1+
+
VREF δ
⎟⎜
⎟
⎟
⎢
⎥
⎣
⎦
5kΩ
49.9kΩ
100Ω
RTD
V+
8
With VREF constant, the output voltage is linearly proportional
to δ with a gain factor of
2
3
R2
R
R1
R2
R1
R2
⎡
⎠ ⎝
⎣
⎤
⎛
⎜
⎝
⎞ ⎛
⎞
⎟
⎠
⎛
⎜
⎝
⎞
⎟
⎠
1/2
OP2177
1
VREF
1+
+
V
⎟ ⎜
⎢
OUT
⎥
⎦
4
15V
V–
R
F
Figure 65. Low Power Linearized RTD Circuit
0.1µF
ADR421
V+
R
R
R
SINGLE OPERATIONAL AMPLIFIER BRIDGE
7
2
3
V
The low input offset voltage drift of the OP1177 makes it very
effective for bridge amplifier circuits used in RTD signal condi-
tioning. It is often more economical to use a single bridge
operational amplifier as opposed to an instrumentation amplifier.
OUT
6
R(1+δ)
OP1177
4
V–
R
F
In the circuit shown in Figure 66, the output voltage at the
operational amplifier is
Figure 66. Single Bridge Amplifier
⎡
⎢
⎤
⎥
⎥
⎥
⎛
⎜
⎞
⎟
R2
R
δ
R1
R2
⎜
⎜
⎟
⎟
⎢
VO =
VREF
R1
R
⎢
⎛
⎝
⎞
⎟
⎠
+ 1+
(1+ δ)
⎜
⎜
⎝
⎟
⎠
⎢
⎣
⎥
⎦
Rev. G | Page 20 of 24
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