OP1177_07 Datasheet, PDF (20/24 Pages) Analog Devices – Precision Low Noise, Low Input Bias Current Operational Amplifiers

OP1177/OP2177/OP4177
+15V
0.1µF
ADR421
4.12kΩ
4.37kΩ
500Ω
200Ω
4.12kΩ
100Ω
100Ω
20Ω
6
1/2
7
OP2177
5
V_OUT
100Ω
V+
RTD
5kΩ
49.9kΩ
2
8
1/2 1
OP2177
3
4
V_OUT
V–
Figure 65. Low Power Linearized RTD Circuit
SINGLE OPERATIONAL AMPLIFIER BRIDGE
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.
In the circuit shown in Figure 66, the output voltage at the
operational amplifier is
V_O
=
R2
R
⎡
⎢
⎢⎢^VREF
⎢⎣
⎜⎛
⎜
⎜
⎜⎝
δ
⎟⎞⎥⎤
⎟⎥
R1
R
+
⎜⎝⎛^{1 +}
R1
R2
⎟⎠⎞
(1 +
δ)
⎟⎥
⎟⎠⎥⎦
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.
For δ << 1, the preceding expression becomes
V_O
≅
⎜⎝⎛
R2
R
⎟⎠⎞
V_REF
⎜⎛
⎜
⎜⎜⎝ ¹⁺
δ
R1 ₊
R
R1
R2
⎟⎞
⎟
⎟⎟⎠
=
⎢⎣⎡⎜⎝⎛
R2
R
⎟⎠⎞
⎜⎝⎛¹
+
R1
R2
⎟⎠⎞
+
⎜⎝⎛
R1
R2
⎟⎠⎞⎥⎦⎤
V_REF
δ
With VREF constant, the output voltage is linearly proportional
to δ with a gain factor of
V_REF
⎜⎝⎛
R2
R
⎟⎠⎞
⎢⎣⎡⎜⎝⎛^{1 +}
R1
R2
⎟⎠⎞ ⁺ ⎜⎝⎛
R1
R2
⎟⎠⎞⎥⎦⎤
15V
R_F
0.1µF
ADR421
R
R
R(1+δ)
R
V+
2
7
6
OP1177
3
4
V–
V_OUT
R_F
Figure 66. Single Bridge Amplifier
Rev. E | Page 20 of 24