Datasheet OP177 (Analog Devices) - 9

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Data Sheet OP177 APPLICATIONS INFORMATION GAIN LINEARITY THERMOCOUPLE AMPLIFIER WITH COLD-
The actual open-loop gain of most monolithic operational
JUNCTION COMPENSATION
amplifiers varies at different output voltages. This nonlinearity An example of a precision circuit is a thermocouple amplifier causes errors in high closed-loop gain circuits. that must accurately amplify very low level signals without It is important to know that the manufacturer’s A introducing linearity and offset errors to the circuit. In this VO specifica- tion is only a part of the solution because al automated testers circuit, an S-type thermocouple with a Seebeck coefficient of use endpoint testing and, therefore, show only the average gain. 10.3 μV/°C produces 10.3 mV of output voltage at a temperature For example, Figure 24 shows a typical precision operational of 1000°C. The amplifier gain is set at 973.16, thus, it produces amplifier with a respectable open-loop gain of 650 V/mV. an output voltage of 10.024 V. Extended temperature ranges However, the gain is not constant through the output voltage beyond 1500°C are accomplished by reducing the amplifier range, causing nonlinear errors. An ideal operational amplifier gain. The circuit uses a low cost diode to sense the temperature shows a horizontal scope trace. at the terminating junctions and, in turn, compensates for any ambient temperature change. The OP177, with the high open- Figure 25 shows the OP177 output gain linearity trace with the loop gain plus low offset voltage and drift, combines to yield a truly impressive average AVO of 12,000 V/mV. The output trace precise temperature sensing circuit. Circuit values for other is virtually horizontal at all points, assuring extremely high gain thermocouple types are listed in Table 5. accuracy. Analog Devices, Inc., also performs additional testing to ensure consistent high open-loop gain at various output
Table 5.
voltages. Figure 26 is a simple open-loop gain test circuit.
Thermocouple Seebeck Type Coefficient R1 R2 R7 R9
K 39.2 μV/°C 110 Ω 5.76 kΩ 102 kΩ 269 kΩ J 50.2 μV/°C 100 Ω 4.02 kΩ 80.6 kΩ 200 kΩ
VX
S 10.3 μV/°C 100 Ω 20.5 kΩ 392 kΩ 1.07 MΩ
–10V 0V +10V 2 6 10.000V +15V REF01 4 2.2µF R R R7 9 3 392kΩ
023
1.07MΩ A 47kΩ VO ≥ 650V/mV + 1% 0.05% R 1% L = 2kΩ
00289-
+15V
Figure 24. Typical Precision Operational amplifier
10µF 0.1µF + VY R2 R ISOTHERMAL 8 10µF 20.5kΩ COLD- 1.0kΩ JUNCTIONS 1% 0.05% – – TYPES COPPER V R X 5 OP177 VOUT + COPPER 100Ω –10V 0V +10V (ZERO + 10µF ISOTHERMAL ADJUST- BLOCK MENT) R1 10µF 0.1µF 100Ω R
024
4 A COLD-JUNCTION 1% VO ≥ 12000V/mV 50Ω R COMPENSATION L = 2kΩ
00289-
1%
Figure 25. Output Gain Linearity Trace
–15V ANALOG GROUND
026
VY ANALOG GROUND
00289-
10kΩ 10kΩ
Figure 27. Thermocouple Amplifier with Cold Junction Compensation
1MΩ VIN = ±10V VX – 10Ω OP177 + RL
025 00289- Figure 26. Open-Loop Gain Linearity Test Circuit Rev. H | Page 9 of 16 Document Outline FEATURES PIN CONFIGURATION GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM REVISION HISTORY SPECIFICATIONS ELECTRICAL CHARACTERISTICS TEST CIRCUITS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS APPLICATIONS INFORMATION GAIN LINEARITY THERMOCOUPLE AMPLIFIER WITH COLD-JUNCTION COMPENSATION PRECISION HIGH GAIN DIFFERENTIAL AMPLIFIER ISOLATING LARGE CAPACITIVE LOADS BILATERAL CURRENT SOURCE PRECISION ABSOLUTE VALUE AMPLIFIER PRECISION POSITIVE PEAK DETECTOR PRECISION THRESHOLD DETECTOR/AMPLIFIER OUTLINE DIMENSIONS ORDERING GUIDE