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OP177GSZ-REEL データシート(PDF) 9 Page - Analog Devices |
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OP177GSZ-REEL データシート(HTML) 9 Page - Analog Devices |
9 / 16 page OP177 Rev. E | Page 9 of 16 APPLICATION INFORMATION GAIN LINEARITY The actual open-loop gain of most monolithic op amps varies at different output voltages. This nonlinearity causes errors in high closed-loop gain circuits. It is important to know that the manufacturer’s AVO specifica- tion is only a part of the solution because all automated testers use endpoint testing and, therefore, show only the average gain. For example, Figure 23 shows a typical precision op amp with a respectable open-loop gain of 650 V/mV. However, the gain is not constant through the output voltage range, causing non- linear errors. An ideal op amp shows a horizontal scope trace. Figure 24 shows the OP177 output gain linearity trace with its truly impressive average AVO of 12,000 V/mV. The output trace is virtually horizontal at all points, assuring extremely high gain accuracy. Analog Devices also performs additional testing to ensure consistent high open-loop gain at various output voltages. Figure 25 is a simple open-loop gain test circuit. AVO ≥ 650V/mV RL = 2kΩ VX –10V 0V +10V Figure 23. Typical Precision Op Amp VY VX –10V 0V +10V AVO ≥ 12000V/mV RL = 2kΩ Figure 24. Output Gain Linearity Trace – + VY VX 10kΩ 10kΩ 1MΩ 10Ω RL VIN = ±10V OP177 Figure 25. Open-Loop Gain Linearity Test Circuit THERMOCOUPLE AMPLIFIER WITH COLD- JUNCTION COMPENSATION An example of a precision circuit is a thermocouple amplifier that must accurately amplify very low level signals without introducing linearity and offset errors to the circuit. In this circuit, an S-type thermocouple with a Seebeck coefficient of 10.3 μV/°C produces 10.3 mV of output voltage at a temperature of 1000°C. The amplifier gain is set at 973.16, thus, it produces an output voltage of 10.024 V. Extended temperature ranges beyond 1500°C are accomplished by reducing the amplifier gain. The circuit uses a low cost diode to sense the temperature at the terminating junctions and, in turn, compensates for any ambient temperature change. The OP177, with its high open- loop gain plus low offset voltage and drift, combines to yield a precise temperature sensing circuit. Circuit values for other thermocouple types are listed in Table 5. Table 5. Thermocouple Type Seebeck 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 S 10.3 μV/°C 100 Ω 20.5 kΩ 392 kΩ 1.07 MΩ VOUT –15V 10µF 0.1µF +15V 10µF 0.1µF R4 50Ω 1% R5 100Ω (ZERO ADJUST- MENT) ANALOG GROUND ANALOG GROUND 10µF R8 1.0kΩ 0.05% + 10µF COPPER COPPER ISOTHERMAL BLOCK COLD-JUNCTION COMPENSATION REF01 2.2µF + +15V 6 4 2 10.000V – + TYPES ISOTHERMAL COLD- JUNCTIONS – + OP177 R1 100Ω 1% R2 20.5kΩ 1% R3 47kΩ 1% R7 392kΩ 1% R9 1.07MΩ 0.05% Figure 26. Thermocouple Amplifier with Cold Junction Compensation |
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