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OP177 Scheda tecnica(PDF) 8 Page - Analog Devices |
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OP177 Scheda tecnica(HTML) 8 Page - Analog Devices |
8 / 12 page OP177 APPLICATIONS 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 specifi- cation is only a part of the solution, since all automated testers use endpoint testing and, therefore, only show the average gain. For example, Figure 22 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 nonlinear errors. An ideal op amp would show a horizontal scope trace. Figure 22. Typical Precision Op Amp Figure 23. OP177’s Output Gain Linearity Trace Figure 24. Open-Loop Gain Linearity Test Circuit Figure 23 shows the OP177’s output gain linearity trace with its truly impressive average AVO of 12000 V/mV. The output trace is virtually horizontal at all points, assuring extremely high gain accuracy. PMI also performs additional testing to insure consistent high open-loop gain at various output voltages. Figure 24 is a simple open-loop gain test circuit for your own evaluation. THERMOCOUPLE AMPLIFIER WITH COLD-JUNCTION COMPENSATION An example of a precision circuit is a thermocouple amplifier that must amplify very low level signals accurately without introducing linearity and offset errors to the circuit. In this circuit, an S-type thermocouple, which has a Seebeck coefficient of 10.3 µV/°C, produces 10.3 mV of output voltage at a temperature of 1,000°C. The amplifier gain is set at 973.16. Thus, it will produce an output voltage of 10.024 V. Extended temperature ranges to beyond 1,500°C can be 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 very precision temperature sensing circuit. Cir- cuit values for other thermocouple types are shown in Table I. Table I. Thermo- Seebeck couple 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 S 10.3 µV/°C 100 Ω 20.5 kΩ 392 kΩ 1.07 MΩ Figure 25. Thermocouple Amplifier with Cold Junction Compensation PRECISION HIGH GAIN DIFFERENTIAL AMPLIFIER The high gain, gain linearity, CMRR, and low TCVOS of the OP177 make it possible to obtain performance not previously available in single stage, very high-gain amplifier applications. See Figure 26. For best CMR, R1 R2 must equal R3 R4 . In this example, with a 10 mV differential signal, the maximum errors are as listed in Table II. |
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