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OP295GS-REEL7 Datasheet(PDF) 11 Page - Analog Devices |
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OP295GS-REEL7 Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page OP295/OP495 Rev. E | Page 11 of 16 V+ – + – + 5 3 2 1 6 8 4 7 1/2 OP295/ OP495 1/2 OP295/ OP495 + – VIN VO R4 100kΩ R3 20kΩ R2 20kΩ R1 100kΩ VREF RG VO = (5+ 200kΩ) VIN+VREF RG Figure 23. Single-Supply Instrumentation Amplifier Resistor RG sets the gain of the instrumentation amplifier. Minimum gain is 6 (with no RG). All resistors should be matched in absolute value as well as temperature coefficient to maximize common-mode rejection performance and minimize drift. This instrumentation amplifier can operate from a supply voltage as low as 3 V. SINGLE-SUPPLY RTD THERMOMETER AMPLIFIER This RTD amplifier takes advantage of the rail-to-rail swing of the OP295/OP495 to achieve a high bridge voltage in spite of a low 5 V supply. The OP295/OP495 amplifier servos a constant 200 μA current to the bridge. The return current drops across the parallel resistors 6.19 kΩ and 2.55 MΩ, developing a voltage that is servoed to 1.235 V, which is established by the AD589 band gap reference. The 3-wire RTD provides an equal line resistance drop in both 100 Ω legs of the bridge, thus improving the accuracy. The AMP04 amplifies the differential bridge signal and converts it to a single-ended output. The gain is set by the series resis- tance of the 332 Ω resistor plus the 50 Ω potentiometer. The gain scales the output to produce a 4.5 V full scale. The 0.22 μF capacitor to the output provides a 7 Hz low-pass filter to keep noise at a minimum. ZERO ADJ AD589 37.4kΩ 5V 1.235 3 2 4 5 6 8 1 7 5V AMP04 50Ω 332Ω 0.22µF VO – + 2 3 1 + – 200Ω 10-TURNS 26.7kΩ 0.5% 26.7kΩ 0.5% 100Ω RTD 100Ω 0.5% 2.55MΩ 1% 6.19kΩ 1% 1/2 OP295/ OP495 4.5V = 450°C 0V = 0°C Figure 24. Low Power RTD Amplifier COLD JUNCTION COMPENSATED, BATTERY- POWERED THERMOCOUPLE AMPLIFIER The 150 μA quiescent current per amplifier consumption of the OP295/OP495 makes them useful for battery-powered temperature measuring instruments. The K-type thermocouple terminates into an isothermal block where the terminated junctions’ ambient temperatures can be continuously monitored and corrected by summing an equal but opposite thermal EMF to the amplifier, thereby canceling the error introduced by the cold junctions. AD589 ALUMEL CHROMEL AL CR 1N914 24.9kΩ 1.235V 9V 1.33MΩ 20kΩ 2 3 4 8 – + – + – + 1 COLD JUNCTIONS ISOTHERMAL BLOCK K-TYPE THERMOCOUPLE 40.7µV/°C VO 5V = 500°C 0V = 0°C OP295/ OP495 SCALE ADJUST 7.15kΩ 1% 1.5MΩ 1% 24.9kΩ 1% 475Ω 1% 2.1kΩ 1% 24.3kΩ 1% 4.99kΩ 1% 500Ω 10-TURN ZERO ADJUST Figure 25. Battery-Powered, Cold-Junction Compensated Thermocouple Amplifier To calibrate, immerse the thermocouple measuring junction in a 0°C ice bath and adjust the 500 Ω zero-adjust potentiometer to 0 V out. Then immerse the thermocouple in a 250°C tem- perature bath or oven and adjust the scale-adjust potentiometer for an output voltage of 2.50 V, which is equivalent to 250°C. Within this temperature range, the K-type thermocouple is quite accurate and produces a fairly linear transfer characteristic. Accuracy of ±3°C is achievable without linearization. Even if the battery voltage is allowed to decay to as low as 7 V, the rail-to-rail swing allows temperature measurements to 700°C. However, linearization may be necessary for temperatures above 250°C, where the thermocouple becomes rather nonlinear. The circuit draws just under 500 μA supply current from a 9 V battery. 5 V ONLY, 12-BIT DAC THAT SWINGS 0 V TO 4.095 V Figure 26 shows a complete voltage output DAC with wide output voltage swing operating off a single 5 V supply. The serial input, 12-bit DAC is configured as a voltage output device with the 1.235 V reference feeding the current output pin (IOUT) of the DAC. The VREF, which is normally the input, now becomes the output. The output voltage from the DAC is the binary weighted voltage of the reference, which is gained up by the output amplifier such that the DAC has a 1 mV per bit transfer function. |
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