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OP113FS Datasheet(PDF) 6 Page - Analog Devices |
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OP113FS Datasheet(HTML) 6 Page - Analog Devices |
6 / 16 page OP113/OP213/OP413 –6– REV. C A High Accuracy Thermocouple Amplifier Figure 4 shows a popular K-type thermocouple amplifier with cold-junction compensation. Operating from a single +12 volt supply, the OP113 family’s low noise allows temperature mea- surement to better than 0.02 °C resolution from 0°C to 1000°C range. The cold-junction error is corrected by using an inexpen- sive silicon diode as a temperature measuring device. It should be placed as close to the two terminating junctions as physically possible. An aluminum block might serve well as an isothermal system. 1/2 OP213 1 3 2 8 4 0V TO 10.00V (0 C TO 1000 C) +12V 0.1 F + 10 F R9 124k R8 453 R5 40.2k R1 10.7k R2 2.74k REF02EZ 0.1 F +12V 2 6 4 ++ –– D1 1N4148 R3 53.6 R4 5.62k +5.000V K-TYPE THERMOCOUPLE 40.7 V/ C R6 200 Figure 4. Accurate K-Type Thermocouple Amplifier R6 should be adjusted for a zero-volt output with the thermo- couple measuring tip immersed in a zero-degree ice bath. When calibrating, be sure to adjust R6 initially to cause the output to swing in the positive direction first. Then back off in the nega- tive direction until the output just stops changing. An Ultralow Noise, Single Supply Instrumentation Amplifier Extremely low noise instrumentation amplifiers can be built using the OP113 family. Such an amplifier that operates off a single supply is shown in Figure 5. Resistors R1–R5 should be of high precision and low drift type to maximize CMRR perfor- mance. Although the two inputs are capable of operating to zero volt, the gain of –100 configuration will limit the amplifier input common mode to not less than 0.33 V. VIN + – *R1 10k 1/2 OP213 1/2 OP213 *R2 10k *R3 10k *R4 10k VOUT +5V TO +36V *RG (200 + 12.7 ) *ALL RESISTORS 0.1%, 25ppm/ C GAIN = + 6 20k RG Figure 5. Ultralow Noise, Single Supply Instrumentation Amplifier A High Accuracy Linearized RTD Thermometer Amplifier Zero suppressing the bridge facilitates simple linearization of the RTD by feeding back a small amount of the output signal to the RTD (Resistor Temperature Device). In Figure 3 the left leg of the bridge is servoed to a virtual ground voltage by amplifier A1, while the right leg of the bridge is also servoed to zero-volt by amplifier A2. This eliminates any error resulting from common-mode voltage change in the amplifier. A three-wire RTD is used to balance the wire resistance on both legs of the bridge, thereby reducing temperature mismatch errors. The 5.000 V bridge excitation is derived from the extremely stable AD588 reference device with 1.5 ppm/ °C drift performance. Linearization of the RTD is done by feeding a fraction of the output voltage back to the RTD in the form of a current. With just the right amount of positive feedback, the amplifier output will be linearly proportional to the temperature of the RTD. 6 5 4 7 A2 R5 4.02k R7 100 8 +15V –15V 1/2 OP213 R4 100 R2 8.25k RG FULL SCALE ADJUST RW1 R1 8.25k R3 50 R8 49.9k R9 5k LINEARITY ADJUST @1/2 F.S. VOUT (10mV/ C) –1.50V = –150 C +5.00V = +500 C A1 3 2 1 RW2 RW3 1/2 OP213 100 RTD 6 4 13 11 12 7 9 8 10 16 2 14 15 1 3 +15V –15V 10 F AD588BD Figure 3. Ultraprecision RTD Amplifier To calibrate the circuit, first immerse the RTD in a zero-degree ice bath or substitute an exact 100 Ω resistor in place of the RTD. Adjust the ZERO ADJUST potentiometer for a 0.000 V output, then set R9 LINEARITY ADJUST potentiometer to the middle of its adjustment range. Substitute a 280.9 Ω resistor (equivalent to 500 °C) in place of the RTD, and adjust the FULL-SCALE ADJUST potentiometer for a full-scale voltage of 5.000 V. To calibrate out the nonlinearity, substitute a 194.07 Ω resistor (equivalent to 250 °C) in place of the RTD, then adjust the LINEARITY ADJUST potentiometer for a 2.500 V output. Check and readjust the full-scale and half-scale as needed. Once calibrated, the amplifier outputs a 10 mV/ °C temperature coefficient with an accuracy better than ±0.5°C over an RTD measurement range of –150 °C to +500°C. Indeed the amplifier can be calibrated to a higher temperature range, up to 850 °C. |
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