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AD670 bảng dữ liệu(PDF) 10 Page - Analog Devices |
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AD670 bảng dữ liệu(HTML) 10 Page - Analog Devices |
10 / 13 page AD670 REV. A –9– APPLYING THE AD670 The AD670 has been designed for ease of use, system compat- ibility, and minimization of external components. Transducer interfaces generally require signal conditioning and preampli- fication before the signal can be converted. The AD670 will reduce and even eliminate this excess circuitry in many cases. To illustrate the flexibility and superior solution that the AD670 can bring to a transducer interface problem, the following dis- cussions are offered. Temperature Measurements Temperature transducers are one of the most common sources of analog signals in data acquisition systems. These sensors re- quire circuitry for excitation and preamplification/buffering. The instrumentation amplifier input of the AD670 eliminates the need for this signal conditioning. The output signals from tem- perature transducers are generally sufficiency slow that a sample/hold amplifier is not required. Figure 12 shows tile AD590 IC temperature transducer interfaced to the AD670. The AD580 voltage reference is used to offset the input for 0 °C calibration. The current output of the AD590 is converted into a voltage by R1. The high impedance unbuffered voltage is ap- plied directly to the AD670 configured in the –128 mV to 127 mV bipolar range. The digital output will have a resolution of 1 °C. Figure 12. AD670 Temperature Transducer lnterface Platinum RTDs are also a popular, temperature transducer. Typical RTDs have a resistance of 100 Ω at 0°C and change re- sistance 0.4 Ω per °C. If a consent excitation current is caused to flow in the RTD, the change in voltage drop will be a mea- sure of the change in temperature. Figure 13 shows such a method and the required connections to the AD670. The AD580 2.5 V reference provides the accurate voltage for the ex- citation current and range offsetting for the RTD. The op amp is configured to force a constant 2.5 mA current through the RTD. The differential inputs of the AD670 measure the differ- ence between a fixed offset voltage and the temperature depen- dent output of the op amp which varies with the resistance of the RTD. The RTD change of approximately 0.4 Ω/°C results in a 1 mV/ °C voltage change. With the AD670 in the 1 mV/LSB range, temperatures from 0 °C to 255°C can be measured. Figure 13. Low Cost RTD Interface Differential temperature measurements can be made using an AD590 connected to each of the inputs as shown in Figure 14. This configuration will allow the user to measure the relative temperature difference between two points with a 1 °C resolu- tion. Although the internal 1k and 9k resistors on the inputs have ±20% tolerance, trimming the AD590 is unnecessary as most differential temperature applications are concerned with the relative differences between the two. However, the user may see up to a 20% scale factor error in the differential temperature to digital output transfer curve. This scale factor error can be eliminated through a software cor- rection. Offset corrections can be made by adjusting for any dif- ference that results when both sensors are held at the same temperature. A span adjustment can then be made by immers- ing one AD590 in an ice bath and one in boiling water and eliminating any deviation from 100 °C. For a low cost version of this setup, the plastic AD592 can be substituted for the AD590. Figure 14. Differential Temperature Measurement Using the AD590 ˜ |
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