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LM134H データシート(PDF) 6 Page - Linear Technology |
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LM134H データシート(HTML) 6 Page - Linear Technology |
6 / 12 page LM134 Series 6 Lead Resistance The sense voltage which determines the operating current of the LM134 is less than 100mV. At this level, thermo- couple or lead resistance effects should be minimized by locating the current setting resistor physically close to the device. Sockets should be avoided if possible. It takes only 0.7 Ω contact resistance to reduce output current by 1% at the 1mA level. Start-Up Time The LM134 is designed to operate at currents as low as 1 µA. This requires that internal biasing current be well below that level because the device achieves its wide operating current range by using part of the operating current as bias current for the internal circuitry. To ensure start-up, however, a fixed trickle current must be provided internally. This is typically in the range of 20nA to 200nA and is provided by the special ultralow IDDS FETs shown in the Schematic Diagrams as Q7 and Q8. The start-up time of the LM134 is determined by the IDSS of these FETs and the capacitor C1. This capacitor must charge to approxi- mately 500mV before Q3 turns on to start normal circuit operation. This takes as long as (500mV)(50pF)/(20nA) = 1.25ms for very low IDSS values. Using the LM134 as a Temperature Sensor Because it has a highly linear output characteristic, the LM134 makes a good temperature sensor. It is particularly useful in remote sensing applications because it is a current output device and is therefore not affected by long wire runs. It is easy to calibrate, has good long term stability and can be interfaced directly with most data acquisition systems, eliminating the expensive preampli- fiers required for thermocouples and platinum sensors. A typical temperature sensor application is shown in Figure 2. The LM134 operating current at 25 °C is set at 298 µA by the 226Ω resistor, giving an output of 1µA/°K. The current flows through the twisted pair sensor leads to the 10k termination resistor, which converts the current output to a voltage of 10mV/ °K referred to ground. The voltage across the 10k resistor will be 2.98V at 25 °C, with a slope of 10mV/ °C. The simplest way to convert this signal to a Centigrade scale is to subtract a constant 2.73V in software. Alternately, a hardware conversion can be used, as shown in Figure 3, using an LT1009 as a level shifter to offset the output to a Centigrade scale. The resistor (RSET) used to set the operating current of the LM134 in temperature sensing applications should have low temperature coefficient and good long term stability. A 30ppm/ °Cdriftintheresistorwillchangetheslopeofthe temperature sensor by 1%, assuming that the resistor is at the same temperature as the sensor, which is usually the case since the resistor should be located physically close to the LM134 to prevent errors due to wire resistance. A long term shift of 0.3% in the resistor will create a 1 °C temperature error. The long term drift of the LM134 is typically much better than this, so stable resistors must be used for best long term performance. Calibration of the LM134 as a temperature sensor is extremely easy. Referring to Figure 2, calibration is achieved by trimming the termination resistor. This theoretically trims both zero and slope simultaneously for Centigrade and Fahrenheit applications. The initial errors in the LM134 are directly proportional to absolute temperature, just like the actual output. This allows the sensor to be trimmed at any temperature and have the slope error be corrected at the same time. Residual slope error is typically less than 1% after this single trim is completed. Figure 2 Kelvin Temperature Sensor TO DATA ACQUISITION SYSTEM 10mV/ °K 9.53k 1k CALIBRATE 134 F02 V+ V– R LM234-3 RSET 226 Ω I = 1 µA/°K VS ≥ 5V APPLICATIO S I FOR ATIO |
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同様の説明 - LM134H |
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