外文翻譯---降低測量噪聲的五個(gè)技巧-在線瀏覽
2025-03-07 00:02本頁面
【正文】 a mon method of sending sensor information over long distances in many processmonitoring applications, as shown in Figure 6.Figure 6 An instrumentation amplifier uses a shunt resistor to convert process current signals into voltage.Each of these current loops contains three ponents – a sensor, a power source, and one or more DAQ devices. The current signal from the sensor is typically between 4 and 20 mA, with 4 mA representing the lowest signal value and 20 mA representing the maximum. This transmission scheme has the advantage of using 0 mA to indicate an open circuit or bad connection. Power supplies are typically in the range of 24 to 30 VDC, depending on the total amount of voltage dropped along the circuit. Finally, the DAQ device uses a highprecision shunt resistor between the leads of the instrumentation amplifier to convert the current signal into a voltage measurement. Because all the current that flows from one lead of the power supply must return to the other, current loop signals are immune to most sources of electrical noise and voltage (IR) drops along extensive cable lengths. Furthermore, the leads that provide power to the sensor also carry the measurement signal, greatly simplifying field wiring.An isolation barrier such as the one shown in Figure 6 provides two main benefits in current loop applications. First, because power supply voltages typically exceed the maximum input range of most instrumentation amplifiers, isolation is essential for levelshifting the amplifier ground away from earth ground to an acceptable voltage. Second, current loops operate on the principal that current never leaves the circuit. Therefore, isolating the current loop from any path to ground prevents degradation of the signal. Devices such as the NI 6238 and NI 6239 industrial M Series DAQ devices provide a builtin shunt resistor and up to 60 VDC of isolation from earth ground for 420 mA current loop applications.E. Use 24 V Digital LogicMeasurement noise is not limited to analog signals. Digital logic may also be affected by a noisy electrical environment, possibly indicating false on/off values or accidental triggers. There are many voltage levels and logic families associated with digital I/O, some more noise resistant than others. Transistortransistor logic (TTL) is by far the most mon logic family, powering everything from microprocessors to LEDs. Though it is widely available, TTL may not always be the best choice for all digital applications.For industrial applications, TTL has the inherent disadvantage of small noise margins. With high and lowlogic levels of V and V, respectively, there is little room for error. For example, the lowlevel noise margin for a TTL input is V (the difference between V, the maximum lowlevel TTL input, and V, the maximum lowlevel TTL output). Any noise coupled to the digital signal in excess of V may shift the voltage into the undefined region between V and V. Here, the behavior of the digital input is uncertain and may produce incorrect values (see Figure 7).Figure 7 24V logic has better noise margins than TTL.24 V logic, however, offers increased noise margins and better overall noise immunity. Because most industrial sensors, actuators, and control logic already operate off 24
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