外文翻譯---降低測(cè)量噪聲的五個(gè)技巧-展示頁(yè)
2025-01-27 00:02本頁(yè)面
【正文】 h a system would pletely reject any ACcoupled noise. A practical device, however, specifies the degree to which it can reject monmode voltage with a monmode rejection ratio (CMRR). The CMRR is the ratio of the measured signal gain to the monmode gain applied by the amplifier, as noted by the following equation:Choosing a DAQ device with a better CMRR over a broader range of frequencies can make a significant difference in your system’s overall noise immunity. For example, Figure 3 shows the CMRR for a lowcost M Series device pared with that of an industrial M Series device.Figure 3 The NI 6230 provides a much higher CMRR than the NI 6220 relative to earth ground.At 60 Hz, NI 6230 industrial M Series devices have 20 dB greater CMRR than NI 6220 lowcost M Series devices. This is equivalent to a 10 times better attenuation of 60 Hz noise.Any application may benefit from rejecting 60 Hz noise. However, those with large rotating machinery or motors require noise immunity at higher frequencies. At 1 kHz, NI 6230 devices reject noise 100 times better than NI 6220 devices, making them ideal for industrial applications.C. Break Ground LoopsGround loops are arguably the most mon source of noise in data acquisition systems. Proper grounding is essential for accurate measurements, yet it is a frequently misunderstood concept. A ground loop forms when two connected terminals in a circuit are at different ground potentials. This difference causes a current to flow in the interconnection, which can produce offset errors. Further plicating matters, the voltage potential between signal source ground and DAQ device ground is generally not a DC level. This results in a signal that reveals powerline frequency ponents in the readings. Consider the simple thermocouple application in Figure 4.Figure 4 A differential thermocouple measurement with a grounded signal source can create a ground loop.Here, an otherwise straightforward temperature measurement is plicated by the device under test (DUT) being at a different ground potential than that of the DAQ device. Though both devices share the same building ground, the difference in ground potential could be 200 mV or more if the power distribution circuits are not properly connected. The difference appears as a monmode voltage with an AC ponent in the resulting measurement.Recall that isolation is a means of electrically separating signal source ground from the instrumentation amplifier ground reference (see Figure 5).Figure 5 Isolation eliminates ground loops by separating earth ground from the amplifier ground reference.Because current cannot flow across the isolation barrier, the amplifier ground reference can be at a higher or lower potential than earth ground. You cannot inadvertently create a ground loop with this circuit. Using an isolated measurement device removes the ambiguity of properly grounding a measurement system, ensuring more accurate results.D. Use 420 mA Current LoopsLong cable lengths and the presence of noise in industrial or electrically harsh environments can make accurate voltage measurements difficult. As a result, industrial transducers that sense pressure, flow, proximity, and so on often emit current signals instead of voltage. A 420 mA current loop is
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