Ni usb-6001/6002/6003 User Guide National Instruments
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| Signal Source Type
Analog Input Mode * Floating Signal Source (Not Connected to Building Ground) Ground-Referenced Signal Source † Examples: • Ungrounded thermocouples • Signal conditioning with isolated outputs • Battery devices Example:
• Plug-in instruments with non-isolated outputs Differential (DIFF) Referenced Single-Ended (RSE) * Refer to the Taking Differential Measurements section for descriptions of the RSE and DIFF modes and software considerations. † Refer to the Ground-Referenced Signal Sources section for more information. + –
– AI+
AI– AI GND
Signal Source DAQ Device + –
– AI+
AI– AI GND
Signal Source DAQ Device V cm
– + – AI AI GND
Signal Source DAQ Device Ground-loop potential (V A – V B ) are added to measured signal. + – + – AI AI GND V B V A DAQ Device Signal Source NI USB-6001/6002/6003 User Guide | | 15 Floating Signal Sources An instrument or device that has an isolated output is a floating signal source. A floating signal source is not connected to the building ground system, but has an isolated ground-reference point. Some examples of floating signal sources are outputs of transformers, thermocouples, battery-powered devices, optical isolators, and isolation amplifiers. For more information about field wiring and noise considerations for analog signals, go to ni.com/info and enter Info Code rdfwn3
. When to Use Differential Connections with Floating Signal Sources Use DIFF input connections for any channel that meets any of the following conditions: • The input signal is low level and requires greater accuracy. • The leads connecting the signal to the device are greater than 3 m (10 ft). • The input signal requires a separate ground-reference point or return signal. • The signal leads travel through noisy environments. • Two analog input channels, AI+ and AI-, are available for the signal. DIFF signal connections reduce noise pickup and increase common-mode noise rejection. DIFF signal connections also allow input signals to float within the working voltage of the device. With this type of connection, the instrumentation amplifier (IA) rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground. Refer to the Taking Differential Measurements section for more information about differential connections. When to Use Referenced Single-Ended (RSE) Connections with Floating Signal Sources Only use RSE input connections if the input signal meets all of the following conditions: • The input signal can share a common reference point, AI GND, with other signals that use RSE. • The leads connecting the signal to the device are less than 3 m (10 ft). DIFF input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions. In the single-ended modes, more electrostatic and magnetic noise couples into the signal connections than in DIFF configurations. The coupling is the result of differences in the signal path. Magnetic coupling is proportional to the area between the two signal conductors. Electrical coupling is a function of how much the electric field differs between the two conductors. Refer to the
section for more information about RSE connections.
16 | ni.com | NI USB-6001/6002/6003 User Guide Ground-Referenced Signal Sources A ground-referenced signal source is a signal source connected to the building system ground. It is already connected to a common ground point with respect to the device, assuming that the computer is plugged into the same power system as the source. Non-isolated outputs of instruments and devices that plug into the building power system fall into this category. The difference in ground potential between two instruments connected to the same building power system is typically between 1 and 100 mV, but the difference can be much higher if power distribution circuits are improperly connected. If a grounded signal source is incorrectly measured, this difference can appear as measurement error. Follow the connection instructions for grounded signal sources to eliminate this ground potential difference from the measured signal. For more information about field wiring and noise considerations for analog signals, go to ni.com/info and enter Info Code rdfwn3 .
Sources Use DIFF input connections for any channel that meets any of the following conditions: • The input signal is low level and requires greater accuracy. • The leads connecting the signal to the device are greater than 3 m (10 ft). • The input signal requires a separate ground-reference point or return signal. • The signal leads travel through noisy environments. • Two analog input channels, AI+ and AI-, are available for the signal. DIFF signal connections reduce noise pickup and increase common-mode noise rejection. DIFF signal connections also allow input signals to float within the working voltage of the device. Refer to the
section for more information about differential connections. When to Use Referenced Single-Ended (RSE) Connections with Ground-Referenced Signal Sources Do not use RSE connections with ground-referenced signal sources. Use differential connections instead. As shown in the bottom right cell of Table 6, there can be a potential difference between AI GND and the ground of the sensor. In RSE mode, this ground loop causes measurement errors.
NI USB-6001/6002/6003 User Guide | © National Instruments | 17 Input Range The NI DAQ device has an input range of ±10 V. For differential mode, each AI should stay within ±10 V with respect to AI GND, and the voltage between positive and negative inputs should be lower or equal to ±10 V. For RSE mode, signals of ±10 V at any analog input terminal with respect to AI GND are accurately measured. tải về 1.5 Mb. Chia sẻ với bạn bè của bạn:
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