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Input Coupling
You can configure the FieldDAQ device for either AC or DC coupling in software.
•
DC coupling
—Passes any DC offset present in the source signal to the ADC. Choose this
configuration when the signal source has only small amounts of offset voltage or if the
DC content of the acquired signal is important.
•
AC coupling
—Enables a highpass resistor-capacitor (RC) filter into the signal
conditioning path. The filter time constant is 300 ms. The highpass RC filter settles to
0.5% accuracy in 1.6 s in response to a step input. It takes 5 s to settle to 24-bit accuracy
in response to a step input. Signal output impedances greater than 100 kΩ can noticeably
lengthen settling times. Choose this configuration to take full advantage of the input
dynamic range for sources with a significant amount of unwanted offset.
IEPE Excitation
You can program the FieldDAQ device to supply 4 mA of IEPE excitation current to an AI
channel for applications using an IEPE accelerometer or microphone. You can independently
configure IEPE signal conditioning on a per-channel basis on the FieldDAQ device.
When you enable IEPE signal conditioning, the sensor generates a DC voltage offset that is
dependent on its design; enable AC coupling, described in
, to remove this
unwanted DC offset.
Open/Short Circuit Detection
When you enable IEPE signal conditioning, the FieldDAQ device monitors each channel for
open sensor and shorted sensor conditions:
•
Open Circuit Detection
—Indicates whether a sensor is connected. An open circuit is
detected when the IEPE excitation current falls below 4 mA. When an open circuit is
detected, the Open Current Loop status is set in NI-DAQmx and the Signal Input LED
lights red.
•
Short Circuit Detection
—Indicates if a sensor or wiring has shorted. A short circuit is
detected when the voltage between AI+ and AI- drops below 180 mV. Refer to the
FD-11634 Specifications
for additional short circuit thresholds. When a short circuit is
detected, the Overcurrent status is set in NI-DAQmx and the Signal Input LED lights red.
Filtering
You can configure a filter for each bank on the FieldDAQ device. FieldDAQ supports three
types of lowpass filtering:
•
Brickwall (default)
•
Butterworth
•
Comb
FD-11634 User Guide
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© National Instruments
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