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Chapter 2
Using Your NI 4472
Generating Onboard Current Excitation with ICP Circuitry
Input Coupling
NI 4472 User Manual
If you attach an ICP-type accelerometer or microphone preamplifier to an
analog input channel, you must enable the ICP circuitry for that channel in
order to generate the required excitation current. The ICP circuitry of any
input channel can be enabled or disabled independently of that of any other
input channel.
When ICP signal conditioning is enabled, large DC-offset voltages
can occur on signal inputs due to the output bias voltage requirements
of the ICP transducer you are using. To remove this offset you must enable
AC coupling on the affected input channels of the NI 4472. Using
DC coupling with ICP is appropriate only if the impedance of the sensor
does not exceed 2.5 kΩ and you are acquiring very low frequency signals.
In LabVIEW, use AI Parameter.vi to control the onboard excitation.
You can control onboard excitation only for the channels that are
configured for an acquisition in AI Config.vi. If you are programming in
the NI-DAQ text application program interface (API), you should use the
AI_Change_Parameter
parameter.
You can configure each analog input channel of the NI 4472 to be
AC- or DC-coupled. If you select DC coupling, any DC offset present
in the source signal is passed to the ADC. The DC-coupled configuration
is usually best if the signal source has only small amounts of offset
voltage (less than ±100 mV) or if the DC content of the acquired signal
is important.
If the source has a significant amount of unwanted offset (bias voltage),
you must select AC coupling to take full advantage of the input signal
range. Using AC coupling results in a drop in the low-frequency response
of the analog input. The −3 dB cutoff frequency is approximately 3.4 Hz,
but the −0.01 dB cutoff frequency, for instance, is considerably higher at
approximately 70.5 Hz.
function with the ND_DSA_EXCITATION
2-8
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