Generating Onboard Current Excitation With Iepe Circuitry; Input Coupling - National Instruments NI 447 Series User Manual

Generating Onboard Current Excitation with IEPE Circuitry

Input Coupling

© National Instruments Corporation
If you attach an IEPE accelerometer or microphone preamplifier to an AI
channel, you must enable the IEPE circuitry for that channel to generate the
required excitation current. The IEPE circuitry of any input channel can be
enabled or disabled independently of that of any other input channel.
When IEPE signal conditioning is enabled, large DC-offset voltages
can occur on signal inputs due to the output bias voltage requirements
of the IEPE transducer you are using. To remove this offset, you must
enable AC coupling on the affected input channels of the NI 447X. Using
DC coupling with IEPE 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
AI_CHANNEL_DSA_EXCITATION parameter.
You can configure each AI channel of the NI 447X to be
AC- or DC-coupled. If you select DC coupling, any DC offset present
in the source signal is passed to the A/D converter (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 should 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 for
the NI 447X (except the NI PXI-4472B). The −0.1 dB cutoff frequency is
approximately 22.2 Hz.
♦
NI PXI-4472B
The −3 dB cutoff frequency is approximately 0.5 Hz for the NI PXI-4472B.
The −0.1 dB cutoff frequency is approximately 3.2 Hz.
function with the
2-9
Chapter 2 Using the NI 447X
NI 447X User Manual