© National Instruments Corporation 11 NI USB-9233 User Guide and Specifications
NI USB-9233 Circuitry
The NI USB-9233 analog input channels are referenced to chassis ground
through a 50 Ω resistor. Because the chassis ground for the NI USB-9233
is provided through the USB cable, the grounding of the host computer can
affect your measurement. Each channel is protected from overvoltages.
The NI USB-9233 provides an IEPE excitation current for each input
signal. The signal is AC coupled, buffered, and conditioned. The signal is
then sampled by 24-bit ADCs. The NI USB-9233 IEPE excitation current
and AC coupling are always enabled.
Figure 9. Input Circuitry for One Channel
Understanding NI USB-9233 Filtering
The NI USB-9233 uses a combination of analog and digital filtering to
provide an accurate representation of desirable signals while rejecting
out-of-band signals. The filters discriminate between signals based on
the frequency range, or bandwidth of the signal. The three important
bandwidths to consider are the passband, the stopband, and the alias-free
bandwidth.
The NI USB-9233 represents signals within the passband as accurately as
possible, as quantified primarily by passband ripple and phase nonlinearity.
The filters reject frequencies within the stopband as much as possible, as
quantified by stopband rejection. All signals that appear in the alias-free
bandwidth are either unaliased signals or signals that have been filtered by
at least the amount of the stopband rejection.
NI USB-9233
AI+
AI–
ADC
Amplifier
and
Prefilter
+
–
AC
Coupling
Excitation
Current
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