National Instruments PCI-445 Series User Manual page 69

Chapter 6 Theory of Analog Operation
PCI-4451/4452/4453/4454 User Manual
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PCI-4453/4454
The PCI-4453 has two identical analog input channels. The PCI-4454 has
four identical analog input channels. An analog input channel is illustrated
in Figure 4-5, Analog Input Stage of the
channels have 16-bit resolution and are simultaneously sampled at
software-programmable rates from 5 to 51.2 kS/s in 47.484 µS/s
increments. This flexibility in sample rates makes the device well suited for
a wide variety of applications, including audio and vibration analysis. The
analog inputs have AC/DC coupling. The input stage has been designed for
high performance measurement of single-ended signals.
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PCI-4451/4452/4453/4454
The analog inputs have both analog and real-time digital filters
implemented in hardware to prevent aliasing. Input signals first pass
through lowpass analog filters to attenuate signals with frequency
components beyond the range of the ADCs. Then digital antialiasing filters
automatically adjust their cutoff frequency to remove frequency
components above half the programmed sampling rate. These filters cause
a delay of 42 conversion periods between the input analog data and the
digitized data.
The 95 dB dynamic range of the PCI-445X is the result of low noise and
distortion and makes possible high-accuracy measurements. The device has
an amplitude flatness of ±0.1 dB, and have a maximum total harmonic
distortion (THD) specification of −90 dB at 1 kHz and a worst case THD
of −80 dB at higher frequencies.
State-of-the-art, 128-times oversampling, delta-sigma modulating ADCs
achieve the low noise and low distortion of the PCI-445X. Because these
ADCs sample at 128 times the specified sampling rate with 1-bit resolution,
they produce nearly perfect linearity. Extremely flat, linear-phase, lowpass
digital filters then remove the quantization noise from outside the band of
interest, divide the sample rate by 128, and increase the resolution to
16 bits. Using the delta-sigma modulating ADCs, the PCI-445X is immune
to the DNL distortion associated with conventional data acquisition
devices.
PCI-4453/4454. These input
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