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Chapter 6 Theory of Analog Operation
PCI-4451/4452 User Manual 6-2
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National Instruments Corporation
The 90 dB dynamic range of the PCI-4451/4452 devices is the result of low
noise and distortion and makes possible high-accuracy measurements. The
devices have excellent amplitude flatness of ±0.1 dB, and have a maximum
total harmonic distortion (THD) specification of −92 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-4451/4452. 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-4451/4452 are immune to the DNL distortion associated with
conventional data acquisition devices.
Input Coupling
The PCI-4451/4452 has a software programmable switch to individually
configure each input channel for AC or DC coupling. If the switch is set for
DC, the capacitor is bypassed, and any DC offset present in the source
signal being used passes to the ADC. The DC configuration is preferred
because it places one less component in the signal path and thus has higher
fidelity. The DC configuration is recommended 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 (or bias voltage),
you must set the switch for 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. The input coupling switch can connect
the input circuitry to ground instead of to the signal source. This connection
is usually made during offset calibration, which is described in Chapter 5,
Calibration.
Calibration
The PCI-4451/4452 analog inputs have calibration adjustments. Onboard
calibration DACs remove the offset and gain errors for each channel. For
complete calibration instructions, refer to Chapter 5, Calibration.
User.book Page 2 Tuesday, April 14, 1998 10:20 AM