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Chapter 2 Hardware Overview
NI 5911 User Manual 2-6 ni.com
Like any other type of converter that uses noise shaping to enhance
resolution, the frequency response of the converter is only flat to its
maximum useful bandwidth. The NI 5911 has a bandwidth of 4 MHz.
Beyond this frequency, there is a span where the converter acts resonant
and where a signal is amplified before being converted. These signals are
attenuated in the subsequent digital filter to prevent aliasing. However,
if the applied signal contains major signal components in this frequency
range, such as harmonics or noise, the converter may overload and signal
data will be invalid. In this case, you will receive a warning signaling
overload. You then need to either select a higher input range or attenuate
the signal.
How Flexible Resolution Works
The ADC can be sourced through a noise shaping circuit that moves
quantization noise on the output of the ADC from lower frequencies to
higher frequencies. A digital lowpass filter applied to the data removes all
but a fraction of the original shaped quantization noise. The signal is then
resampled to a lower sampling frequency and a higher resolution. Flexible
resolution provides antialiasing protection due to the digital lowpass filter.
Calibration
The NI 5911 can be calibrated for very high accuracy and resolution due
to an advanced calibration scheme. There are two different types of
calibration: Internal, or self, calibration and external calibration. Internal
calibration is performed via a software command that compensates for
drifts caused by environmental temperature changes. You can internally
calibrate yourNI 5911 without anyexternal equipment connected. External
calibration recalibrates the device when the specified calibration interval
has expired. See Appendix A, Specifications, for the calibration interval.
External calibration requires you to connect an external precision voltage
reference to the device.
20 kS/s 20 bits 8kHz
10 kS/s 21 bits 4kHz
Table 2-2. Available Sampling Rates and Corresponding Bandwidth
in Flexible Resolution Mode (Continued)
Sampling Rate Resolution Bandwidth