Chapter2 HardwareOverview
NI5911 User Manual 2-6 ni.com
Likeany other type of converter that uses noise shaping to enhance
resolution, the frequencyrespon se oft he converteris only flat to its
maximum useful bandwidth. The NI 5911 has a bandwidth of 4 MHz.
Beyondthis 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 preventaliasing. However,
if the applied signal contains major signal components in this frequency
range, such as harmonics or noise, the convertermay 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
higherfrequ encies. A digital lowpass filter applied to the data removes all
buta fractionof the original shaped quantization noise. The signal is then
resampledto a lower sampling frequency and a higher resolution. Flexible
resolution provides antialiasing protection due to the digital lowpass filter.
Calibration
The NI5911 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
driftscaused by environmental temperature changes.Y oucan internally
calibrateyour NI 5911 without any external equipment connected. External
calibration recalibrates the device when the specifiedcalibration interval
has expired. See Appendix A, Specifications, for the calibration interval.
Externalcalibration requires you to connect an external precision voltage
reference to the device.
20kS/s 20bits 8kHz
10kS/s 21bits 4kHz
Table2-2. Available Sampling Rates and Corresponding Bandwidth
inFlexible Resolution Mode (Continued)
Sampling Rate Resolution Bandwidth