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
datawill be invalid. In this case, you will receive an overload warning. You
must then 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.
Table2-2. Available Sampling Rates and Corresponding Bandwidth
inFlexible Resolution Mode
Sampling Rate Resolution Bandwidth
12.5MS/s 11 Bits 3.75 MHz
5MS/s 14Bits 2MHz
2.5 MS/s 15.5 Bits 1MHz
1MS/s 17.5Bits 400kHz
500 kS/s 18 Bits 200 kHz
200 kS/s 18.5 Bits 80 kHz
100 kS/s 19 Bits 40 kHz
50kS/s 19.5 Bits 20 kHz
20kS/s 20.5 Bits 8kHz
10kS/s 21 Bits 4kHz