National Instruments NI 5911 User Manual

Also see for NI 5911: Getting started guide

Contents

National Instruments NI 5911 User Manual Manual pdf 17 page image

Chapter 2 Hardware OverviewNI 5911 User Manual 2-6 ni.comLike any other type of converter that uses noise shaping to enhanceresolution, the frequency response of the converter is only flat to itsmaximum useful bandwidth. The NI 5911 has a bandwidth of 4 MHz.Beyond this frequency, there is a span where the converter acts resonantand where a signal is amplified before being converted. These signals areattenuated in the subsequent digital filter to prevent aliasing. However,if the applied signal contains major signal components in this frequencyrange, such as harmonics or noise, the converter may overload and signaldata will be invalid. In this case, you will receive an overload warning. Youmust then either select a higher input range or attenuate the signal.How Flexible Resolution WorksThe ADC can be sourced through a noise shaping circuit that movesquantization noise on the output of the ADC from lower frequencies tohigher frequencies. A digital lowpass filter applied to the data removes allbut a fraction of the original shaped quantization noise. The signal is thenresampled to a lower sampling frequency and a higher resolution. Flexibleresolution provides antialiasing protection due to the digital lowpass filter.Table 2-2. Available Sampling Rates and Corresponding Bandwidthin Flexible Resolution ModeSampling Rate Resolution Bandwidth12.5 MS/s 11 Bits 3.75 MHz5 MS/s 14 Bits 2 MHz2.5 MS/s 15.5 Bits 1 MHz1 MS/s 17.5 Bits 400 kHz500 kS/s 18 Bits 200 kHz200 kS/s 18.5 Bits 80 kHz100 kS/s 19 Bits 40 kHz50 kS/s 19.5 Bits 20 kHz20 kS/s 20.5 Bits 8 kHz10 kS/s 21 Bits 4 kHz