National Instruments NI 5911 Measurement Modes

Chapter 2 Hardware Overview

NI 5911 User Manual 2-4

National Instruments Corporation

The inputs of the PGIA typically draw an input bias current of 1nA at

25°C. Attaching a device with a very high source impedance can cause an

offset voltage to be added to the signal you measure, according to the

formula Rs × 1 nA, where Rs is the external source impedance. For

example, if the device you have attached to the NI591 1 has an output

impedance of 10 kΩ, typically the offset voltage is 10µV (10 kΩ x 1 nA).

The NI5 911 features input-protection circuits that protect both the positive

and negative analog input from damage from AC and DC signals up to

±42V.

If the voltage at one of these inputs exceeds a threshold voltage, Vtr, the

input clamps to Vtr and a resistance of 100 kΩ is inserted in the path to

minimize input currents to a nonharmful level.

The protection voltage, Vtr, is input range dependent, as shown in

Table 2 -1.

When you need to measure a small AC signal on top of a large DC

component, you can use AC coupling. AC coupling rejects any DC

component in your signal before it enters into the PGIA. Activating AC

coupling inserts a capacitor in series with the input impedance. Input

coupling can be selected via software. See AppendixB, Digitizer Basics,

for more information on input coupling.

Measurement Modes

The ADC samples at a constant rate of 100M S/s with a vertical resolution

of 8 bits. Using random interleaved sampling (RIS), the sample rate can be

increased to 1GS/s. In this conventional mode of operation called

oscilloscope mode, the analog bandwidth is 100 MHz.

For sampling signals with lower bandwidth, 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. This mode, called flexible resolution

mode, provides antialiasing protection due to the digital lowpass filter.

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