User Guide

Chapter 2 Hardware Overview

Input Ranges

To optimize the ADC resolution, you can select different gains for the

PGIA. In this way, you can scale your input signal to match the full input

range of the converter. The NI 5911 PGIA offers seven different input

ranges, from ±0.1 V to ±10 V, as shown in Table 2-1.

Note

If you try to acquire a signal below the set input range the sensitive front-end

components of the NI 5911 may become unstable and begin returning invalid data. To

return the digitizer to a stable configuration, switch to the maximum input range setting and

acquire an AC-coupled or 0 V signal.

Input Impedance

The input impedance of the NI 5911 PGIA is 1 MΩ between the positive

and negative input, ±2% depending on input capacitance. The output

impedance of the device connected to the NI 5911 and the input impedance

of the NI 5911 form an impedance divider, which attenuates the input

signal according to the following formula:

where V

is the measured voltage, V

is the source voltage, R

is the external

source impedance, and R

is the input impedance.

If the device you are measuring has a very large output impedance, your

measurements will be affected by this impedance divider. For example,

Table 2-1. Input Ranges for the NI 5911

Range Input Protection Threshold

±10 V ±10 V

±5 V ±5 V

±2 V ±5 V

±1 V ±5 V

±0.5 V ±5 V

±0.2 V ±5 V

±0.1 V ±5 V

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