Chapter2 HardwareOverview
NI5911 User Manual 2-4 ni.com

Input Bias

The inputs of the PGIA typically draw an input bias current of 1 nA at
25 °C.Attaching a device with a very high source impedance can cause
an offsetvoltage to be added to the signalyou m easure,according to
the formula Rs×1nA,whereRsisthe externalsource im pedance. For
example, if thedevice you have attached to the NI 5911 has an output
impedance of 10 kΩ, typically the offset voltage is 10 µV (10 kΩ×1nA).

Input Protection

TheNI 5911features input-protection circuits that protect both the positive
and negative analog input from damage from AC and DC signals up to
±42 V.
Ifthe voltage at one of these inputs exceeds a threshold voltage, Vtr,the
input clamps to Vtr and a resistance of 100 kΩis inserted in the path to
minimize input currents to a nonharmful level.
Theprotection voltage, Vtr, is input rangedependent, as sh own in Table 2-1.

AC Coupling

When you needto measure a smallAC signal on top of a large DC
component, you can use AC coupling. AC coupling rejects any DC
componentin your signal before it enters into t hePGIA. A ctivating AC
coupling inserts a capacitor in series with the input impedance. Input
coupling canbe selected via software. See Appendix B, Digiti zer Basics,
for more information on input coupling.
Oscilloscope and Flexible Resolution Modes
In oscilloscope mode, the NI 5911 works as a conventional desktop
oscilloscope,acquiring data at 100 MS/s with a vertical resolution of 8 bits.
This mode is useful for displaying waveforms and for deriving waveform
parameters such as slew rate, rise time, and settling time.
Flexibleresolution differs from oscilloscope mode in two ways: it has
higherresolution (sampling rate dependent), and the signal bandwidth is
limited to provide antialiasing protection. This mode is useful for spectral
analysis,distortion analysis, and other measurements for which high
resolution is crucial.