Chapter 2
Hardware Overview
2-4
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if the device has 1 M
Ω
output impedance, your measured signal will be
one-half the actual signal value.
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 offset voltage to be added to the signal you measure, according to
the formula
R
s
× 1 nA
, where
R
s
is the external source impedance. For
example, if the device you have attached to the NI 5911 has an output
impedance of 10 k
Ω
, typically the offset voltage is 10
µ
V (10 k
Ω
×
1 nA).
Input Protection
The NI 5911 features input-protection circuits that protect both the positive
and negative analog input from damage from AC and DC signals up to
±42 V.
If the voltage at one of these inputs exceeds a threshold voltage, V
tr
, the
input clamps to V
tr
and a resistance of 100 k
Ω
is inserted in the path to
minimize input currents to a nonharmful level.
The protection voltage, V
tr
, is input range dependent, as shown in Table 2-1.
AC Coupling
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 the
Digitizer Basics
appendix in
your
NI-SCOPE Software User Manual
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.
Flexible resolution differs from oscilloscope mode in two ways: it has
higher resolution (sampling rate dependent), and the signal bandwidth is
limited to provide antialiasing protection. This mode is useful for spectral