National Instruments NI 5911 - Manual (Page 14)

Chapter 2 Hardware Overview
© National Instruments Corporation 2-3 NI 5911 User Manual
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 M
Ω 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
m
is the measured voltage, V
s
is the source voltage, R
s
is the external
source impedance, and R
in
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
V
m
V
s
R
in
R
s
R
in
+
-------------------- =