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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.
Input Impedance
The input impedance of the NI 5911 PGIA is 1 MΩ between the positive
and negative input. 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, 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,
if the device has 1 MΩ output impedance, your measured signal will be
one-half the actual signal value.
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
+
--------------------=