Chapter2 HardwareOverview
©NationalInstruments Corporation 2-3 NI5911 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 inputimpedance of the NI 5911 PGIA is 1 MΩbetween the p ositive
and negative input.The output impedance of the device connected to the
NI5 911 and the input impedance of the NI 5911 form an impedance
divider, which attenuates the input signal according to the following
formula:
whereVmis the measured voltage, Vsis the source voltage, Rsis the external
source,and Rin is the input impedance.
Ifthe device you are measuring has a very large output impedance, your
measurements will beaffected by this impedance divider. For example,
ifthe device has 1 MΩ output impedance, your measured signal will be
one-half the actual signal value.
Table2-1. Input Ranges for the NI 5911
Range InputProtection Threshold
±10 V ±10V
±5V ±5 V
±2V ±5 V
±1V ±5 V
±0.5V ±5V
±0.2V ±5V
±0.1V ±5V
Vm
VsRin
RsRin
+
--------------------=