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.
Note If you try to acquire a signal belowthe set input range the sensitive front-end
components of the NI 5911 may become unstable and begin returning invaliddata. To
returnthe digitizer to a stable configuration, switch to the maximum input range setting and
acquire an AC-coupledor 0 V signal.
Input Impedance
The inputimpedance of the NI 5911 PGIA is 1 MΩbetween the p ositive
and negative input, ±2% depending on input capacitance. The output
impedanceof the device connected to the NI 5911and the input impedance
of the NI 5911 form an impedance divider, which attenuates the input
signalaccording to the following formula:
whereVmis the measured voltage, Vsis the source voltage, Rsis the external
sourceimpedance, 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,
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
+
--------------------=