Analog Input Range

An input range is a set of input voltages that an analog input channel can digitize with the specified accuracy. NI sbRIO devices with selectable input ranges have a programmable gain instrumentation amplifier (PGIA), which amplifies or attenuates the AI signal depending on the input range. You can program the input range for each AI channel independently on the sbRIO-9637.

The ADC converts analog inputs into discrete digital values. For a 16-bit ADC there are 2¹⁶ (65,536) possible values. These values are spread evenly across the input range, and the voltage difference between values is proportional to the input range selected for the channel. The voltage difference between values is the size of the least significant bit (LSB size) for the channel. The following equation shows how to calculate the LSB size for a channel set to the -10 V to 10 V input range with a 16-bit ADC.

The sbRIO-9637 uses a scaling method that requires some ADC codes (typically about 5% of the codes) to lie outside the specified range. This method improves absolute accuracy, but it increases the LSB size by about 5% over the calculated value.

Choose an input range that matches the expected range of your input signal. A large input range accommodates large variations in signals but results in a larger LSB and, therefore, lower resolution. A smaller input range improves the resolution, but large input signals may go out of range.

For more information about selecting ranges, refer to the LabVIEW Help.

The following table shows the input ranges and resulting LSB sizes for AI channels on each NI sbRIO device.