Analog Input Range

Input range refers to the set of input voltages that an analog input channel can digitize with the specified accuracy. The NI programmable gain instrumentation amplifier (NI-PGIA) amplifies or attenuates the AI signal depending on the input range.

The input range affects the resolution of the USB-6423 for an AI channel. Resolution refers to the voltage of one ADC code. For example, a 16-bit analog-to-digital converter (ADC) converts analog inputs into one of 65,536 (= 2¹⁶) codes—that is, one of 65,536 possible digital values. These values are spread fairly evenly across the input range. So, for an input range of -10 V to 10 V, the voltage of each code of a 16-bit ADC is:

The USB-6423 uses a calibration method that requires some codes to lie outside of the specified range. This calibration method improves absolute accuracy, but it increases the nominal resolution of input ranges by about 5% over what the formula shown above indicates.

Choose an input range that matches the expected input range of your signal. A large input range can accommodate a large signal variation, but reduces the voltage resolution. Choosing a smaller input range improves the voltage resolution, but may result in the input signal going out of range.