Input range refers to the set of input voltages that an analog input channel can digitize with the specified accuracy. The NI-PGIA amplifies or attenuates the AI signal depending on the input range. You can individually program the input range of each AI channel on your USB-6451.

The input range affects the resolution of the USB-6451 for an AI channel. Resolution refers to the voltage of one ADC code. For example, a 20-bit ADC converts analog inputs into one of 1,048,576 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 20-bit ADC is:

10   V   -   - 10   V 2 20 = 19 . 07   µ V

The USB-6451 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 4% over what the formula shown above would indicate.

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.

Table 14. USB-6451 Input Range and Nominal Resolution
Input Range (V) Nominal Resolution (µV)
10 19.87
5 9.94
2.5 4.97
0.2 0.40