The possible range of the on-board analog-to-digital converter (ADC) depends on the DAQ card, and the DAQmx driver will maximize the input signal across the ADC range. This allows the best possible resolution to be used to convert the signal to a digital format.
The input range limits of an Analog Input (AI) DAQmx task are specified when configuring the task either in Measurement & Automation Explorer (NI MAX) or when using the DAQmx Create Virtual Channel VI. When configuring in NI MAX, the range limits are specified in the Input Range section (located in the Settings tab). When using the DAQmx Create Virtual Channel VI, the range can be set by using the Minimum value and Maximum value inputs. For more information regarding manually setting the range with virtual channels in LabVIEW or in NI MAX, refer to Setting NI-DAQmx Channel Gain by Min/Max Value or Range High/Low in LabVIEW
The gain selections are specific to each card and the ranges given are for bipolar acquisitions. Bipolar acquisition ranges are on either side of zero (e.g. ±10 or ±1). On some models, these ranges are also available in unipolar modes. For example, most DAQ devices that have an ADC with input range of 10 V are designed with the following bipolar signal ranges.
|Device Gain ||Range|
|0.5||± 10 V|
|1||± 5 V|
|2.5||± 2 V|
|5||± 1 V|
|10||± 500 mV|
|25||± 200 mV|
|50||± 100 mV|
The unipolar signal ranges available to this card are listed below:
|Device Gain ||Range|
When you specify a minimum and maximum value of -4 volts and 4 volts for an acquisition on most devices, DAQmx will automatically select a signal range of ± 5 V and a device gain of 1.
When you specify a minimum and maximum value of 0 and 0.6 volts, DAQmx will automatically select a unipolar signal range of 0 –1 V and a gain of 10. Using unipolar will keep the true resolution over this range. For example, in this example let's say your hardware has 16 bits of resolution. Your 16 bits will be across the 0 –1 V range, not the ± 1 V range.
For Traditional DAQ, the high limit for the gain is the highest expected level of the signal you want to measure, and the low limit is the lowest expected level of the signal. If you leave the input limits blank, or set them to zero, LabVIEW will use the limits that you specified in the NI-DAQ Configuration Utility. If you use channel names, LabVIEW will select the best gain for your channel configuration.