Set Analog Input Gain for DAQ Device

Updated Apr 2, 2018

Issue Details

How do I set the Analog Input Gain for my DAQ Device?

What internal gain selection process is followed when I set my input range for a DAQ signal acquisition?

Solution

For the DAQmx driver, the gain for an acquisition is automatically selected when the user specifies the input range.

For hardware using the Traditional NI-DAQ (Legacy) driver that have software-selectable gain, you will find an input limits parameter in the AI Config VI, but it is also present in higher-level VIs and examples in LabVIEW depending on the DAQ driver you are using.

Additional Information

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 Minimu
m value and Maximum value inputs. For more information regarding manually setting the range with virtual channels in LabVIEW or in NI MAX, read In LabVIEW, Using Min/Max or High/Low to Set NI-DAQmx Channel Gain. 

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. +/-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
10-10 V
20-5 V
50-2 V
100-1 V
200-500 mV
500-200mV
1000-100mV

 
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 0-1V 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.

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