How Does the ADC Configurable Input Range Work on NI Hardware?

Updated Apr 10, 2019

Issue Details

I'm used to hardware that has a fixed ADC input range and I saw that using many of the NI multifunctional DAQ you can select the ADC input range instead of having to do it applying some gain externally and converting the acquired value back to the real signal value.

Does NI hardware really change programmatically the ADC range?

Solution

Actually, NI uses a component called programmable gain instrumentation amplifier (NI-PGIA) that amplifies the signal according to the configured range (max value and min value) on DAQmx Create Virtual Channel.vi. Below you can see a simplified diagram representation of the common architecture in our hardware.
Let's suppose a specific hardware uses a ±10V ADC (fixed) and you have configured DAQmx task with ±100mV of input range, what will happen is that the PGIA board will be set to 100x gain so the signal will use the whole ADC resolution and will have a smaller code width. After the signal digitalization, DAQmx driver itself will convert the acquired value (±10V range signal) back to the real signal value (±100mV range signal).

In summary, that abstraction encapsulates the signal convertion and makes not necessary to us know what is the input range of each ADC on NI hardware, making it easier for you to have your signal acquired and ready for your analysis, log and anything else your application demands.

 

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