When reading analog input voltages on my NI 9207, I am seeing voltage stair-steps that show lower resolution than what I expect the resolution to be for the 24-bit ADC (see picture below). The voltage stair-steps seem to happen about 19 to 20 times per second (20 Hz). Why do I see this low resolution stair-step when trying to sample at higher rates?
Using the datasheet, you can find the NI 9207 has a Delta-Sigma ADC onboard, and, when the module is set to High Resolution mode, the quickest conversion time is 52 ms. As a result, you will not be able to read samples at a rate above approximately 19.2 Hz, and will therefore see the stair-step behavior on quickly changing signals. If you need to read at faster rates, you can switch to High Speed mode for a conversion time of 2 ms per channel.
You can change the mode to High Speed over the default High Resolution mode by using a DAQmx Property Node or in the NI Measurement and Automation Explorer (NI MAX) DAQmx Task. For more information on how to change this property, see Additional Information section on Repeated Samples from C Series Module with NI-DAQmx.
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