How to Set the Sampling Rate of the NI-9217/9219 Hardware: CompactDAQ>>Analog Input Modules>>NI 9219
Problem: When I try to take measurements with the NI-9217 or NI-9219 module , I see many repeated samples. In effect, I only receive new samples at a fraction of the rate that I expect. How can I control the sampling rate of the module? Solution: The sampling rate of the NI-9217 and NI-9219 modules depend on a number of factors. Since all modules inside a given cDAQ chassis share a common backplane clock (for a given type of task such as analog input), the fastest configured sample clock in your cDAQ chassis will actually be used for all modules. Any modules configured for a slower sample rate will show repeated samples; these samples can be discarded within your application. Even if you are using only one NI-9217/9219 module within your cDAQ chassis, you may still experience repeated samples. In order to understand this, it is helpful to reference the following tables from the Operating Instructions for each module. From page 13 of the NI-9217 Operating Instructions (see the related links section below):
From page 22 of the NI-9219 Operating Instructions (see the related links section below):
Each option listed in the Specifications sheet (e.g. High speed) is actually a user-programmable setting that can be selected in LabVIEW. This setting is called the ADC Timing Mode and can be programmed using the DAQmx Channel Property Node (located on the Functions palette), as shown below.
On this property node, you can choose from High Speed, Best 60 Hz Rejection, Best 50 Hz Rejection, and High Resolution modes. For example, assume you are measuring a single thermocouple channel on your NI-9219 module, and you have configured the DAQmx Timing VI to take measurements at a rate of 50 Hz (the maximum allowed according to the specifications). By default, you will receive data points at a rate of 50 Hz, though many of the data points will be repeated. In fact, the data will only actually change at a rate of around 2 Hz. This is due to the default ADC Timing Mode being set to High Resolution for thermocouple measurements; setting the ADC Timing Mode to High Speed will allow you to obtain changing thermocouple data at a rate up to 50 Hz. Since temperature is typically a slow varying phenomenon it is usually not necessary to measure more than a couple of times a second. For this reason, thermocouples are normally measured at a slow rate with a low-pass filter to ensure accuracy, LabVIEW SignalExpress A feature has been added in NI-DAQmx 8.7.1 that allows you to select the sampling mode from the High Resolution, High Speed, Best 50 Hz Rejection, and Best 60 Hz Rejection modes.  Please ensure that you have the latest version of the NI-DAQmx drivers installed to use this feature. Related Links: Product Manuals: NI 9217 Operating Instructions (November 2006) Product Manuals: NI 9219 Operating Instructions (March 2007) Product Manuals: NI cDAQ-9172 User Guide and Specifications (June 2007) KnowledgeBase 3UN91NB9: C Series Modules Supported in the NI cDAQ-9172 CompactDAQ KnowledgeBase 48NC2QAY: Incorrect Initial Sample with NI 9211, NI 9217, or NI 9219 in NI CompactDAQ Attachments:
Report Date: 05/04/2007 Last Updated: 05/05/2008 Document ID: 493DGV0E |
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