Incorrect Measurements with SCXI-1122 when Scanning Two or More ChannelsHardware: SCXI>>Modules>>SCXI-1122
Problem: When I try reading one analog input channel of the SCXI-1122 module, the value is correct. But when I read more than one channel, the values read are incorrect. Why? Solution: This issue is caused by scanning the channels too quickly for the voltages to settle. SCXI-1122 channels must be scanned extremely slowly because the module has only one filter and one amplifier for all the channels which are switched through a slow settling mechanical relay. If you have the SCXI-1122 configured to use a 4 Hz filter (the default), then you must scan slowly with a large interchannel delay (perhaps using round robin scanning as described in the related links below). The interchannel delay for scanning channels on the SCXI-1122 with the 4 Hz filter engaged should be approximately 1 second. For instance, if you have three channels, then you should not try to scan any faster than around 0.33 Hz. The SCXI-1122 was not actually intended to be used in a scanning fashion. It was intended to be used by scanning a buffer on one channel, then switching to the next channel and scanning another buffer. This is described in the SCXI-1122 User Manual chapter 3 as well as in the Getting Started with SCXI User Manual chapter 3 (linked below). In LabVIEW 7.x or earlier, refer to chapter 9 of the LabVIEW Measurements Manual (linked below) for more information about how to correctly program the SCXI-1122 in LabVIEW. In LabVIEW 8.0 or later, refer to the NI SCXI-1122 Supported Properties topic in the LabVIEW Help. If you need a faster sampling rate and must scan your channels for your application, use a different SCXI module that has independent filters and amplifiers on each channel. For isolation use the SCXI-1125, for strain use the SCXI-1520, for thermocouples use the SCXI-1102, and for RTDs or thermistors use the SCXI-1581 for excitation and the SCXI-1102 to measure. The SCXI-1121 is an all purpose module similar to the SCXI-1122 but with independent signal conditioning for each channel. This enables the SCXI-1121 to scan channels faster, but has many jumpers to configure the modules properties for correct behavior. In conclusion, if you are scanning channels using the SCXI-1122 with the 4 Hz filter engaged, do not select a sampling rate any faster than 1 second per channel. It is preferable to sample buffers on each channel before switching to the next. If neither of these is acceptable for your application, then you must use another SCXI module. Related Links: Product Manuals: SCXI-1122 User Manual KnowledgeBase 1DAIG7W3: What Is the Difference between Interval Scanning and Round Robin Scanning? KnowledgeBase 2D6CTML8: Data Acquisition Sampling Terminology Product Manuals: Getting Started with SCXI Productc Manuals: LabVIEW Measurements Manual (chapter 9) Attachments:
Report Date: 08/08/1997 Last Updated: 01/15/2010 Document ID: 107C2818 |
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