Multichannel SCXI Convert Clock Default Rate Change in NI-DAQmxPrimary Software: Driver Software>>NI-DAQmxPrimary Software Version: 8.5 Primary Software Fixed Version: N/A Secondary Software: N/A Hardware: Multifunction DAQ (MIO)
Problem: How does NI-DAQmx choose the convert clock rate for a multichannel SCXI acquisition? Solution: Starting in NI-DAQmx 8.5, a total delay of 20 µs was added to the fastest possible channel conversion period on an SCXI chassis connected to a 16-bit or 18-bit E or M Series device. If a sample rate is set fast enough to require a convert clock rate greater than the default rate, warning 200011 is generated, informing the user that the accuracy of the measurement might be compromised. However, since this is a warning, the convert clock used is based on the rate needed by the sample clock set by the user (that is, convert clock = sample clock * number of channels). For SCXI chassis containing a module with track-and-hold circuitry, 10 µs of padding is now added to the fastest possible conversion rate when connected to a 12-bit E Series device. These changes ensure the most accurate data is being acquired by maximizing the settling time between each channel. However, if the affect on the performance due to this change is too great (such as on an RT system using Hardware Timed Single Point in a control loop), then the convert clock rate attribute can be explicitly set, overriding the default rate. To calculate the fastest sample rate where no accuracy is lost due to settling time (and no warning is thrown), use the following forumula: Sample Rate = 1 / (0.000020 * Number_of_Channels). If you need to run your application at a faster sample rate, check the user manual of your SCXI module to get the "minimum scan interval" accuracy specifications. This change affects only multichannel acquisitions. In NI-DAQmx 8.4 and earlier, only 10 µs of delay was added to the fastest possible channel conversion rate on an SCXI chassis without a track-and-hold module connected to any DAQ device. In these versions of NI-DAQmx, no warning is generated when the sample rate needed a faster convert clock than was chosen by default. When an SCXI chassis with a track-and-hold module was present, the convert clock rate selected was the fastest possible rate that the DAQ device can handle, up to 333kHz. Related Links: Drivers and Updates: NI-DAQmx 8.9 ReadMe Attachments:
Report Date: 02/26/2007 Last Updated: 03/19/2009 Document ID: 46PAPQE1 |
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