Artem,
Thank you for contacting National Instruments support. This is expected behavior. When the wait mode is set to Sleep, DAQmx will only sleep if and only if there is no data available to process. So, for example, at 200 kS/s, data is filling up the buffer fairly quickly and should always be available to read, which means that the process will not sleep at all. When data is always available, the CPU is going to handle it as quickly as possible and thus the CPU will appear to be running at near 100%.
Specifying a wait mode of Sleep is really only beneficial when the acquisition is running slowly enough that a lot of time is spent in DAQmx Read waiting for the data to arrive. In the past, the CPu was maxed out even at extremely slow sampling rates. Sleep mode alleviates this problem by putting the process to sleep when no data is ready for processing; however, if data is available, it will be handled immediately and no sleeping will occur. If data is constantly available for processing, the CPU is expected to deal with it right away and the usage will appear to be high.
Regards,
Kenn North
Principal Product Manager - Search, Digital Analytics
http://ni.com/search