National Instruments DAQ Series User Manual

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National Instruments DAQ Series User Manual Manual pdf 45 page image

Chapter 4 Theory of Operation© National Instruments Corporation 4-7 DAQCard-700 User ManualNotice that only counter 0 is required for DAQ operations. The softwaremust track the number of conversions that have occurred and turn offcounter 0 after the required number of conversions has been obtained.Single-Channel Data AcquisitionDuring single-channel data acquisition, a control register is set to select theanalog input channel before data acquisition is initiated. This multiplexersetting remains constant during the entire data acquisition process;therefore, all A/D conversion data is read from a single channel.Multichannel (Scanned) Data AcquisitionMultichannel data acquisition is performed by enabling scanning duringdata acquisition. Multichannel scanning is controlled by a scan counter.For scanning operations, the scan counter decrements from the highestnumbered channel (selected by the user) through channel 0 and then repeatsthe sequence. For RSE input mode, therefore, any number of channels from2 to 16 can be scanned. For DIFF input mode, any number of channels from2 to 8 can be scanned. Notice that the same AI range is used for all channelsin the scan sequence.Data Acquisition RatesThe maximum data acquisition rate (number of samples per second) isdetermined by the conversion period of the ADC plus the acquisitiontime of its track-and-hold stage. During multichannel scanning, the dataacquisition rate is further limited by the settling time of the inputmultiplexers and operational amplifier. After the input multiplexers areswitched, the amplifier must be able to settle to the new input signal valueto within 12-bit accuracy before an A/D conversion is performed, or 12-bitaccuracy cannot be achieved.If the chosen data acquisition rate does not allow the specified settling time,the analog input circuitry may not perform at 12-bit accuracy. Furthermore,if the maximum data acquisition rate is exceeded, A/D conversions may belost. The maximum data acquisition rate and settling time specifications atvarious input ranges are listed in Appendix A, Specifications.