Chapter 4 Theory of OperationDAQCard-1200 User Manual 4-8 © National Instruments Corporationwith a range of –2,048 to +2,047. In this mode, the MSB of the ADC resultis modified to make it two’s complement. The output from the ADC is thensign extended to 16 bits, causing either a leading 0 or a leading F (hex) tobe added, depending on the coding and the sign. Thus, data values readfrom the FIFO are 16 bits wide.Data Acquisition TimingA data acquisition operation refers to the process of taking a sequence ofA/D conversions when the sample interval (the time between successiveA/D conversions) is carefully timed. A data acquisition operation can eitheracquire a finite number of samples (controlled run) or an infinite numberof samples (freerun). The DAQCard-1200 unit can perform bothsingle-channel data acquisition and multiple-channel (scanned) dataacquisition in two modes—continuous and interval.The data acquisition timing circuitry consists of various clocks and timingsignals that control the data acquisition operation. Data acquisition timingconsists of signals that initiate a data acquisition operation, time theindividual A/D conversions, gate the data acquisition operation, andgenerate scanning clocks. The data acquisition operation can either betimed by the timing circuitry or by externally generated signals. Thesetwo modes are software configurable.Data Acquisition OperationData acquisition operations are initiated either externally throughEXTTRIG or through software control. The data acquisition operation isterminated either internally by counter A1 of the 82C53(A) counter/timercircuitry, which counts the total number of samples taken during acontrolled operation, or through software control in a freerun operation.Continuous Data AcquisitionIn a continuous data acquisition operation, samples are taken at regularsample intervals without any delays. Therefore, each sample is takenwith the same sample interval. This applies to both single-channel andmultiple-channel data acquisition in either freerun or controlled operation.The sample interval is either controlled externally by EXTCONV* orinternally by counter A0 of the timing circuitry.
