Chapter 4 Theory of Operation© National Instruments Corporation 4-9 DAQCard-1200 User ManualInterval Data AcquisitionIn an interval data acquisition operation, a new period called the intervalcycle is defined. Within each interval cycle, a predefined number ofsamples is taken at regular sample intervals and conversions are stopped forthe remainder of the interval cycle. A data acquisition operation consists ofback-to-back interval cycles. In single-channel interval data acquisition,you select the number of samples you want in each interval cycle from asingle channel. In multiple-channel interval data acquisition, the channelsare scanned once during each interval cycle. The interval cycle timing canbe externally controlled through OUTB1 or controlled internally bycounter B1 of the timing circuitry.For an example of a multiple-channel interval data acquisition operation,refer to the description of interval scanning signal timing in the DAQTiming Connections section in Chapter 3, Signal Connections.Data Acquisition RatesMaximum data acquisition rates (number of samples per second) aredetermined by the conversion period of the ADC plus the sample-and-holdacquisition time. During multiple-channel scanning, the data acquisitionrates are further limited by the settling time of the input multiplexers andprogrammable gain amplifier. After the input multiplexers are switched,the amplifier must be allowed to settle to the new input signal value towithin 12-bit accuracy before you perform an A/D conversion, or 12-bitaccuracy will not be achieved. The settling time is a function of the gainselected.Table 4-1 shows the recommended settling time for each gain settingduring multiple-channel scanning. Table 4-2 shows the maximumrecommended data acquisition rates for both single-channel andmultiple-channel data acquisition. For single-channel scanning, this rate islimited only by the ADC conversion period plus the sample-and-holdacquisition time, which is specified at 10 μs (11 μs if EXTCONV* is used).
