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Chapter 4 Theory of Operation

Notice that only counter 0 is required for DAQ operations. The software

must track the number of conversions that have occurred and turn off

counter 0 after the required number of conversions has been obtained.

Single-Channel Data Acquisition

During single-channel data acquisition, a control register is set to select the

analog input channel before data acquisition is initiated. This multiplexer

setting remains constant during the entire data acquisition process;

therefore, all A/D conversion data is read from a single channel.

Multichannel (Scanned) Data Acquisition

Multichannel data acquisition is performed by enabling scanning during

data acquisition. Multichannel scanning is controlled by a scan counter.

For scanning operations, the scan counter decrements from the highest

numbered channel (selected by the user) through channel 0 and then repeats

the sequence. For RSE input mode, therefore, any number of channels from

2 to 16 can be scanned. For DIFF input mode, any number of channels from

2 to 8 can be scanned. Notice that the same AI range is used for all channels

in the scan sequence.

Data Acquisition Rates

The maximum data acquisition rate (number of samples per second) is

determined by the conversion period of the ADC plus the acquisition

time of its track-and-hold stage. During multichannel scanning, the data

acquisition rate is further limited by the settling time of the input

multiplexers and operational amplifier. After the input multiplexers are

switched, the amplifier must be able to settle to the new input signal value

to within 12-bit accuracy before an A/D conversion is performed, or 12-bit

accuracy 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 be

lost. The maximum data acquisition rate and settling time specifications at

various input ranges are listed in Appendix A, Specifications.

National Instruments DAQ Series User Manual

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