Multichannel (Scanned) Data Acquisition; Data Acquisition Rates; Digital I/O Circuitry - National Instruments Corporation DAQCard-500 User Manual

Chapter 4 Theory of Operation

Multichannel (Scanned) Data Acquisition

You can perform multichannel data acquisition by enabling scanning during data acquisition.
Multichannel scanning is controlled by a scan counter.
For scanning operations, the DAQCard-500 decrements from the highest numbered channel
(selected by the user) through channel 0 and then repeats the sequence. Any number of channels
from 2 to 8 can be scanned.

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
multiplexer and operational amplifier. After the input multiplexer is 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 not be achieved.
If your 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 are listed in Appendix A, Specifications.
These settling time specifications assume that voltage levels on all the channels included in the
scan sequence are within range and are driven by low-impedance sources. Signal levels outside
the ranges on the channels included in the scan sequence adversely affect the input settling time.
Similarly, greater settling time may be required for channels driven by high-impedance signal
sources.

Digital I/O Circuitry

The DAQCard-500 has eight digital I/O lines that are TTL-compatible. Pins DIN<0..3> of the
I/O connector are digital input lines, and pins DOUT<0..3> are digital output lines. These lines
are monitored or driven by the Digital Input Register or the Digital Output Register, respectively.
Reading the Digital Input Register returns the current state of DIN<0..3> lines. Writing to the
Digital Output Register drives the new value onto the DOUT<0..3> lines. Figure 4-4 shows a
diagram of this circuitry.
© National Instruments Corporation 4-5 DAQCard-500 User Manual