National Instruments AT-MIO-16D, Руководство пользователя

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Chapter 3 Theory of Operation
© National Instruments Corporation 3-9 AT-MIO-16D User Manual
Single-Channel Data Acquisition
During single-channel data acquisition, the mux-gain memory is set up to select the gain and analog
input channel before data acquisition is initiated. These gain and multiplexer settings remain
constant during the entire data acquisition process; therefore, all A/D conversion data is read from a
single channel.
Multiple-Channel (Scanned) Data Acquisition
You perform multiple-channel data acquisition by enabling scanning during data acquisition. You
control multiple-channel scanning with the multiplexer counter and the mux-gain memory.
The mux-gain memory consists of 16 words of memory. Each word of memory contains a
multiplexer address (4 bits) for input analog channel selection, a gain setting (2 bits), and a bit
indicating if the entry is the last in the scan sequence. The mux-gain memory address is controlled
by the multiplexer counter. Whenever a mux-gain memory address location is selected, the
multiplexer and gain control bits contained in that memory location are applied to the analog input
circuitry. For scanning operations, the multiplexer counter steps through successive locations in
the mux-gain memory at a rate determined by the scan clock. With the mux-gain memory,
therefore, an arbitrary sequence of channels (16 maximum) with a separate gain setting for each
channel can be clocked through during a scanning operation.
Both the multiplexer counter and the mux-gain memory can be directly written to through
AT-MIO-16D registers. For writing purposes, the multiplexer counter serves as a pointer to the
mux-gain memory. The counter can be loaded with any 4-bit value to point to any mux-gain
memory location. With this counter, scanning can start at any location in the mux-gain memory.
The SCAN CLK signal is generated from the sample-interval counter. This signal pulses once at
the beginning of each A/D conversion and is supplied at the I/O connector. During multiple-
channel scanning, the multiplexer counter is incremented repeatedly, thereby sequencing through
the mux-gain memory and automatically selecting new channel and gain settings during data
acquisition. The MUX CTR CLK signal is generated from the SCAN CLK and provides the
pulses that increment the multiplexer counter. MUX CTR CLK can be identical to SCAN CLK,
incrementing the multiplexer counter once after every A/D conversion. MUX CTR CLK can also
be generated by dividing SCAN CLK by Counter 1 of the Am9513A Counter/Timer. With this
method, the multiplexer counter can be incremented once every
N A/D conversions such that N
conversions can be performed on a single channel and gain selection before switching to the next
channel and gain selection.
Data Acquisition Rates
Data acquisition rates (number of samples per second) are determined by the conversion period of
the ADC plus the sample-and-hold acquisition time. During multiple-channel scanning, the data
acquisition rates are further limited by the settling time of the input multiplexers and
instrumentation amplifier. After the input multiplexers are switched, the instrumentation amplifier
should be allowed to settle to the new input signal value before an A/D conversion is performed or
else high accuracy will not be achieved. The settling time is determined by the gain selected.
Table 3-1 shows the maximum recommended data acquisition rates for both single-channel and
multiple-channel data acquisition. The rates in Table 3-1 refer to typical settling accuracies of 0.5
LSBs of the final value.