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Chapter 3
Theory of Operation
Single-Channel Data Acquisition
During single-channel data acquisition, the channel-select bits in
Command Register 1 select the analog input channel before data
acquisition begins. This multiplexer setting remains constant during the
entire data acquisition process; therefore, all A/D conversion data is
read from a single channel.
Multichannel Scanning Data Acquisition
Multichannel data acquisition is performed when you enable scanned
data acquisition. A scan counter on the board controls multichannel
scanning.
For multichannel scanning operations, the scan counter decrements
from the highest channel which you select through channel 0. Thus, the
board can scan any number of channels from 2 to 16. Notice that the
same analog input 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
settling time of the input multiplexers and operational amplifier further
limits the data acquisition rate. After the input multiplexers switch
channels, the amplifier must be able to settle to the new input signal
value to within 12-bit accuracy before performing an A/D conversion,
or else it will not achieve 12-bit accuracy. The maximum data
acquisition rate for both single-channel and multichannel operation is
50 kS/s. The signal will settle to 1 LSB for any range if you do not
exceed a signal sampling frequency of 50 kS/s. If you exceed the
recommended data acquisition rate, the analog input circuitry may not
perform at 12-bit accuracy. If you exceed this rate, an error condition
called overrun occurs and you will lose some conversion data.
This recommended rate of 50 kS/s assumes 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, channels driven by high-impedance signal
sources should be allowed for greater settling time.
National Instruments Corporation
3-7
PC-LPM-16/PnP User Manual
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