National Instruments PCI-6035E, Руководство пользователя

Страница: 46 / 265

Chapter 2 Analog Input
E Series User Manual 2-12 ni.com
Connect channel 2 to AI GND (or you can use the internal ground signal;
refer to
Internal Channels for E Series Devices in the NI-DAQmx Help or
the LabVIEW 8.x Help . Set the input range of channel 2 to 0–100 mV to
match channel 1. Then scan channels in the order: 0, 2, 1.
Inserting a grounded channel between signal channels improves settling
time because the NI-PGIA adjusts to the new input range setting faster
when the input is grounded.
Minimize Voltage Step between Adjacent Channels
When scanning between channels, the settling time increases when the
voltage step is larger between channels. This is true even if all channels
being scanned have the same input range. If you know the expected input
range of your signals, you can group signals with similar expected ranges
together in your scan list.
For example, suppose all channels in a system use a –5 to 5 V input range.
The signals on channels 0, 2, and 4 vary between 4.3 V and 5 V. The signals
on channels 1, 3, and 5 vary between –4 V and 0 V. Scanning channels in
the order 0, 2, 4, 1, 3, 5 will produce more accurate results than scanning
channels in the order 0, 1, 2, 3, 4, 5.
Avoid Scanning Faster than Necessary
Designing your system to scan at slower speeds gives the PGIA more time
to settle to a more accurate level. Consider the following examples.
Example 1
Averaging many AI samples can increase the accuracy of the reading by
decreasing noise effects. In general, the more points you average, the more
accurate the final result will be. However, you may choose to decrease the
number of points you average and slow down the scanning rate.
Suppose you want to sample 10 channels over a period of 20 ms and
average the results. You could acquire 500 points from each channel at a
scan rate of 250 kS/s. Another method would be to acquire 1,000 points
from each channel at a scan rate of 500 kS/s. Both methods take the same
amount of time. Doubling the number of samples averaged (from 500 to
1,000) decreases the effect of noise by a factor of 1.4 (the square root of 2).
However, doubling the number of samples (in this example) decreases the
time the PGIA has to settle from 4
µ s to 2 µ s. In some cases, the slower scan
rate system returns more accurate results.