National Instruments NI USB-621x - Manual (Page 34)

Chapter 4 Analog Input
NI USB-621x User Manual 4-8 ni.com
You can connect channel 2 to AI GND (or you can use the internal ground
signal; refer to Internal Channels in the NI-DAQmx Help). Set the input
range of channel 2 to –200 mV to 200 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 that have the same input range, the
settling time increases with the voltage step between the channels. 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 produces 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 NI-PGIA more
time to settle to a more accurate level. Here are two examples to consider.
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. 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 250 points from each channel at a
scan rate of 125 kS/s. Another method would be to acquire 500 points from
each channel at a scan rate of 250 kS/s. Both methods take the same amount
of time. Doubling the number of samples averaged (from 250 to 500)
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 NI-PGIA has to settle from 8 µs to 4 µs. In some cases, the slower
scan rate system returns more accurate results.