Insert grounded channel between signal channels, Minimize voltage step between adjacent channels – National Instruments NI 6232 User Manual
Page 35
Chapter 4
Analog Input
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1/50 LSB) of the ±10 V range. Some devices can take many microseconds
for the circuitry to settle this much.
To avoid this effect, you should arrange your channel scanning order so that
transitions from large to small input ranges are infrequent.
In general, you do not need this extra settling time when the PGIA is
switching from a small input range to a larger input range.
Insert Grounded Channel between Signal Channels
Another technique to improve settling time is to connect an input channel
to ground. Then insert this channel in the scan list between two of your
signal channels. The input range of the grounded channel should match the
input range of the signal after the grounded channel in the scan list.
Consider again the example above where a 4 V signal is connected to
channel 0 and a 1 mV signal is connected to channel 1. Suppose the input
range for channel 0 is –10 V to 10 V and the input range of channel 1 is
–200 mV to 200 mV.
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 or the
LabVIEW 8.x 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 will produce more accurate results than scanning
channels in the order 0, 1, 2, 3, 4, 5.