Chapter 4
Analog Input
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National Instruments Corporation
4-9
Carefully Choose the Channel Scanning Order
Avoid Switching from a Large to a Small Input Range
Switching from a channel with a large input range to a channel with a small
input range can greatly increase the settling time.
Suppose a 4 V signal is connected to channel 0 and a 1 mV signal is
connected to channel 1. 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.
When the multiplexer switches from channel 0 to channel 1, the input to the
NI-PGIA switches from 4 V to 1 mV. The approximately 4 V step from 4 V
to 1 mV is 1,000% of the new full-scale range. For a 16-bit device to settle
within 0.0015% (15 ppm or 1 LSB) of the ±200 mV full-scale range on
channel 1, the input circuitry must settle to within 0.000031% (0.31 ppm or
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 NI-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
). 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.
