Chapter 2 Analog Input
© National Instruments Corporation 2-11 E Series User Manual
Use Short High-Quality Cabling
Using short high-quality cables can decrease several effects that decrease
accuracy including crosstalk, transmission line effects, and noise. The
capacitance of the cable can also effectively increase the settling time.
National Instruments recommends using individually shielded,
twisted-pair wires that are 2 m or less to connect AI signals to the device.
Refer to the
Connecting Analog Input Signals section for more information.
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 0–10 V and the
input range of channel 1 is 0–100 mV.
When the multiplexer switches from channel 0 to channel 1, the input to the
PGIA switches from 4 V to 1 mV. The approximately 4 V step from 4 V to
1 mV is 4,000% of the new full-scale range. For a 12-bit device to settle
within 0.012% (120 ppm or 1/2 LSB) of the 100 mV full-scale range on
channel 1, the input circuitry must settle to within 0.0003% (3 ppm or
1/80 LSB) of the 4 V step. Some devices can take as long as 100 µs 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. Another useful
technique is to insert a grounded channel between signal channels.
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. The input range for
channel 0 is 0–10 V and the input range of channel 1 is 0–100 mV.
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