Carefully Choose The Channel Scanning Order; Avoid Switching From A Large To A Small Input Range; Insert Grounded Channel Between Signal Channels - National Instruments NI USB-621x User Manual

Carefully Choose the Channel Scanning Order

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If your source impedance is high, you can decrease the scan rate to allow
the NI-PGIA more time to settle. Another option is to use a voltage follower
circuit external to your DAQ device to decrease the impedance seen by the
DAQ device. Refer to the KnowledgeBase document, How Do I Create a
Buffer to Decrease the Source Impedance of My Analog Input Signal?,
by going to
ni.com/info

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.
and entering the info code
4-7
Chapter 4 Analog Input
.
rdbbis
NI USB-621x User Manual