National Instruments Data Acquisition Device NI USB-621x, Руководство пользователя

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Chapter 4 Analog Input
© National Instruments Corporation 4-7 NI USB-621x User Manual
approximately 4 V step from 4 V to 1 mV is 1,000% of the new
f
u ll-scale range. For a 16-bit device to settle within 0.0015%
(15 ppm or 1 LSB) of the ±200 mV f u ll-scale range on channel 1,
the inp u t circ u itry m u st 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 circ
u itry to settle this m u ch.
To avoid this effect, yo u sho u ld arrange yo u r channel scanning
order so that transitions from large to small inp u t ranges are
infreq u ent.
In general, yo u do not need this extra settling time when the
NI-PGIA is switching from a small inp u t range to a larger inp u t
range.
– Insert Gro u nded Channel between Signal Channels—Another
techniq u e to improve settling time is to connect an inp u t channel
to gro u nd. Then insert this channel in the scan list between two of
yo u r signal channels. The inp u t range of the gro u nded channel
sho u ld match the inp u t range of the signal after the gro u nded
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. S
u ppose
the inp u t range for channel 0 is –10 V to 10 V and the inp u t range
of channel 1 is –200 mV to 200 mV.
Yo u can connect channel 2 to AI GND (or yo u can u se the internal
gro u nd signal; refer to Internal Channels in the NI-DAQmx Help ).
Set the inp u t 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 gro u nded channel between signal channels improves
settling time beca u se the NI-PGIA adj u sts to the new inp u t range
setting faster when the inp u t is gro u nded.
– Minimize Voltage Step between Adjacent Channels—When
scanning between channels that have the same inp
u t range, the
settling time increases with the voltage step between the channels.
If yo
u know the expected inp u t range of yo u r signals, yo u can
gro u p signals with similar expected ranges together in yo u r scan
list.
For example, s u ppose all channels in a system u se a –5 to 5 V
inp u t 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
prod
u ces more acc u rate res u lts than scanning channels in the
order 0, 1, 2, 3, 4, 5.