NI DAQ 6052E, Руководство пользователя

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Chapter 3 Hardware Overview
© National Instruments Corporation 3-5 NI 6052E User Manual
Multiple-Channel Scanning Considerations
The NI 6052E can sample multiple channels at the same maximum rate as
the single-channel rate; however, pay careful attention to the
settling time .
Settling time is the time required for an amplifier, relays, or other circuits
to reach a stable mode of operation. The settling time is independent of the
selected gain, even at the maximum sampling rate. The settling time for
high-speed devices is gain dependent, which can affect the useful sampling
rate for a given gain. However, as long as the gain is constant and source
impedances are low, no extra settling time is necessary between channels.
Refer to Appendix A,
Specifications , for a complete list of settling times.
Settling times can increase when scanning channels with various gains.
When the PGIA switches to a higher gain, the signal on the previous
channel can be well outside the new, smaller range. For example, suppose
a 4 V signal is connected to channel 0 and a 1 mV signal is connected to
channel 1, and the PGIA is programmed to apply a gain of 1 to channel 0
and a gain of 100 to channel 1. When the multiplexer switches to channel 1
and the PGIA switches to a gain of 100, the new full-scale range is 100 mV
if the ADC is in unipolar mode. The 4 V step from 4 V to 1 mV is 4,000%
of the new full-scale range. For a 16-bit device to settle within 0.0015%
(15 ppm or 1 LSB) of the full-scale range on channel 1, the input circuitry
must settle within 0.00004% (0.4 ppm or 1/400 LSB) of the 4 V step. The
circuitry can take up to 200
µ
s to settle this much. In general, this extra
settling time is unneeded when the PGIA switches to a lower gain.
A phenomenon called charge injection , in which the AI multiplexer
injects a small amount of charge into each signal source when that
source is selected, can cause settling times to increase when scanning
high-impedance signals. If the impedance of the source is too high, the
effect of the charge—a voltage error—has not decayed by the time the
ADC samples the signal. For this reason, keep source impedances under
1 k
Ω
to perform high-speed scanning.
Multiple-channel scanning is not recommended unless sampling rates are
low or you must sample several signals almost simultaneously. The data is
more accurate and channel-to-channel independent if you independently
acquire data from each channel (for example, 100 points from channel 0,
then 100 points from channel 1, then 100 points from channel 2, and so on).