National Instruments AT-MIO-16X - Manual (Page 228)

Appendix A Specifications
© National Instruments Corporation A-5 AT-MIO-16X User Manual
Multiple-Channel Scanning Acquisition Rates
When scanning among channels with different voltages, the analog
circuitry on the AT-MIO-16X needs time to settle from one voltage to
the next. Because of its complex transient response, the AT-MIO-16X
is not always able to settle to full 16-bit accuracy within 10
µ
sec, which
is the shortest guaranteed sampling interval. Table A-3 lists the typical
voltage settling times to within three different percentages of full-scale
range.
When scanning among channels at various gains, the settling times may
further increase. The settling times given in Table A-3 are for signals
changing from and to voltages within in the same full-scale range.
When the PGIA switches to a higher gain, the signal on the previous
channel may be well outside the new, smaller range. For instance,
suppose a 4-V signal is connected to Channel 0 and a 1-mV signal is
connected to Channel 1, and suppose 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 bipolar
mode). The approximately 4-V step from 4 V to 1 mV is 2,000% of the
new full-scale range. To settle within 0.0015% (15 ppm) of the ±100
mV full-scale range on Channel 1, the input circuitry has to settle to
within 0.000075% (0.75 ppm) of the 4-V step. It may take as long as
4,000
µ
sec for the circuitry to settle this much. In general this extra
settling time is not needed when the PGIA is switching to a lower gain.
Because of the problems with settling times, multiple-channel scanning
is not recommended unless sampling rates are low enough or it is
necessary to simultaneously sample several signals as close as possible.
The data is much more accurate (and channel-to-channel independent)
if you acquire data from each channel independently (for example, 100
points from Channel 0, then 100 points from Channel 1, then 100 points
from Channel 2, and so on). If, however, all the channels are scanned at
Table A-3. Typical Multiple-Channel Scanning Settling Times
Accuracy
±0.0061% FSR
(±4 LSB)
±0.0015% FSR
(±1 LSB)
±0.00076% FSR
(±0.5 LSB)
Typical settling time 10
µ
sec 20
µ
sec 40
µ
sec
Maximum per-channel
acquisition rate 100 ksamples/sec 50 ksamples/sec 25 ksamples/sec