National Instruments DAQ PCI E Series User Manual page 34

Chapter 3 Hardware Overview
between channels as long as the gain is constant and source impedances
are low. Refer to Appendix A, Specifications, for a complete listing of
settling times for each of the PCI E Series boards.
When scanning among channels at various gains, the settling times may
increase. 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 one 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 approximately 4 V step from 4 V to 1 mV is 4,000% of the new
full-scale range. For a 16-bit board to settle within 0.0015% (15 ppm or
1 LSB) of the 100 mV full-scale range on channel 1, the input circuitry
has to settle within 0.00004% (0.4 ppm or 1/400 LSB) of the 4 V step.
It may take as long as 200 µs 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.
Settling times can also increase when scanning high-impedance signals
due to a phenomenon called charge injection, where the analog input
multiplexer injects a small amount of charge into each signal source
when that source is selected. If the impedance of the source is not low
enough, the effect of the charge—a voltage error—will not have
decayed by the time the ADC samples the signal. For this reason, keep
source impedances under 1 k to perform high-speed scanning.
Due to the previously described limitations of settling times resulting
from these conditions, multichannel scanning is not recommended
unless sampling rates are low enough or it is necessary to sample
several signals as nearly simultaneously 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).
© National Instruments Corporation 3-11 PCI E Series User Manual