Chapter 2
Installing and Configuring the DAQPad-MIO-16XE-50
DAQPad-MIO-16XE-50 User Manual
2-8
ni.com
You cannot disable dither on the DAQPad-MIO-16XE-50 because the
resolution of the ADC is so fine that the ADC and the PGIA inherently
produce almost 0.5 LSB
rms
of noise. This is equivalent to having a dither
circuit that is always enabled.
Multiple-Channel Scanning Considerations
The maximum multiple-channel scanning acquisition rate is identical to
the single-channel acquisition rate for all gains. No extra settling time is
necessary between channels as long as the gain is constant and source
impedances are low.
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 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
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 AI 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—has not decayed by the time the ADC samples the
signal. For this reason, you should keep source impedances under 10 k
Ω
to
perform high-speed scanning.
Due to the previously described limitations of settling times resulting from
these conditions, multiple-channel 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).
