National Instruments VXI-MIO Series User Manual
Page 38
Chapter 3 Hardware Overview
National Instruments Corporation
3-9
VXI-MIO Series User Manual
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 12-bit module to settle within 0.012% (120 ppm
or 1/2 LSB) of the 100 mV full-scale range on channel 1, the input
circuitry has to settle to within 0.0003% (3 ppm or 1/80 LSB) of the
4 V step. It may take as long as 100
µ
s for the circuitry to settle to this
accuracy. For a 16-bit module 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
µ
s for the circuitry to settle to this accuracy.
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, you
should keep source impedances under 1 k
Ω
to perform high-speed
scanning.
Multichannel scanning is not recommended unless sampling rates are
low enough or it is necessary to sample several signals as near to
simultaneously as possible. Single-channel scanning yields more
accurate settling times. 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).