Chapter3 HardwareOverview
ATE Series User Manual 3-12 ni.com
When scanning among channels at variousgains, the settling times may
increase.When the PGIA switches to a higher gain, the signal on the
previouschannel 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 channel1, and suppose the PGIA is programmed to apply
again of o ne to channel 0 and a gain of 100 to channel 1. When the
multiplexerswitches to channel 1 and the PGIA switches to a gain of 100,
the newfull-scale range is 100 m V(if t he 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 device to settle within 0.012% (120 ppm or
1/2 LSB) of the 100 mV full-scale range on channel 1, the input circuitry
hasto settle to within 0.0003% (3 ppm or 1/80 LSB) of the 4 V step. It may
takeas long as 100 µs for the circuitry to settle this much. For a 16-bit
deviceto 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 lowergain.
Settlingtimes can also increase when scanning high-impedance signals due
toa phenomenon called charge injection, where the AI multiplexer injects
asmall amount o f chargeinto each signal source when that source is
selected.If the impedance of the source is not low enough, the effect of the
charge—avoltage error—does not have decayed by the time the ADC
samples the signal. Forthis reaso n,y ou should keepsource impedances
under 1 kΩto perform high-speed scanning.
Dueto the previously described limitations of settling times resultin g from
these conditions, multiple-channel scanning is not recommended unless
sampling rates are lowenough or it is necessary to sam ple severalsignals
asnearly simultaneously as possible. The data is much more accurate and
channel-to-channelindependent 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).