Appendix A
Specifications
© National Instruments Corporation
A-3
AT-MIO-16D User Manual
System noise is the amount of noise seen by the ADC when there is no signal present at the input
of the board. The amount of noise that is reported directly (without any analysis) by the ADC is
not necessarily the amount of real noise present in the system, unless the noise is
≥
0.5 LSB root
mean square (rms). Noise that is less than this magnitude produces varying amounts of flicker,
and the amount of flicker seen is a function of how near the real mean of the noise is to a code
transition. If the mean is near or at a transition between codes, the ADC flickers evenly between
the two codes, and the noise is seen as very nearly 0.5 LSB. If the mean is near the center of a
code and the noise is relatively small, very little or no flicker is seen, and the noise is reported by
the ADC as nearly 0 LSB. From the relationship between the mean of the noise and the measured
rms magnitude of the noise, the character of the noise can be determined. National Instruments has
determined that the character of the noise in the AT-MIO-16D is fairly Gaussian, and so the noise
specifications given are the amounts of pure Gaussian noise required to produce our readings.
Analog Data Acquisition Rates
Single-Channel Acquisition Rates
The maximum data acquisition rate for the AT-MIO-16D is 100 ksamples/sec. Permissible data
acquisition rates are determined by the minimum A/D conversion time of the system. This
minimum conversion time is the sum of the conversion time of the ADC and the settling time of the
analog input front end. When data acquisition is performed on a single analog input channel, the
time required for the input sample-and-hold amplifier to acquire the input signal and settle to 12-bit
accuracy (0.01%) is added to the conversion time of the ADC. The sample-and-hold amplifier in
the AT-MIO-16D takes 1
µ
sec typical and 1.5
µ
sec maximum to settle to 0.01% for a 10 V step.
The data acquisition rates shown in the preceding table are the best rates for single-channel
acquisition. These rates take into account the specified typical and maximum (worst-case)
conversion times of the ADC plus 2
µ
sec to allow for sample-and-hold settling time.
Multiple-Channel Scanning Acquisition Rates
The following are the maximum multiple-channel scan rates recommended for the AT-MIO-16D:
Gain
Data Acquisition Rate
1, 2, 4, 8
100 ksamples/sec
10
100 ksamples/sec
100
70 ksamples/sec
500
20 ksamples/sec
Recommended multiple-channel scanning rates are slower than single-channel acquisition rates for
higher gains, because as gain is increased, the AT-MIO-16D circuitry takes longer to settle from
one channel voltage to the next. The recommended settling time for gains of 1 through 10 is 10
µ
sec; for a gain of 100, 14
µ
sec is recommended; and 50
µ
sec is recommended for a gain of 500.
For these settling times, the circuitry on the AT-MIO-16D boards will settle to 0.5 LSBs of the
final value, or to 0.01%, for a full-scale step.
A full-scale difference between input channels is the worst-case switching condition for channel
scanning settling time, with one channel at the positive end of the full-scale range and the other
channel at the negative end of the full-scale range. The lower the analog input source impedance,
the better the settling time performance.
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