National Instruments AT-MIO-64F-5, Руководство пользователя

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Specifications Appendix A
AT-MIO-64F-5 User Manual A-4 © National Instruments Corporation
Differential nonlinearity (DNL) is a measure of deviation of code widths from their theoretical
value of 1 LSB. The width of a given code is the size of the range of analog values that can be
input to produce that code, ideally 1 LSB. A specification of ±1 LSB differential nonlinearity
ensures that no code has a width of 0 LSBs (that is, no missing codes) and that no code width
exceeds 2 LSBs.
Noise
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 considerably
greater than 0.5 LSB 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-64F-5 is fairly
Gaussian, so the noise specifications given are the amounts of pure Gaussian noise required to
produce our readings.
Overvoltage Protection
The amount of input overvoltage the AT-MIO-64F-5 can tolerate is limited primarily by the
current handling of the input multiplexers. While each input can safely handle its rated
overvoltage, it would be unwise to stress all inputs simultaneously. Survival with more than one
input of any multiplexer at maximum overvoltage is not guaranteed, unless the overvoltages are
of the opposite sign. The 64 input channels are divided among the four input multiplexers in the
following groupsÐACH0-ACH15; ACH16-ACH23 and ACH40-ACH47; ACH24-ACH31 and
ACH48-ACH55; and ACH32-ACH39 and ACH56-ACH63.
It is important to realize that the overvoltage specification is a survival specification only. This
means that the board will not be damaged by application of the specified overvoltage. It does not
mean that measurement integrity will be preserved, even if the channel being measured is not the
one being subjected to overvoltage stress.
Analog Data Acquisition Rates
Single-Channel Acquisition Rates
The AT-MIO-64F-5 operates at a data acquisition rate of at least 200 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 time required
for the input sample-and-hold amplifier to acquire the input signal and settle to 12-bit accuracy
(0.01%). The sum of conversion time and acquisition time for the sampling ADC used on the
AT-MIO-64F-5 is guaranteed to be less than 5 µsec and is typically 4.6 µsec.