National Instruments Low-Cost Multifunction I/O Board for ISA Lab-PC+ User Manual download pdf (Page 84)

Appendix A Specifications

Explanation of Analog Input Specifications

Relative accuracy is a measure of the linearity of an ADC. However, relative accuracy is a

tighter specification than a nonlinearity specification. Relative accuracy indicates the maximum

deviation from a straight line for the analog input-to-digital output transfer curve. If an ADC has

been calibrated perfectly, then this straight line is the ideal transfer function, and the relative

accuracy specification indicates the worst deviation from the ideal that the ADC permits.

A relative accuracy specification of ±1 LSB is roughly equivalent to (but not the same as) a

/2 LSB nonlinearity or integral nonlinearity specification because relative accuracy

encompasses both nonlinearity and variable quantization uncertainty, a quantity often mistakenly

assumed to be exactly ±

/2 LSB. Although quantization uncertainty is ideally ±

/2 LSB, it can be

different for each possible digital code and is actually the analog width of each code. Thus, it is

more specific to use relative accuracy as a measure of linearity than it is to use what is normally

called nonlinearity, because relative accuracy ensures that the sum of quantization uncertainty

and A/D conversion error does not exceed a given amount.

Integral nonlinearity in an ADC is an often ill-defined specification that is supposed to indicate a

converter's overall A/D transfer linearity. The manufacturers of the ADC chips used by National

Instruments specify their integral nonlinearity by stating that the analog center of any code will

not deviate from a straight line by more than ±

/2 LSB. This specification is misleading because,

although the center of a particularly wide code may be found within ±

/2 LSB of the ideal, one of

its edges may be well beyond ±1 LSB; thus, the ADC would have a relative accuracy of that

amount. National Instruments tests its boards to ensure that they meet all three linearity

specifications defined in this appendix; specifications for integral nonlinearity are included

primarily to maintain compatibility with a convention of specifications used by other board

manufacturers. Relative accuracy, however, is much more useful.

Differential nonlinearity 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.

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 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 Lab-PC+ is fairly Gaussian, and so the noise

specifications given are the amounts of pure Gaussian noise required to produce our readings.

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