National Instruments PC-LPM-16/PnP - page 48

Appendix A Specifications
PC-LPM-16/PnP User Manual A-6  National Instruments Corporation
To illustrate these definitions, Figure A-1 shows a portion of the
analog-input-to-digital-output transfer curve for an ideal, ADC overlaid
on the transfer curve of a hypothetical, typical, ADC. As shown in
Figure A-1, the relative accuracy is the deviation of the code transition
voltage from the center of the code for an ideal ADC, expressed in terms
of LSBs. Notice that in this case, an ideal ADC has a relative accuracy
of ± 1/2 LSB, because this definition of relative accuracy encompasses
both nonlinearity and quantization uncertainties. Integral nonlinearity
is the worst case deviation of the center of the code from the ideal
center, expressed in terms of LSBs. Finally, the differential
nonlinearity is deviation of a code width from ideal code width,
expressed in terms of LSBs.
Figure A-1. ADC Errors
Re1
1
xx
INLe
Re2
2
Output Code (LSB)
-1
12 -1
-2
Cideal Cactual
Vactual
Videal
 Practical ADC 
 analog-input-to-digital-output curve
 Ideal ADC
 analog-input-to-digital-output curve
Cideal = Center of code1 for ideal ADC
Cactual = Center of code1 for practical ADC
INL error = INLe = Cactual-Cideal
Relative Accuracy = Maximum (Rel, Re2)
DNL error = Vactual-Videal
KEY
Input Voltage (LSBs)
a.Book : I.Appendix A Page 6 Wednesday, November 20, 1996 6:36 PM