of Microprocessors

DIGITAL-TO-ANALOG AND ANALOG-TO-DIGITAL CONVERTERS 223
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INPUT CODE
Fig. 7-1. DAC linearity errors
arithmetic and is just as fast on microcomputers. This topic will be discussed
in Chapter 18.) When a number is sent to the DAC to be converted, all 16
bits are sent out. The DAC in turn is interfaced so that it sees the most
significant N bits of the word, N being the DAC's resolution. An ADC
likewise would connect to the most significant bits of the word and supply
zeroes for the unused low order bits. The ultimate resolution implied by 16
bits is an astounding 305 JLV. An 8-bit microcomputer would probably
handle things in a similar manner unless converter resolutions of8 bits or less
are being used.
Linearity
Another term used in specifying DACs is /inearity. Linearity is related
to accuracy but is definitely not the same thing. Although the voltage levels
ofan ideal DAC are perfectly equally spaced, real DACs have severe difficulty
even approaching the ideal for reasonably high resolutions. The most common
linearity error is called differentia/linearity error. Although the physical
reason for this will become clear later, Fig. 7-1 illustrates this error. The
stepped plot shown represents the output that would occur if the DAC were
driven by a binary counter. Differential linearity refers to the actual difference
in step position between any two adjacent steps compared to the ideal
difference. When a differential linearity error occurs, it is because one step is
either higher or lower than it should be. The diagram shows a differential
linearity error of one-half of the ideal step size, which is equivalent to
one-half of the least significant bit (LSB) of the digital input. If the error