Intel 80C196KB Series, Руководство пользователя, Страница: 62 / 98

80C196KB USER’S GUIDE
scale reference minus 1.5 LSB; and it’s code widths are
all exactly one LSB. These qualities result in a digitiza-
tion without offset, full-scale or linearity errors. In oth-
er words, a perfect conversion.
Figure 11-8 shows an Actual Characteristic of a hypo-
thetical 3-bit converter, which is not perfect. When the
Ideal Characteristic is overlaid with the imperfect char-
acteristic, the actual converter is seen to exhibit errors
in the location of the first and final code transitions and
code widths. The deviation of the first code transition
from ideal is called ‘‘zero offset’’, and the deviation of
the final code transition from ideal is ‘‘full-scale error’’.
The deviation of the code widths from ideal causes two
types of errors. Differential Non-Linearity and Non-
Linearity. Differential Non-Linearity is a local linearity
error measurement, whereas Non-Linearity is an over-
all linearity error measure.
Differential Non-Linearity is the degree to which actual
code widths differ from the ideal one LSB width. It
gives the user a measure of how much the input voltage
may have changed in order to produce a one count
change in the conversion result. Non-Linearity is the
worst case deviation of code transitions from the corre-
sponding code transitions of the Ideal Characteristic.
Non-Linearity describes how much Differential Non-
Linearities could add up to produce an overall maxi-
mum departure from a linear characteristic. If the Dif-
ferential Non-Linearity errors are too large, it is possi-
ble for an A/D converter to miss codes or exhibit non-
monotonicity. Neither behavior is desirable in a closed-
loop system. A converter has no missed codes if there
exists for each output code a unique input voltage range
that produces that code only. A converter is monotonic
if every subsequent code change represents an input
voltage change in the same direction.
Differential Non-Linearity and Non-Linearity are
quantified by measuring the Terminal Based Linearity
Errors. A Terminal Based Characteristic results when
an Actual Characteristic is shifted and rotated to elimi-
nate zero offset and full-scale error (see Figure 11-9).
The Terminal Based Characteristic is similar to the Ac-
tual Characteristic that would be seen if zero offset and
full-scale error were externally trimmed away. In prac-
tice, this is done by using input circuits which include
gain and offset trimming. In addition, V
REF
on the
80C196KB could also be closely regulated and trimmed
within the specified range to affect full-scale error.
Other factors that affect a real A/D Converter system
include sensitivity to temperature, failure to completely
reject all unwanted signals, multiplexer channel dissim-
ilarities and random noise. Fortunately these effects are
small.
Temperature sensitivities are described by the rate at
which typical specifications change with a change in
temperature.
Undesired signals come from three main sources. First,
noise on V
CC
ÐV
CC
Rejection. Second, input signal
changes on the channel being converted after the sam-
ple window has closedÐFeedthrough. Third, signals
applied to channels not selected by the multiplexerÐ
Off-Isolation.
Finally, multiplexer on-channel resistances differ slight-
ly from one channel to the next causing Channel-to-
Channel Matching errors, and random noise in general
results in Repeatability errors.
11.4 A/D Glossary of Terms
Figures 11-7, 11-8, and 11-9 display many of these
terms. Refer to AP-406 ‘MCS-96 Analog Acquisition
Primer‘ for additional information on the A/D terms.
ABSOLUTE ERROR ÐThe maximum difference be-
tween corresponding actual and ideal code transitions.
Absolute Error accounts for all deviations of an actual
converter from an ideal converter.
ACTUAL CHARACTERISTIC ÐThe characteristic of
an actual converter. The characteristic of a given con-
verter may vary over temperature, supply voltage, and
frequency conditions. An Actual Characteristic rarely
has ideal first and last transition locations or ideal code
widths. It may even vary over multiple conversion un-
der the same conditions.
BREAK-BEFORE-MAKE ÐThe property of a multi-
plexer which guarantees that a previously selected
channel will be deselected before a new channel is se-
lected. (e.g. the converter will not short inputs togeth-
er.)
CHANNEL-TO-CHANNEL MATCHING ÐThe dif-
ference between corresponding code transitions of actu-
al characteristics taken from different channels under
the same temperature, voltage and frequency condi-
tions.
CHARACTERISTIC ÐA graph of input voltage ver-
sus the resultant output code for an A/D converter. It
describes the transfer function of the A/D converter.
CODE ÐThe digital value output by the converter.
CODE CENTER ÐThe voltage corresponding to the
midpoint between two adjacent code transitions.
CODE TRANSITION ÐThe point at which the con-
verter changes from an output code of Q, to a code of
Q
a
1. The input voltage corresponding to a code tran-
sition is defined to be that voltage which is equally like-
ly to produce either of two adjacent codes.
CODE WIDTH ÐThe voltage corresponding to the
difference between two adjacent code transitions.
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