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WHAT DESIGNERS SHOULD KNOW
ABOUT DATA CONVERTER DRIFT
Understanding the Components of Worst-Case Degradation Can Help in Avoiding Overspecification
Exactly how inaccurate will a change in temperature make
an analog-to-digital or digital-to-analog converter? As de-
signers are well aware, a 12-bit device may provide a much
lower accuracy at its operating-temperature extremes, per-
haps only to 9 or even 8 bits. But for lack of more precise
knowledge, many play it safe (and expensive) and
overspecify.
Yet it is fairly simple to determine a converter鈥檚 absolute
worst-case degradation from its various drift specifications.
Considering these specifications separately and examining
their bases will help to unravel the labyrinth of converter
drift and show how to go about calculating the actual worst-
case drift error for most devices.
Accuracy drift for a D/A converter or a successive-approxi-
mation A/D converter has three primary components: its
gain, offset, and nonlinearity temperature coefficients. In-
stead of calling out the gain and offset drifts separately,
some manufacturers specify a full-scale drift, which takes
both into account. Another important specification in many
applications is differential nonlinearity, which reflects the
equality (or rather, the inequality) of the analog steps be-
tween adjacent digital codes. But, since this parameter is
really describing only the distribution of the linearity error,
its temperature coefficient does not contribute to the
converter鈥檚 worst-case accuracy drift.
EXAMINING THE COMPONENTS OF DRIFT
The transfer function of a D/A converter will illustrate how
the different kinds of drift degrade accuracy.
In a bipolar D/A converter, which produces both positive
and negative analog voltages, offset drift changes all the
output voltages by an equal amount, moving the entire
transfer function up or down from the ideal in parallel to it
(Figure 1a). The drift of the converter鈥檚 voltage reference is
the main cause of this error鈥攚hich may also be called the
minus-full-scale drift, since it occurs even when all the input
bits are logic 0 or off. In a unipolar unit, the offset drift is
usually much smaller, being due mostly to drift in the offset
voltage of the output operational amplifier and secondarily
to leakage in the current switches.
Unlike offset drift, gain drift rotates the transfer function
(Figure 1b). In a bipolar unit it does so around minus full
scale (all bits off), and in a unipolar unit it does so around
zero (again all bits off). The gain drift affects each output
voltage by the same percentage (not the same amount),
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tipping the transfer function at an angle to the ideal. In
general, about 70% of this drift is caused by the drift of the
converter鈥檚 voltage reference.
Obviously, then, reference drift is a major contributor to
total inaccuracy due to gain and offset drift. A positive
temperature coefficient for the reference causes the transfer
function to rotate about zero, as shown in Figure 1c for a
bipolar converter. Since the gain and bipolar offset drifts due
to the reference will always be opposite in direction, the
worst-case accuracy drift may be less than half the sum of
the individual drift specifications. In a unipolar converter,
the gain and offset drifts may well add together, but the
unipolar offset drift is usually insignificant compared to the
magnitude of the gain drift, so it is not so important a factor.
Full-scale drift describes the change in the output voltage
when all bits are on. For a unipolar converter, it is simply the
sum of the offset and gain drifts. In contrast, for a bipolar
converter, the full-scale drift is the sum of half the reference
drift, the gain drift exclusive of the reference, and the offset
drift exclusive of the reference.
POOR TRACKING CAUSES LINEARITY DRIFT
Finally, linearity drift reflects the shift in the analog output
voltage from the straight line drawn between the output
value when all the bits are off (minus full scale) and the
output value when all the bits are on (plus full scale). This
error is caused by the varying temperature coefficients of the
ratio resistances of the converter鈥檚 current-weighting (scal-
ing) resistor, as well as the ratio drifts of the base-emitter
voltages and betas of its transistor current switches.
Since the change in linearity with temperature depends on
how closely various parameters track each other, and not on
absolute parameters values, it is fairly easy to control with
present-day hybrid and monolithic technologies. As a result,
linearity drift is usually much smaller than either the gain or
offset drift. Moreover, it is generally guaranteed to be within
some maximum limit over the converter鈥檚 full operating
temperature range.
Another specification that is important in some applications
is bipolar zero drift, which reflects the change in the output
voltage of a bipolar converter at midscale, when only the
most significant bit is on and all other bits are off. This drift
error at zero is not affected by reference drift at all, but is
caused mainly by poor tracking in the converter鈥檚 scaling
resistors and current switches. Therefore, it appears as a
AB-063
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Printed in U.S.A. September, 1986
1977 Burr-Brown Corporation
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