ADC0808/ADC0809
shows a ground referenced system with a separate
supply and reference. In this system, the supply must be
trimmed to match the reference voltage. For instance, if a
5.12V is used, the supply should be adjusted to the same
voltage within 0.1V.
567207
FIGURE 9. Ratiometric Conversion System
The ADC0808 needs less than a milliamp of supply current
so developing the supply from the reference is readily ac-
complished. In
a ground referenced system is
shown which generates the supply from the reference. The
buffer shown can be an op amp of sufficient drive to supply
the milliamp of supply current and the desired bus drive, or if
a capacitive bus is driven by the outputs a large capacitor will
supply the transient supply current as seen in
The
LM301 is overcompensated to insure stability when loaded by
the 10
μF
output capacitor.
The top and bottom ladder voltages cannot exceed V
CC
and
ground, respectively, but they can be symmetrically less than
V
CC
and greater than ground. The center of the ladder voltage
should always be near the center of the supply. The sensitivity
of the converter can be increased, (i.e., size of the LSB steps
decreased) by using a symmetrical reference system. In
a 2.5V reference is symmetrically centered about
V
CC
/2 since the same current flows in identical resistors. This
system with a 2.5V reference allows the LSB bit to be half the
size of a 5V reference system.
567224
FIGURE 10. Ground Referenced
Conversion System Using Trimmed Supply
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