ADC0808/ADC0809
Applications Information
(Continued)
tem reference must be used which relates the full-scale volt-
age to the standard volt. For example, if V
CC
= V
REF
= 5.12V,
then the full-scale range is divided into 256 standard steps.
The smallest standard step is 1 LSB which is then 20 mV.
2.0 RESISTOR LADDER LIMITATIONS
The voltages from the resistor ladder are compared to the
selected into 8 times in a conversion. These voltages are
coupled to the comparator via an analog switch tree which is
referenced to the supply. The voltages at the top, center and
bottom of the ladder must be controlled to maintain proper
operation.
The top of the ladder, Ref(+), should not be more positive
than the supply, and the bottom of the ladder, Ref(−), should
not be more negative than ground. The center of the ladder
voltage must also be near the center of the supply because
the analog switch tree changes from N-channel switches to
P-channel switches. These limitations are automatically sat-
isfied in ratiometric systems and can be easily met in ground
referenced systems.
Figure 10
shows a ground referenced system with a sepa-
rate 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.
DS005672-7
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
Figure 11
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
Figure 12.
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 volt-
age should always be near the center of the supply. The sen-
sitivity of the converter can be increased, (i.e., size of the
LSB steps decreased) by using a symmetrical reference sys-
tem. In
Figure 13,
a 2.5V reference is symmetrically cen-
tered 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.
9
www.national.com
NSC [ NATIONAL SEMICONDUCTOR ]
