ADC0809 National Semiconductor Corporation, ADC0809 Datasheet - Page 10

• Manufacturer Part Number: ADC0809
• Description: 8-bit Microprocessor Compatible A/d Converters With 8-channel Multiplexer
• Manufacturer: National Semiconductor Corporation

www.national.com
Figure 10 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
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.
Conversion System Using Trimmed Supply
FIGURE 9. Ratiometric Conversion System
FIGURE 10. Ground Referenced
10
5.12V is used, the supply should be adjusted to the same
voltage within 0.1V.
The top and bottom ladder voltages cannot exceed V
ground, respectively, but they can be symmetrically less than
V
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 Fig-
ure 13, a 2.5V reference is symmetrically centered about
V
system with a 2.5V reference allows the LSB bit to be half the
size of a 5V reference system.
CC
CC
/2 since the same current flows in identical resistors. This
and greater than ground. The center of the ladder voltage
567224
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CC
and