tem reference must be used which relates the full-scale volt-

age to the standard volt. For example, if V

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.

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

ground, respectively, but they can be symmetrically less than

V

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

resistors. This system with a 2.5V reference allows the LSB

bit to be half the size of a 5V reference system.

DS005672-7

FIGURE 9. Ratiometric Conversion System

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