ADC0808-N, ADC0809-N

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SNAS535H – OCTOBER 1999 – REVISED MARCH 2013

The ADC0808 needs less than a milliamp of supply current so developing the supply from the reference is

readily accomplished. In Figure 13 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 14. The LM301 is overcompensated to insure stability when loaded by the 10

μF output

capacitor.

The top and bottom ladder voltages cannot exceed VCC and ground, respectively, but they can be symmetrically

less than VCC 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 Figure 15, a 2.5V reference is symmetrically centered about VCC/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.

Figure 12. Ground Referenced

Conversion System Using Trimmed Supply

Figure 13. Ground Referenced Conversion System with

Reference Generating VCC Supply

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