The heart of this single chip data acquisition system is its 8-bit analog-to-digital converter. The converter is designed to give fast, accurate, and repeatable conversions over a wide range of temperatures. The converter is partitioned into 3 major sections: the R ladder network, the successive ap- proximation register, and the comparator. Monotonicity is particularly important in closed loop feedback control systems.
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The smallest standard step is 1 LSB which is then 20 mV. 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 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. The ADC 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 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. In Figure 13, a 2. This system with a 2. Ratiometric Conversion System www.