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ADC0808CCJ Datasheet(PDF) 5 Page - National Semiconductor (TI) |
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ADC0808CCJ Datasheet(HTML) 5 Page - National Semiconductor (TI) |
5 / 14 page Functional Description Multiplexer. The device contains an 8-channel single-ended analog signal multiplexer. A particular input channel is se- lected by using the address decoder. Table 1 shows the input states for the address lines to select any channel. The ad- dress is latched into the decoder on the low-to-high transition of the address latch enable signal. TABLE 1. SELECTED ADDRESS LINE ANALOG CHANNEL CBA IN0 L L L IN1 L L H IN2 L H L IN3 L H H IN4 H L L IN5 H L H IN6 H H L IN7 H H H CONVERTER CHARACTERISTICS The Converter 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 256R ladder network, the successive ap- proximation register, and the comparator. The converter’s digital outputs are positive true. The 256R ladder network approach ( Figure 1) was chosen over the conventional R/2R ladder because of its inherent monotonicity, which guarantees no missing digital codes. Monotonicity is particularly important in closed loop feedback control systems. A non-monotonic relationship can cause os- cillations that will be catastrophic for the system. Additionally, the 256R network does not cause load variations on the ref- erence voltage. The bottom resistor and the top resistor of the ladder net- work in Figure 1 are not the same value as the remainder of the network. The difference in these resistors causes the output characteristic to be symmetrical with the zero and full-scale points of the transfer curve. The first output transi- tion occurs when the analog signal has reached +1⁄2 LSB and succeeding output transitions occur every 1 LSB later up to full-scale. The successive approximation register (SAR) performs 8 it- erations to approximate the input voltage. For any SAR type converter, n-iterations are required for an n-bit converter. Figure 2 shows a typical example of a 3-bit converter. In the ADC0808, ADC0809, the approximation technique is ex- tended to 8 bits using the 256R network. The A/D converter’s successive approximation register (SAR) is reset on the positive edge of the start conversion (SC) pulse. The conversion is begun on the falling edge of the start conversion pulse. A conversion in process will be in- terrupted by receipt of a new start conversion pulse. Con- tinuous conversion may be accomplished by tying the end-of-conversion (EOC) output to the SC input. If used in this mode, an external start conversion pulse should be ap- plied after power up. End-of-conversion will go low between 0 and 8 clock pulses after the rising edge of start conversion. The most important section of the A/D converter is the com- parator. It is this section which is responsible for the ultimate accuracy of the entire converter. It is also the comparator drift which has the greatest influence on the repeatability of the device. A chopper-stabilized comparator provides the most effective method of satisfying all the converter require- ments. The chopper-stabilized comparator converts the DC input signal into an AC signal. This signal is then fed through a high gain AC amplifier and has the DC level restored. This technique limits the drift component of the amplifier since the drift is a DC component which is not passed by the AC am- plifier. This makes the entire A/D converter extremely insen- sitive to temperature, long term drift and input offset errors. Figure 4 shows a typical error curve for the ADC0808 as measured using the procedures outlined in AN-179. www.national.com 5 |
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