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ADC0848CCN Scheda tecnica(PDF) 9 Page - National Semiconductor (TI) |
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ADC0848CCN Scheda tecnica(HTML) 9 Page - National Semiconductor (TI) |
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9 / 20 page  Functional Description The ADC0844 and ADC0848 contain a 4-channel and 8-channel analog input multiplexer (MUX) respectively. Each MUX can be configured into one of three modes of operation differential, pseudo-differential, and single ended. These modes are discussed in the Applications Information Sec- tion. The specific mode is selected by loading the MUX ad- dress latch with the proper address (see Table 1 and Table 2). Inputs to the MUX address latch (MA0-MA4) are common with data bus lines (DB0-DB4) and are enabled when the RD line is high. A conversion is initiated via the CS and WR lines. If the data from a previous conversion is not read, the INTR line will be low. The falling edge of WR will reset the INTR line high and ready the A/D for a conversion cycle. The rising edge of WR, with RD high, strobes the data on the MA0/ DB0-MA4/DB4 inputs into the MUX address latch to select a new input configuration and start a conversion. If the RD line is held low during the entire low period of WR the previous MUX configuration is retained, and the data of the previous conversion is the output on lines DB0-DB7. After the conver- sion cycle (t C ≤ 40 µs), which is set by the internal clock fre- quency, the digital data is transferred to the output latch and the INTR is asserted low. Taking CS and RD low resets INTR output high and outputs the conversion result on the data lines (DB0-DB7). Applications Information 1.0 MULTIPLEXER CONFIGURATION The design of these converters utilizes a sampled-data com- parator structure which allows a differential analog input to be converted by a successive approximation routine. The actual voltage converted is always the difference be- tween an assigned “+” input terminal and a “−” input terminal. The polarity of each input terminal of the pair being con- verted indicates which line the converter expects to be the most positive. If the assigned “+” input is less than the “−” in- put the converter responds with an all zeros output code. A unique input multiplexing scheme has been utilized to pro- vide multiple analog channels. The input channels can be software configured into three modes: differential, single ended, or pseudo-differential. Figure 1 shows the three modes using the 4-channel MUX ADC0844. The eight inputs of the ADC0848 can also be configured in any of the three modes. In the differential mode, the ADC0844 channel inputs are grouped in pairs, CH1 with CH2 and CH3 with CH4. The polarity assignment of each channel in the pair is inter- changeable. The single-ended mode has CH1–CH4 as- signed as the positive input with the negative input being the analog ground (AGND) of the device. Finally, in the pseudo-differential mode CH1–CH3 are positive inputs ref- erenced to CH4 which is now a pseudo-ground. This pseudo-ground input can be set to any potential within the in- put common-mode range of the converter. The analog signal conditioning required in transducer-based data acquisition systems is significantly simplified with this type of input flex- ibility. One converter package can now handle ground refer- enced inputs and true differential inputs as well as signals with some arbitrary reference voltage. The analog input voltages for each channel can range from 50 mV below ground to 50 mV above V CC (typically 5V) with- out degrading conversion accuracy. TABLE 1. ADC0844 MUX ADDRESSING MUX Address CS WR RD Channel# MUX MA3 MA2 MA1 MA0 CH1 CH2 CH3 CH4 AGND Mode X LLL L H + − XL L H L L H − + Differential XL H L L H + − XL H H L H − + LHL L L H + − LHLH L L H + − Single-Ended LH HL L H + − L HHH L H + − H H L L L H + − Pseudo- HH L H L L H + − Differential HHH L L H + − XXXX L L L Previous Channel Configuration X=don’t care www.national.com 9 |
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