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ADC0848CCV 데이터시트(PDF) 10 Page - National Semiconductor (TI) |
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ADC0848CCV 데이터시트(HTML) 10 Page - National Semiconductor (TI) |
10 / 20 page Applications Information (Continued) 2.0 REFERENCE CONSIDERATIONS The voltage applied to the reference input of these convert- ers defines the voltage span of the analog input (the differ- ence between V IN(MAX) and VIN(MIN)) over which the 256 possible output codes apply. The devices can be used in ei- ther ratiometric applications or in systems requiring absolute accuracy. The reference pin must be connected to a voltage source capable of driving the minimum reference input resis- tance of 1.1 k Ω. This pin is the top of a resistor divider string used for the successive approximation conversion. In a ratiometric system ( Figure 2a), the analog input voltage is proportional to the voltage used for the A/D reference. This voltage is typically the system power supply, so the V REF pin can be tied to V CC. This technique relaxes the stability re- quirements of the system reference as the analog input and A/D reference move together maintaining the same output code for a given input condition. For absolute accuracy ( Figure 2b), where the analog input varies between very specific voltage limits, the reference pin can be biased with a time and temperature stable voltage source. The LM385 and LM336 reference diodes are good low current devices to use with these converters. The maximum value of the reference is limited to the V CC supply voltage. The minimum value, however, can be quite small (see Typical Performance Characteristics) to allow di- rect conversions of transducer outputs providing less than a 5V output span. Particular care must be taken with regard to noise pickup, circuit layout and system error voltage sources when operating with a reduced span due to the increased sensitivity of the converter (1 LSB equals V REF/256). 3.0 THE ANALOG INPUTS 3.1 Analog Differential Voltage Inputs and Common-Mode Rejection The differential input of these converters actually reduces the effects of common-mode input noise, a signal common to both selected “+” and “−” inputs for a conversion (60 Hz is most typical). The time interval between sampling the “+” in- put and then the “−” inputs is 1⁄2 of a clock period. The change in the common-mode voltage during this short time interval can cause conversion errors. For a sinusoidal common-mode signal this error is: where f CM is the frequency of the common-mode signal, V peak is its peak voltage value and tC is the conversion time. For a 60 Hz common-mode signal to generate a 1⁄4 LSB error ( ≈5 mV) with the converter running at 40 µS, its peak value would have to be 5.43V. This large a common-mode signal is much greater than that generally found in a well designed data acquisition system. 4 Single-Ended DS005016-12 2 Differential DS005016-13 3 Pseudo-Differential DS005016-14 Combined DS005016-15 FIGURE 1. Analog Input Multiplexer Options DS005016-38 www.national.com 10 |
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