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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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AD7484BST Fiches technique(PDF) 9 Page - Analog Devices |
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AD7484BST Fiches technique(HTML) 9 Page - Analog Devices |
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9 / 12 page  REV. PrC 7/13/01 PRELIMINARY TECHNICAL DATA AD7484 – 9 – OFFSET / OVERRANGE The AD7484 provides a ±8% overrange capability as well as a programmable Offset Register. The overrange capability is achieved by the use of a 15th bit (D14) and the CLIP input. If the CLIP input is at logic high and the contents of the offset register are zero, then the AD7484 operates as a nor- mal 14-bit ADC. If the input voltage is greater than the full-scale voltage, the data output from the ADC will be all 1’s. Similarly, if the input voltage is lower than the zero- scale voltage, the data output from the ADC will be all 0’s. In this case D14 acts as an overrange indicator. It is set to a 1 if the analog input voltage is outside the nominal 0 to +2.5V range. If the Offset Register contains any value other than zero, the contents of the register are added to the SAR result at the end of conversion. This has the effect of shifting the transfer function of the ADC as shown in Figure 9 and Fig- ure 10. However, it should be noted that with the CLIP input set to logic high, the maximum and minimum codes that the AD7484 will ouput will be 0x3FFF and 0x0000 respectively. Further details are given in Table 1 and Table 2. Figure 9 shows the effect of writing a positive value to the Offset Register. If, for example, the contents of the Offset Register contained the value 1024, then the value of the ana- log input voltage for which the ADC would transition from reading all 0’s to 000...001 (the bottom reference point) would be: 0.5LSB - (1024 LSBs) = -156.326mV The analog input voltage for which the ADC would read full-scale (0x3FFF) in this example would be: 2.5V -1.5LSBs - (1024 LSBs) = 2.34352V 000...000 0V ANALOG INPUT 111...111 000...001 000...010 111...110 111...000 011...111 +VREF-1.5LSB -OFFSET 1LSB = VREF/16384 Figure 9. Transfer Characteristic With Positive Offset The effect of writing a negative value to the Offset Register is shown in Figure 10. If a value of -512 was written to the Offset Register, the bottom end reference point would now occur at: 0.5LSB - (-512 LSBs)= +78.20mV Following from this, the analog input voltage needed to produce a full-scale (0x3FFF) result from the ADC would now be: 2.5V - 1.5LSBs - (-512 LSBs) = 2.5779V 000...000 0V ANALOG INPUT 111...111 000...001 000...010 111...110 111...000 011...111 0.5LSB -OFFSET +VREF-1.5LSB -OFFSET 1LSB = VREF/16384 Figure 10. Transfer Characteristic With NegativeOffset Table 1 below shows the expected ADC result for a given analog input voltage with different offset values and with CLIP tied to logic high. The combined advantages of the offset and overrange features of the AD7484 are shown clearly in Table 2. It shows the same range of analog in- put and offset values as Table 1 but with the clipping feature disabled. Values from -1310 to +1310 may be written to the Offset Register. These values correspond to an offset of ±200mV. A write to the Offset Register is performed by writing a 15-bit word to the part as detailed in the Interfacing sections. The 12 LSBs of the 15-bit word contain the offset value, the 3 MSBs must be set to zero. Failure to write zeros to the 3 MSBs may result in the incorrect operation of the device. Table 1. Clipping Enabled (CLIP = 1) OFFSET -512 0 +1024 VIN ADC DATA, D[0:14] -200m V -1822 -1310 -286 -156.3m V -1536 -1024 0 0V -512 0 1024 +78.2m V 0 512 1536 +2.3435V 14847 15359 16383 +2.5V 15871 16383 17407 +2.5779V 16383 16895 17919 +2.7V 17182 17694 18718 Table 2. Clipping Disabled (CLIP = 0) OFFSET -512 0 +1024 VIN ADC DATA, D[0:13] D14 -200mV 00 0 1 -156.3mV 00 0 1 0V 0 0 1024 0 +78.2mV 0 512 1536 0 +2.3435V 14847 15359 16383 0 +2.5V 15871 16383 16383 0 +2.5779V 16383 16383 16383 1 +2.7V 16383 16383 16383 1 |
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