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ADC1241CIJ Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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ADC1241CIJ Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 14 page  20 Functional Description (Continued) Digital Control Inputs AD Function CS WR RD CAL AZ 1 1 1 Start Conversion without Auto-Zero 1 1 1 Read Conversion Result without Auto-Zero 1 1 0 Start Conversion with Auto-Zero 1 1 0 Read Conversion Result with Auto-Zero 1X X X Start Calibration Cycle 0 X 1 0 X Test Mode (DB2 DB3 DB5 and DB6 become active) FIGURE 1 Function of the AD Control Inputs The table in Figure 1 summarizes the effect of the digital control inputs on the function of the ADC1241 The Test Mode where RD is high and CS and CAL are low is used by the factory to thoroughly check out the operation of the ADC1241 Care should be taken not to inadvertently be in this mode since DB2 DB3 DB5 and DB6 become active outputs which may cause data bus contention 22 RESETTING THE AD All internal logic can be reset which will abort any conver- sion in process The AD is reset whenever a new conver- sion is started by taking CS and WR low If this is done when the analog input is being sampled or when EOC is low the Auto-Cal correction factors may be corrupted therefore making it necessary to do an Auto-Cal cycle before the next conversion This is true with or without Auto-Zero The Cali- bration Cycle cannot be reset once started On power-up the ADC1241 automatically goes through a Calibration Cy- cle that takes typically 1396 clock cycles 30 Analog Considerations 31 REFERENCE VOLTAGE The voltage applied to the reference input of the converter defines the voltage span of the analog input (the difference between VIN and AGND) over which 4095 positive output codes and 4096 negative output codes exist The A-to-D can be used in either ratiometric or absolute reference ap- plications The voltage source driving VREF must have a very low output impedance and very low noise The circuit in Figure 2 is an example of a very stable reference that is appropriate for use with the ADC1241 In a ratiometric system the analog input voltage is propor- tional to the voltage used for the AD reference When this voltage is the system power supply the VREF pin can be tied to VCC This technique relaxes the stability requirement of the system reference as the analog input and AD refer- ence move together maintaining the same output code for given input condition TLH10554 – 17 Tantalum FIGURE 2 Low Drift Extremely Stable Reference Circuit 11 |
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