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AD694BR Datasheet(PDF) 10 Page - Analog Devices |
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AD694BR Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page AD694 –10– REV. B Figure 11. Buffer Amplifier VOS Adjustment APPLICATIONS CURRENT OUTPUT DAC INTERFACE The AD694 can be easily interfaced to current output DACs such as the AD566A to construct a digital to 4–20 mA interface as shown in Figure 13. The AD694 provides the voltage refer- ence and the buffer amplifier necessary to operate the DAC. Only simple connections are necessary to construct the circuit. The 10 V reference of the AD694 supplies reference input of the AD566. The buffer amplifier converts the full-scale current to +10 V utilizing the internal resistors in the DAC; therefore the AD694 is configured for a 10 V full-scale input. A 10 pF capaci- tor compensates for the 25 pF output capacitance of the DAC. An optional 100 Ω trim resistor (R T) allows the full-scale to be trimmed, a 50 Ω resistor may be substituted if a trim is not re- quired; accuracy will be typically ±1 LSB and the trim does not affect the 4 mA offset. Care should be taken in managing the circuit grounds. Connections from AD694 Pins 9, 3 and AD566 Pins 3 and 7 should be as short as possible and to a single point close to Pin 5 of the AD694. Best practice would have separate connections to the star ground from each pin; this is essential Figure 12. Using the Alarm to Drive a TTL Gate ALARM CIRCUIT The AD694 has an alarm circuit which warns of open circuit conditions at IOUT (Pin 11), or of attempts to drive the voltage at IOUT higher than VS – 2 V. The alarm transistor will pull down if an out of control condition is sensed. The alarm current is limited to about 20 mA. Figure 12 shows a typical application. In a digital/analog system the alarm can provide a TTL signal to a controller. The collec- tor of the alarm transistor is tied to the system logic supply through a 20 k Ω pull-up resistor. The alarm is off in normal op- eration and the voltage at the alarm pin is high. In the event that the wire from IOUT (Pin 11) is opened, or if a large input over- drive forces IOUT too close to VS, then the alarm pin is driven low. This configuration is compatible with CMOS or TTL logic levels. The alarm transistor can also be used to directly drive an LED or other indicators. for the AD566 power ground from Pin 12. The 4–20 mA output (Pin 11) must have a return path to the power ground. The re- turn line from the load may be connected to the power ground, or to the –15 V supply based upon the size of the load to be driven, and on power dissipation considerations. SINGLE-SUPPLY DIGITAL TO 4–20 mA INTERFACE A 12 bit input to 4–20 mA output interface can be constructed that operates on a single 15 V supply. The DAC is operated in its voltage switching mode; this allows the DAC, when supplied with a voltage reference of less than 2.5 V, to provide an output voltage that is proportional to the digital input code and ranges from 0 V to VREF. The AD694 voltage reference is connected to supply 2 V and the input stage is set to a 2 V full scale; the input buffer amplifier serves to buffer the voltage output from the DAC. Connected in this manner, a full-scale DAC input code will result in a 20 mA output and an all 0 code will result in a 4 mA output. The loading on the AD694 voltage reference is |
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