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AD7306 Datasheet(PDF) 6 Page - Analog Devices |
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AD7306 Datasheet(HTML) 6 Page - Analog Devices |
6 / 8 page AD7306 –6– REV. B GENERAL DESCRIPTION The AD7306 drivers/receivers provide an interface which is compatible with RS-232/RS-422 standard interfaces. As both standards are widely accepted it is often necessary to provide an interface which is compatible with both. The AD7306 is ideally suited to this type of application as both standards may be met using a single package. This part contains two RS-232 drivers, one RS-422 driver, one RS-232 receiver, and one receiver path which can be configured as either RS-232 or RS-422. This re- ceiver is configured using the 232/422 SEL pin. This part also contains an internal charge pump voltage con- verter which facilitates operation using a single +5 V power supply. Charge Pump DC-DC Voltage Generator The charge pump voltage generator uses a switched capacitor technique to develop ±10 V levels from an input +5 V supply. A highly efficient charge pump design coupled with a high fre- quency internal oscillator permit operation using four 0.1 µF capacitors. ±10V POWER SUPPLY GENERATOR +5V INPUT SUPPLY +10V OUTPUT –10V OUTPUT C3 0.1 µF C4 0.1 µF C1+ C1– C2+ C2– V– V+ C1 0.1 µF C2 0.1 µF VCC Figure 7. Charge Pump Voltage Generator Capacitors C1 and C2 act as charge storage capacitors while C3 and C4 provide output smoothing. For correct operation all four capacitors must be included. Either polarized or nonpolarized capacitors may be used for C1–C4. If a polarized type is used, then the correct polarity should be observed. This may be ig- nored with nonpolarized type capacitors. The charge pump output voltages, V+ and V–, are used inter- nally to power the RS-232 transmitters. This permits RS-232 output levels to be developed on the RS-232 transmitter out- puts. The charge pump output voltages may also be used to power external circuitry if the current requirements are small. Please refer to the Typical Performance Characteristics. The generator rise time after power up is 200 µs typical. This time is necessary to completely charge the storage capacitors in the charge pump. Therefore, RS-232 data transmission should not be initiated until this time has elapsed after switch on. This will ensure that valid data is always transmitted. RS-232 Drivers The RS-232 drivers in the AD7306 meet the EIA RS-232 specifications. The drivers are inverting level shifters which convert TTL/CMOS levels into RS-232 output levels. The in- put switching threshold is typically 1.3 V. With a typical RS-232 load, the output levels are ±7.5 V. Under worst case load conditions, the drivers are guaranteed to provide ±5 V which meets the minimum RS-232 requirement. The output slew rate is internally limited to <30 V/ µs without the need for an external slew limiting capacitor. Short circuit protection is also provided which prevents damage in the event of output fault conditions. Active current limiting is used which limits the output short circuit current to less than 12 mA in the event of an output fault. This type of current limiting does not degrade the output voltage swing under normal loading conditions as would be the case with conventional passive limiting. The powered-down output impedance is typically 10 M Ω. This is considerably larger than the 300 Ω minimum value required by the RS-232 specification. It provides additional protection under fault conditions where another powered-up transmitter output is inadvertently shorted to the powered-down device. RS-232 Receivers The receivers are inverting level shifters which accept RS-232 input levels ( ±3 V to ±15 V) and translates them into 5 V TTL/CMOS levels. The input switching thresholds are 0.8 V minimum and 2.4 V maximum which are well within the RS-232 requirement of ±3 V. Internal 5 kΩ pull-down resistors to GND are provided on the receiver inputs. This ensures that an unconnected input will be interpreted as a low level giving a Logic “1” on the TTL/CMOS output. Excellent noise immu- nity is achieved by the use of hysteresis and internal filtering circuitry. The filter rejects noise glitches of up to 0.5 µs in duration. RS-422 Driver The RS-422 driver on the AD7306 accepts a TTL/CMOS in- put and translates it into a differential RS-422 level signal. The input switching threshold is typically 1.3 V. The unloaded out- put differential voltage is typically ±5 V (see Typical Perfor- mance Characteristics). Short circuit protection is provided on the output which limits the current to less than 150 mA. RS-422 Receiver The RS-422 receiver on the AD7306 accepts a differential in- put signal and translates it into a TTL/CMOS output level. The input resistance on both differential inputs is 5 k Ω typical. With the receiver inputs unconnected (floating), internal bias- ing ensures that the receiver output is a Logic “1.” Excellent noise immunity and high transmission speed is achieved using the differential configuration. |
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