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AD7391AR Datasheet(PDF) 9 Page - Analog Devices |
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AD7391AR Datasheet(HTML) 9 Page - Analog Devices |
9 / 12 page REV. A AD7390/AD7391 –9– The AD7390 should be powered directly from the system power supply. This arrangement, shown in Figure 6, employs an LC filter and separate power and ground connections to isolate the analog section from the logic switching transients. FERRITE BEAD: TWO TURNS, FAIR-RITE #2677006301 TTL/CMOS LOGIC CIRCUITS 5V POWER SUPPLY 100 F ELECT. 10 F–22 F TANTALUM 0.1 F CERAMIC CAPACITOR 5V 5V RETURN Figure 6. Use Separate Traces to Reduce Power Supply Noise Whether or not a separate power supply trace is available, how- ever, generous supply bypassing will reduce supply-line induced errors. Local supply bypassing consisting of a 10 µF tantalum electrolytic in parallel with a 0.1 µF ceramic capacitor is recom- mended in all applications (Figure 7). AD7390 OR AD7391 0.1 F CLK VOUT REF VDD GND C * 10 F 6 7 8 5 1 2 3 4 SDI CLR LD *OPTIONAL EXTERNAL REFERENCE BYPASS 2.7V TO 5.5V Figure 7. Recommended Supply Bypassing INPUT LOGIC LEVELS All digital inputs are protected with a Zener-type ESD protection structure (Figure 8) that allows logic input voltages to exceed the VDD supply voltage. This feature can be useful if the user is driving one or more of the digital inputs with a 5 V CMOS logic input- voltage level while operating the AD7390/AD7391 on a 3 V power supply. If this mode of interface is used, make sure that the VOL of the 5 V CMOS meets the VIL input requirement of the AD7390/ AD7391 operating at 3 V. See TPC 6 for a graph for digital logic input threshold versus operating VDD supply voltage. LOGIC IN VDD GND Figure 8. Equivalent Digital Input ESD Protection In order to minimize power dissipation from input-logic levels that are near the VIH and VIL logic input voltage specifications, a Schmitt trigger design was used that minimizes the input-buffer current consumption compared to traditional CMOS input stages. TPC 5 shows a plot of incremental input voltage versus supply current showing that negligible current consumption takes place when logic levels are in their quiescent state. The normal crossover current still occurs during logic transitions. A secondary advantage of this Schmitt trigger is the prevention of false triggers that would occur with slow moving logic transi- tions when a standard CMOS logic interface or opto isolators are used. The logic inputs SDI, CLK, LD, CLR all contain the Schmitt trigger circuits. DIGITAL INTERFACE The AD7390/AD7391 have a double-buffered serial data input. The serial-input register is separate from the DAC register, which allows preloading of a new data value into the serial regis- ter without disturbing the present DAC values. A functional block diagram of the digital section is shown in Figure 4, while Table I contains the truth table for the control logic inputs. Three pins control the serial data input. Data at the Serial Data Input (SDI) is clocked into the shift register on the rising edge of CLK. Data is entered in MSB-first format. Twelve clock pulses are required to load the 12-bit AD7390 DAC value. If additional bits are clocked into the shift register, for example when a microcontroller sends two 8-bit bytes, the MSBs are ignored (Figure 9). The CLK pin is only enabled when Load ( LD) is high. The lower resolution 10-bit AD7391 contains a 10-bit shift register. The AD7391 is also loaded MSB first with 10 bits of data. Again if additional bits are clocked into the shift register, only the last 10 bits clocked in are used. The Load pin ( LD) controls the flow of data from the shift register to the DAC register. After a new value is clocked into the serial-input register, it will be transferred to the DAC register by the negative transition of the Load pin ( LD). BYTE 1 BYTE 0 MSB LSB MSB LSB B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 X X X X D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 X X X X X X D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 D11–D0: 12-BIT AD7390 DAC VALUE; D9–D0: 10-BIT AD7391 DAC VALUE X = DON’T CARE THE MSB OF BYTE 1 IS THE FIRST BIT THAT IS LOADED INTO THE DAC Figure 9. Typical AD7390-Microprocessor Serial Data Input Forms RESET ( CLR) PIN Forcing the CLR pin low will set the DAC register to all zeros and the DAC output voltage will be zero volts. The reset function is useful for setting the DAC outputs to zero at power-up or after a power supply interruption. Test systems and motor controllers are two of many applications which benefit from powering up to a known state. The external reset pulse can be generated by the microprocessor’s power-on RESET signal, by an output from the microprocessor, or by an external resistor and capacitor. CLR has a Schmitt trigger input which results in a clean reset function when using external resistor/capacitor generated pulses. The CLR input overrides other logic inputs, specifically LD. However, LD should be set high before CLR goes high. If CLR is kept low, then the contents of the shift register will be transferred to the DAC register as soon as CLR returns high. See the Control-Logic Truth Table I. |
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