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AD7741YR датащи(PDF) 9 Page - Analog Devices |
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AD7741YR датащи(HTML) 9 Page - Analog Devices |
9 / 12 page REV. 0 AD7741/AD7742 –9– AD7741/AD7742 CLKOUT CLKIN C1 C2 TO OTHER CIRCUITRY 5M Figure 8. On-Chip Oscillator The on-chip oscillator circuit also has a start-up time associated with it before it oscillates at its correct frequency and correct voltage levels. The typical start-up time for the circuit is 5 ms (with a 6.144 MHz crystal). The AD7741/AD7742 master clock appears on the CLKOUT pin of the device. The maximum recommended load on this pin is one CMOS load. When using a crystal to generate the AD7741/ AD7742 clock it may be desirable to then use this clock as the clock source for the system. In this case it is recommended that the CLKOUT signal be buffered with a CMOS buffer before being applied to the rest of the circuit. Reference Input The AD7741/AD7742 performs conversion relative to an applied reference voltage that allows easy interfacing to ratiometric systems. This reference may be applied using the internal 2.5 V bandgap reference. For the AD7741, this is done by simply leaving REFIN/OUT unconnected. For the AD7742, REFIN is tied to REFOUT. Alternatively, an external reference, e.g., REF192 or AD780, may be used. For the AD7741, this is con- nected to REFIN/OUT and will overdrive the internal refer- ence. For the AD7742, it is connected directly to the REFIN pin. While the internal reference will be adequate for most applica- tions, power supply rejection and overall regulation may be improved through the use of an external precision reference. The process of selecting an external voltage reference should include consideration of drive capability, initial error, noise and drift characteristics. A suitable choice would be the AD780 or REF192. Power-Down Mode The low power standby mode is initiated by taking the PD pin low, which shuts down most of the analog and digital circuitry. This reduces the power consumption to 185 µW max. The digital data that represents the analog input voltage is con- tained in the duty cycle of the pulse train appearing at the out- put of the comparator. The output is a fixed-width pulse whose frequency depends on the analog input signal. The input voltage is offset internally so that a full-scale input gives an output fre- quency of 0.45 fCLKIN and zero-scale input gives an output fre- quency of 0.05 fCLKIN. The output allows simple interfacing to either standard logic families or opto-couplers. The clock high period controls the pulsewidth of the frequency output. The pulse is initiated by the edge of the clock signal. The delay time between the edge of the clock and the edge of the frequency output is typically 9 ns. Figure 7 shows the waveform of this frequency output. After power-up, or if there is a step change in input voltage, there is a settling time that must elapse before valid data is obtained. This is typically 2 CLKIN cycles on the AD7742 and 10 CLKIN cycles on the AD7741. 6 TCLK 7 TCLK AVERAGE fOUT IS fCLKIN *3/20 BUT THE ACTUAL PULSE STREAM VARIES BETWEEN fCLKIN/6 AND fCLKIN/7 fCLKIN fOUT = fCLKIN/4 VIN = VREF/2 fOUT = fCLKIN/10 VIN = VREF/8 fOUT = fCLKIN*3/20 VIN = VREF/4 Figure 7. AD7741/AD7742 Frequency Output Waveforms Clock Generation As distinct from the asynchronous VFCs which rely on the stability of an external capacitor to set their full-scale frequency, the AD7741/AD7742 uses an external clock to define the full-scale output frequency. The result is a more stable, more linear trans- fer function and also allows the designer to determine the sys- tem stability and drift based upon the external clock selected. A crystal oscillator may also be used if desired. The AD7741/AD7742 requires a master clock input, which may be an external CMOS-compatible clock signal applied to the CLKIN pin (CLKOUT not used). Alternatively, a crystal of the correct frequency can be connected between CLKIN and CLKOUT, when the clock circuit will function as a crystal controlled oscillator. Figure 8 shows a simple model of the on- chip oscillator. |
Аналогичный номер детали - AD7741YR |
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Аналогичное описание - AD7741YR |
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