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PCM1740 Datasheet(PDF) 8 Page - Texas Instruments |
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PCM1740 Datasheet(HTML) 8 Page - Texas Instruments |
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8 / 25 page  PCM1740 8 SBAS128B www.ti.com The 27MHz clock is available at the MCKO output (pin 5). The VCXO output frequency can be precisely tuned using a control voltage at the XTUN input (pin 3). The tuning range is 27MHz ±150ppm typical for a 0V to +3V control voltage range. Figure 3 shows the VCXO equivalent circuit, while Figure 4 shows the typical tuning curve. At power up, the VCXO requires 5ms start up time. The VCXO also exhibits a 10 µs settling time in response to changes in the XTUN control voltage. VCXO operation and the MCKO output are not affected by the power on or external reset functions, continuing to operate during the initialization sequence. Crystal Selection The VCXO connects to an external 27MHz crystal via XT1 (pin 1) and XT2 (pin 24). The crystal should be AT-cut, fundamental mode with ±30ppm accuracy and less than 50Ω motional resistance. Crystal shunt capacitance should be 3pF maximum, while load capacitance should be less than 7pF. Miniature lead type or surface-mount devices are recom- mended. External load capacitors are not needed, since they are provided on-chip. The crystal should be placed as close as possible to the XT1 and XT2 pins to reduce effects of parasitic capacitance and land resistance. PROGRAMMABLE PHASE LOCKED LOOP (PLL) The PCM1740 includes an on-chip PLL for generating a 256fS or 384fS audio system clock from the 27MHz VCXO output. A block diagram of the PLL section is shown in Figure 5. The PLL output clock is used by the digital filter and delta-sigma modulator circuitry, and is made available at the SCKO output (pin 21) for use with additional audio converters and signal processors. Tuning Voltage (V) 27.005 27.004 27.003 27.002 27.001 27.000 26.999 26.998 26.997 26.996 26.995 0.0 0.5 1.5 1.0 2.0 2.5 3.0 3.5 4.0 27MHz Crystal XTUN 0V to +3V VCXO MCKO 27MHz ±150ppm SCKO 256/384f S Frequency Selection ROM Phase Detector and Loop Filter PLL N Counter Frequency Selection Control Register 3 VCO M Counter FIGURE 3. VCXO Equivalent Circuit. FIGURE 4. VCXO Output Frequency (MCKO) versus Tuning Voltage (XTUN). XT1 27MHz Crystal 27MHz Tuned Clock Voltage Range 0 to 3V 1 24 3 XTUN C LV XT2 C L FIGURE 5. PLL Block Diagram. |
Similar Part No. - PCM1740_09 |
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Similar Description - PCM1740_09 |
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