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LTC1272-8ACSW Datasheet(PDF) 9 Page - Linear Technology |
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LTC1272-8ACSW Datasheet(HTML) 9 Page - Linear Technology |
9 / 20 page 9 LTC1272 1272fb S APPLICATI I FOR ATIO Figure 5. RD and CLK IN for Synchronous Operation LTC1272 • TA06 CS & RD BUSY CLK IN ≥40ns* t2 t14 tCONV t13 DB0 (LSB) DB1 DB10 DB11 (MSB) UNCERTAIN CONVERSION TIME FOR 30ns <t 14 < 180ns THE LTC1272 IS ALSO COMPATIBLE WITH THE AD7572 SYNCHRONIZATION MODES. * Figure 5. Nevertheless, even without observing this guide- line, the LTC1272 is still compatible with AD7572 synchro- nization modes, with no increase in linearity error. This means that either the falling or rising edge of CLK IN may be near RD’s falling edge. Driving the Analog Input The analog input of the LTC1272 is much easier to drive than that of the AD7572. The input current is not modu- lated by the DAC as in the AD7572. It has only one small current spike from charging the sample-and-hold capaci- tor at the end of the conversion. During the conversion the analog input draws only DC current. The only requirement is that the amplifier driving the analog input must settle after the small current spike before the next conversion is started. Any op amp that settles in 1 µs to small current transients will allow maximum speed operation. If slower op amps are used, more settling time can be provided by increasing the time between conversions. Suitable de- vices capable of driving the LTC1272 AIN input include the LT1006 and LT1007 op amps. Internal Clock Oscillator Figure 6 shows the LTC1272 internal clock circuit. A crystal or ceramic resonator may be connected between CLK IN (Pin 17) and CLK OUT (Pin 18) to provide a clock oscillator for ADC timing. Alternatively the crystal/resona- tor may be omitted and an external clock source may be connected to CLK IN. For an external clock the duty cycle is not critical. An inverted CLK IN signal will appear at the CLK OUT pin as shown in the operating waveforms of Figure 7. Capacitance on the CLK OUT pin should be minimized for best analog performance. Internal Reference The LTC1272 has an on-chip, temperature compensated, curvature corrected, bandgap reference, which is factory trimmed to 2.42V ±1%. It is internally connected to the DAC and is also available at pin 2 to provide up to 1mA current to an external load. For minimum code transition noise the reference output should be decoupled with a capacitor to filter wideband noise from the reference (10 µF tantalum in parallel with a 0.1 µF ceramic). A simplified schematic of the reference with its recommended decoupling is shown in Figure 8. Figure 6. LTC1272 Internal Clock Circuit LTC1272 • TA09 CLK OUT CLK IN C1 C2 18 17 1M CLOCK LTC1272 NOTES: LTC1272-3 – 4MHz CRYSTAL/CERAMIC RESONATOR LTC1272-8 – 1.6MHz CRYSTAL/CERAMIC RESONATOR |
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