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AD7569AQ 数据表(PDF) 12 Page - Analog Devices |
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AD7569AQ 数据表(HTML) 12 Page - Analog Devices |
12 / 20 page –12– REV. B AD7569/AD7669 MODE 1 INTERFACE The timing diagram for the first mode is shown in Figure 10. It can be used in digital signal processing and other applications where precise sampling in time is required. In these applica- tions, it is important that the signal sampling occurs at exactly equal intervals to minimize errors due to sampling uncertainty or jitter. In these cases, the ST line is driven by a timer or some precise clock source. The falling edge of the ST pulse starts conversion and drives the AD7569/AD7669 track-and-hold amplifier into its hold mode. BUSY stays low for the duration of conversion and returns high at the end of conversion and the track-and hold amplifier reverts to its tracking mode on this rising edge of BUSY. The INT line can be used to interrupt the microprocessor. A READ to the AD7569/AD7669 address accesses the data, and the INT line is reset on the rising edge of CS or RD. Alternatively, the INT can be used to trigger a pulse that drives the CS and RD and places the data into a FIFO or buffer memory. The microprocessor can then read a batch of data from the FIFO or buffer memory at some convenient time. The ST input should not be high when RD is brought low; otherwise, the part will not operate correctly in this mode. It is important, especially in systems where the conversion start (ST pulse) is asynchronous to the microprocessor, that a READ does not occur during a conversion. Trying to read data from the device during a conversion can cause errors to the conver- sion in progress. Also, pulsing the ST line a second time before conversion ends should be avoided since it too can cause errors in the conversion result. In applications where precise sampling is not critical, the ST pulse can be generated from a micropro- cessor WR or RD line gated with a decoded address (different from AD7569/AD7669 CS address). Figure 11. Multichannel Inputs This interface mode is also useful in applications where a num- ber of input channels are required to be converted by the ADC. Figure 11 shows the circuit configuration for such an applica- tion. The signal that drives the ST input of the AD7569/ AD7669 is also used to drive the ENABLE input of the multi- plexer. The multiplexer is enabled on the rising edge of the ST pulse while the input signal is held on the falling edge; therefore, the signal must have settled to within 8 bits over the duration of this ST pulse. The settling time, including tON (ENABLE) of the multiplexer plus the T/H acquisition time (typically 200 ns), thus determines the width of the ST pulse. This is suited to ap- plications where a number of input channels needs to be succes- sively sampled or scanned. MODE 2 INTERFACE The second interface mode is intended for use with micropro- cessors, which can be forced into a WAIT state for at least 2 µs. The ST line of the AD7569/AD7669 must be hardwired high to achieve this mode. The microprocessor starts a conversion and is halted until the result of the conversion is read from the con- verter. Conversion is initiated by executing a memory READ to the AD7569/AD7669 address, bringing CS and RD low. BUSY subsequently goes low (forcing the microprocessor READY or WAIT input low), placing the microprocessor into a WAIT state. The input signal is held on the falling edge of RD (assum- ing CS is already low or is coincident with RD). When the con- version is complete (BUSY goes high), the processor completes the memory READ and acquires the newly converted data. While conversion is in progress, the ADC places old data (from the previous conversion) on the data bus. The timing diagram for this interface is shown in Figure 12. Figure 12. ADC Mode 2 Interface Timing The major advantage of this interface is that it allows the micro- processor to start conversion, WAIT, and then READ data with a single READ instruction. The user does not have to worry about servicing interrupts or ensuring that software delays are long enough to avoid reading during conversion. The fast con- version time of the ADC ensures that for many microprocessors, the processor is not placed in a WAIT state for an excessive amount of time. DIGITAL SIGNAL PROCESSING APPLICATIONS In Digital Signal Processing (DSP) application areas such as voice recognition, echo cancellation and adaptive filtering, the dynamic characteristics (SNR, Harmonic Distortion, Intermod- ulation Distortion) of both the ADC and DAC are critical. The AD7569/AD7669 is specified dynamically as well as with stan- dard dc specifications. Because the track/hold amplifier has a wide bandwidth, an antialiasing filter should be placed on the VIN input to avoid aliasing of high-frequency noise back into the band of interest. The dynamic performance of the ADC is evaluated by applying a sine-wave signal of very low distortion to the VIN input, which is sampled at a 409.6 kHz sampling rate. A Fast Fourier Transform (FFT) plot or Histogram plot is then generated from which SNR, harmonic distortion and dynamic differential nonlinearity data can be obtained. For the DAC, the codes for an ideal sine wave are stored in PROM and loaded down to the DAC. The output spectrum is analyzed, using a spectrum analyzer to evaluate SNR |
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类似说明 - AD7569AQ |
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