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AD7769 Arkusz danych(PDF) 10 Page - Analog Devices |
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AD7769 Arkusz danych(HTML) 10 Page - Analog Devices |
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10 / 16 page  AD7769 –10– REV. A 3. If RD is held low but CS is taken high during the conversion, the device will be de-selected and DB0–DB7 will revert to their high impedance state. This will not affect completion of the conversion, but the data cannot be read, or any other operation performed, until CS is taken low again. 4. Note that the state of RD should not be changed during a conversion. Figure 11. Timing for ADC Channel Select and Conversion Start Figure 12. Timing for ADC Data Read Figure 13. Timing for DAC Channel Select and Data Write DIGITAL SIGNAL PROCESSING APPLICATIONS In Digital Signal Processing (DSP) application areas like voice recognition, echo cancellation and adaptive filtering, the dy- namic characteristics (SNR, Harmonic Distortion, Intermodula- tion Distortion) of both the ADC and DACs are critical. The AD7769 is specified dynamically as well as with standard dc specifications. Because the track/hold amplifier has a wide band- width, an antialiasing filter should be placed on the VINA and VINB inputs to avoid aliasing of high frequency noise back into the bands of interest. The dynamic performance of the ADC is evaluated by applying a sine wave signal of very low distortion to the VINA or VINB input which is sampled at a 409.6 kHz sampling rate. A Fast Fourier Transform (FFT) plot or Histogram plot is then gener- ated from which SNR, harmonic distortion and dynamic differ- ential nonlinearity data can be obtained. For the DACs, 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 and harmonic distortion performance. Similarly, for intermodulation distortion, an input (either to VIN or DAC code) consisting of pure sine waves at two frequencies is applied to the AD7769. Figure 14 shows a 2048 point FFT plot of the ADC with an in- put signal of 130 kHz. The SNR is 49.2 dB. It can be seen that most of the harmonics are buried in the noise floor. It should be noted that the harmonics are taken into account when calculat- ing the SNR. The relationship between SNR and resolution (N) is expressed by the following equation: SNR = (6.02N + 1.76) dB Figure 14. ADC FFT Plot |
Podobny numer części - AD7769_15 |
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Podobny opis - AD7769_15 |
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