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AD7871 Datasheet(PDF) 11 Page - Analog Devices |
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AD7871 Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page AD7871/AD7872 –11– REV. D microprocessor and the conversion result is read from the ADC with the following instruction: ADSP-2100 MR0 = DM(ADC) TMS32020/C25: IN D,ADC MR0 = ADSP-2100 MR0 Register D = Data Memory Address ADC = AD7871 Address Figure 17. AD7871 to ADSP-2100 Parallel Interface Figure 18. AD7871 to TMS32020/C25 Interface Some applications may require that conversions be initiated by the microprocessor rather than an external timer. One option is to decode the AD7871 CONVST from the address bus so that a write operation to the ADC starts a conversion. Data is read at the end of conversion as described earlier. Note, a read opera- tion must not be attempted during conversion. Using the CCIF standard where two input frequencies near the top end of the input bandwidth are used, the second and third or- der terms are of different significance. The second order terms are usually distanced in frequency from the original sine waves while the third order terms are usually at a frequency close to the input frequencies. As a result, the second and third order terms are specified separately. The calculation of the intermodulation distortion is as per the THD specification where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the fundamental expressed in dBs. In this case, the input consists of two, equal amplitude, low distortion sine waves. Figure 16 shows a typical IMD plot for the AD7871/AD7872. Figure 16. IMD Plot Peak Harmonic or Spurious Noise Peak Harmonic or Spurious Noise is defined as the ratio of the rms value of the next largest component in the ADC output spectrum (up to fs/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification will be determined by the largest harmonic in the spectrum, but for parts where the harmonics are buried in the noise floor, peak will be a noise peak. MICROPROCESSOR INTERFACE The AD7871 and AD7872 have a wide variety of interfacing op- tions. The AD7871 offers two operating modes and three data-output formats, while the AD7872 is a dedicated serial output device. The fast data access times on the parallel modes of the AD7871 allow interfacing to the very fast DSPs. The se- rial mode on both the AD7871 and AD7872 is compatible with the serial port structures on all the popular DSPs. Parallel Read Interfacing Figures 17 and 18 show interfaces to the ADSP-2100 and the TMS32020/C25 DSP processors. The AD7871 is operating in Mode 1, parallel read for both interfaces. An external timer con- trols conversion start asynchronously to the microprocessor. At the end of each conversion the ADC BUSY/INT interrupts the |
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