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AD73360L Datasheet(PDF) 9 Page - Analog Devices |
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AD73360L Datasheet(HTML) 9 Page - Analog Devices |
9 / 32 page ![]() REV. 0 AD73360L –9– Figure 7 shows the various stages of filtering that are employed in a typical AD73360L application. In Figure 7a we see the trans- fer function of the external analog antialias filter. Even though it is a single RC pole, its cutoff frequency is sufficiently far away from the initial sampling frequency (DMCLK/8) that it takes care of any signals that could be aliased by the sampling frequency. This also shows the major difference between the initial oversam- pling rate and the bandwidth of interest. In Figure 7b, the signal and noise-shaping responses of the sigma-delta modulator are shown. The signal response provides further rejection of any high-frequency signals while the noise-shaping will push the inherent quantization noise to an out-of-band position. The detail of Figure 7c shows the response of the digital decimation filter (sinc-cubed response) with nulls every multiple of DMCLK/ 256, which is the decimation filter update rate. The final detail in Figure 7d shows the application of a final antialias filter in the DSP engine. This has the advantage of being implemented accord- ing to the user’s requirements and available MIPS. The filtering in Figures 7a through 7c is implemented in the AD73360L. FB = 4kHz FSINIT = DMCLK/8 a. Analog Antialias Filter Transfer Function FB = 4kHz FSINIT = DMCLK/8 NOISE TRANSFER FUNCTION SIGNAL TRANSFER FUNCTION b. Analog Sigma-Delta Modulator Transfer Function FB = 4kHz FSINTER = DMCLK/256 c. Digital Decimator Transfer Function FB = 4kHz FSINTER = DMCLK/256 FSFINAL = 8kHz d. Final Filter LPF (HPF) Transfer Function Figure 7. DC Frequency Responses Decimation Filter The digital filter used in the AD73360L carries out two impor- tant functions. Firstly, it removes the out-of-band quantization noise, which is shaped by the analog modulator and secondly, it decimates the high-frequency bitstream to a lower rate 15-bit word. The antialiasing decimation filter is a sinc-cubed digital filter that reduces the sampling rate from DMCLK/8 to DMCLK/ 256, and increases the resolution from a single bit to 15 bits. Its Z transform is given as: [(1–Z–32)/(1–Z–1)]3. This ensures a mini- mal group delay of 25 µs. Word growth in the decimator is determined by the sampling rate. At 64 kHz sampling, where the oversampling ratio between the sigma-delta modulator and decimator output equals 32, there are five bits per stage of the three-stage Sinc 3 filter. Due to symmetry within the sigma-delta modulator, the LSB will always be a zero; therefore, the 16-bit ADC output word will have 2 LSBs equal to zero, one due to the sigma-delta symmetry and the other being a padded zero to make up a 16-bit word. At lower sampling rates, decimator word growth will be greater than the 16-bit sample word, therefore truncation occurs in trans- ferring the decimator output as the ADC word. For example at 8 kHz sampling, word growth reaches 24 bits due to the OSR of 256 between sigma-delta modulator and decimator. This yields eight bits per stage of the three stage Sinc 3 filter. ADC Coding The ADC coding scheme is in two’s complement format (see Figure 8). The output words are formed by the decimation filter, which grows the word length from the single-bit output of the sigma-delta modulator to a 15-bit word, which is the final output of the ADC block. In 16-bit Data Mode this value is left shifted with the LSB being set to 0. For input values equal to or greater than positive full scale, however, the output word is set at 0x7FFF, which has the LSB set to 1. In mixed Control/Data Mode, the resolution is fixed at 15 bits, with the MSB of the 16-bit transfer being used as a flag bit to indicate either control or data in the frame. VREF + (VREF 0.32875) VREF VREF – (VREF 0.32875) 10...00 00...00 01...11 ADC CODE DIFFERENTIAL ANALOG INPUT VINN VINP VREF + (VREF 0.6575) VREF – (VREF 0.6575) 10...00 00...00 01...11 ADC CODE SINGLE-ENDED ANALOG INPUT VINP VINN Figure 8. ADC Transfer Function |
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