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ADUM7703 Datasheet(PDF) 19 Page - Analog Devices |
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ADUM7703 Datasheet(HTML) 19 Page - Analog Devices |
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19 / 22 page  Data Sheet ADuM7703 Rev. 0 | Page 19 of 22 The input filter configuration for the ADuM7703 is not limited to the low-pass structure shown in Figure 29. The differential RC filter configuration shown in Figure 30 also achieves excellent performance. Recommended values for the resistors and capacitor are 22 Ω and 47 pF, respectively. R VIN– R VIN+ C ADuM7703 Figure 30. Differential RC Filter Network DIGITAL FILTER The output of the ADuM7703 is a continuous digital bit stream. To reconstruct the original input signal information, this output bit stream must be digitally filtered and decimated. A sinc filter is recommended due to simplicity of the filter. A sinc3 filter is recommended because the filter is one order higher than that of the ADuM7703 modulator, which is a second-order modulator. The type of filter selected, the decimation rate, and the modulator clock used determines the overall system resolution and throughput rate. The higher the decimation rate, the greater the system accuracy, as shown in Figure 31. However, there is a trade-off between accuracy and throughput rate and, therefore, higher decimation rates result in lower throughput solutions. Note that for a given bandwidth requirement, a higher MCLKIN frequency can allow higher decimation rates to be used, resulting in higher SNR performance. 10 100 DECIMATION RATE (MHz) 1000 fIN = 1kHz Figure 31. SNR vs. Decimation Rate of Sinc3 Filter Order A sinc3 filter is recommended for the ADuM7703. This filter can be implemented on a field programmable gate array (FPGA) or a digital signal processor (DSP). Equation 1 describes the transfer function of a sinc filter. DR 1 1 1 () = 1 N Z HZ DR Z (1) where: Z is the sample. DR is the decimation rate. N is the sinc filter order. The throughput rate of the sinc filter is determined by the modulator clock and the decimation rate selected. Throughput = MCLK/DR (2) where MCLK is the modulator clock frequency. As the decimation rate increases, the data output size from the sinc filter increases. The output data size is expressed in Equation 3. The 16 most significant bits are used to return a 16-bit result. Data Size = N × log2 DR (3) For a sinc3 filter, the −3 dB filter response point can be derived from the filter transfer function, Equation 1, and is 0.262 times the throughput rate. The filter characteristics for a third-order sinc filter are summarized in Table 14. Table 14. Sinc3 Filter Characteristics for 20 MHz MCLKIN Decimation Ratio (DR) Throughput Rate (kHz) Output Data Size (Bits) Filter Response (kHz) 32 625 15 163.7 64 312.5 18 81.8 128 156.2 21 40.9 256 78.1 24 20.4 512 39.1 27 10.2 |
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