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AD7225LP Arkusz danych(PDF) 10 Page - Analog Devices |
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AD7225LP Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 12 page AD7225 REV. B –10– FILTER I/P I/P SAMPLES Am29520 TLD AD7820 ADC SAMPLES AD7225 QUAD DAC DELAYED I/P AD7226 QUAD DAC VREF A h1 h2 VOUT A VOUT A VOUT B VOUT C VOUT D ACCUMULATOR O/P AD585 SHA FILTER O/P TAP WEIGHTS Am7224 DAC AD584 REF GAIN SET +10V VOUT VREF VREF + T T T 1 234 h4 h3 h2 h1 + FILTER O/P Yn FILTER I/P Xn–1 Xn–2 Xn Xn–3 VREF A h3 VOUT A VREF A h4 VOUT A VREF A VOUT A Figure 18. Programmable Transversal Filter A 4-tap programmable transversal filter may be implemented using the AD7225 (Figure 18). The input signal is first sampled and converted to allow the tapped delay line function to be pro- vided by the Am29520. The multiplication of delayed input samples by fixed, programmable up weights is accomplished by the AD7225, the four coefficients or reference inputs being set by the digital codes stored in the AD7226. The resultant prod- ucts are accumulated to yield the convolution sum output sample which is held by the AD585. 0 –100 0.5 –70 –90 0.05 –80 0 –40 –60 –50 –30 –20 –10 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 NORMALIZED FREQUENCY – f/fs h1 = 0.117 h2 = 0.417 h3 = 0.417 h4 = 0.417 Figure 19. Predicted (Theoretical) Response Figure 20. Actual Response Low pass, bandpass and high pass filters may be synthesized us- ing this arrangement. The particular up weights needed for any desired transfer function may be obtained using the standard Remez Exchange Algorithm. Figure 19 shows the theoretical low pass frequency response produced by a 4-tap transversal filter with the coefficients indicated. Although the theoretical prediction does not take into account the quantization of the in- put samples and the truncation of the coefficients, nevertheless, there exists a good correlation with the actual performance of the transversal filter (Figure 20). DIGITAL WORD MULTIPLICATION Since each DAC of the AD7225 has a separate reference input, the output of one DAC can be used as the reference input for another. This means that multiplication of digital words can be performed (with the result given in analog form). For example, if the output from DACA is applied to VREF B then the output from DACB, VOUT B, can be expressed as: VOUT B = DA • DB • VREF A where DA and DB are the fractional representations of the digital words in DAC latches A and B respectively. If DA = DB = D then the result is D 2 • V REF A In this manner, the four DACs can be used on their own or in conjunction with an external summing amplifier to generate complex waveforms. Figure 21 shows one such application. In this case the output waveform, Y, is represented by: Y = –(x 4 + 2x3 + 3x2 + 2x + 4) • V IN where x is the digital code which is applied to all four DAC latches. V OUT A V OUT B V OUT C V OUT D V REF A V REF B V REF C V REF D +15V V DD AD7225* DGND AGND V SS V IN 25k Ω 50k Ω 33k Ω 50k Ω 100k Ω 100k Ω Y *DIGITAL INPUTS OMITTED FOR CLARITY Figure 21. Complex Waveform Generation |
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