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AD9856/PCB Datasheet(PDF) 25 Page - Analog Devices |
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AD9856/PCB Datasheet(HTML) 25 Page - Analog Devices |
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25 / 37 page  AD9856 Rev. C | Page 24 of 36 Table 6. CIC Interpolation Filter Insertion Loss Table Default Gain 2 × Gain Interpolation Rate (Linear) (dB) (Linear) (dB) 2 1.0000 0.000 2.0000 6.021 3 0.8438 −1.476 1.6875 4.545 4 1.0000 0.000 2.0000 6.021 5 0.9766 −0.206 1.9531 5.815 6 0.8438 −1.476 1.6875 4.545 7 0.6699 −3.480 1.3398 2.541 8 1.0000 0.000 2.0000 6.021 9 0.7119 −2.951 1.4238 3.069 10 0.9766 −0.206 1.9531 5.815 11 0.6499 −3.743 1.2998 2.278 12 0.8438 −1.476 1.6875 4.545 13 0.5364 −5.411 1.0728 0.610 14 0.6699 −3.480 1.3398 2.541 15 0.8240 −1.682 1.6479 4.339 16 1.0000 0.000 2.0000 6.021 17 0.5997 −4.441 1.1995 1.580 18 0.7119 −2.951 1.4238 3.069 19 0.8373 −1.543 1.6746 4.478 20 0.9766 −0.206 1.9531 5.815 21 0.5652 −4.955 1.1305 1.065 22 0.6499 −3.743 1.2998 2.278 23 0.7426 −2.585 1.4852 3.436 24 0.8438 −1.476 1.6875 4.545 25 0.9537 −0.412 1.9073 5.609 26 0.5364 −5.411 1.0728 0.610 27 0.6007 −4.427 1.2014 1.593 28 0.6699 −3.480 1.3398 2.541 29 0.7443 −2.565 1.4886 3.455 30 0.8240 −1.682 1.6479 4.339 31 0.9091 −0.827 1.8183 5.193 32 1.0000 0.000 2.0000 6.021 33 0.5484 −5.219 1.0967 0.802 34 0.5997 −4.441 1.1995 1.580 35 0.6542 −3.686 1.3084 2.335 36 0.7119 −2.951 1.4238 3.069 37 0.7729 −2.237 1.5458 3.783 38 0.8373 −1.543 1.6746 4.478 39 0.9051 −0.866 1.8103 5.155 40 0.9766 −0.206 1.9531 5.815 41 0.5258 −5.583 1.0517 0.437 42 0.5652 −4.955 1.1305 1.065 43 0.6066 −4.342 1.2132 1.679 44 0.6499 −3.743 1.2998 2.278 45 0.6952 −3.157 1.3905 2.863 46 0.7426 −2.585 1.4852 3.436 47 0.7921 −2.024 1.5842 3.996 48 0.8438 −1.476 1.6875 4.545 49 0.8976 −0.938 1.7952 5.082 Default Gain 2 × Gain Interpolation Rate (Linear) (dB) (Linear) (dB) 50 0.9537 −0.412 1.9073 5.609 51 0.5060 −5.917 1.0120 0.104 52 0.5364 −5.411 1.0728 0.610 53 0.5679 −4.914 1.1358 1.106 54 0.6007 −4.427 1.2014 1.593 55 0.6347 −3.949 1.2693 2.072 56 0.6699 −3.480 1.3398 2.541 57 0.7065 −3.018 1.4129 3.002 58 0.7443 −2.565 1.4886 3.455 59 0.7835 −2.120 1.5669 3.901 60 0.8240 −1.682 1.6479 4.339 61 0.8659 −1.251 1.7317 4.770 62 0.9091 −0.827 1.8183 5.193 63 0.9539 −0.410 1.9077 5.610 INVERSE SINC FILTER (ISF) The AD9856 is almost entirely a digital device. The input signal is made up of a time series of digital data-words. These data- words propagate through the device as numbers. Ultimately, this number stream must be converted to an analog signal. To this end, the AD9856 incorporates an integrated DAC. The output waveform of the DAC is the familiar staircase pattern typical of a signal that is sampled and quantized. The staircase pattern is a result of the finite time that the DAC holds a quantized level until the next sampling instant. This is known as a zero-order hold function. The spectrum of the zero-order hold function is the SIN(x)/x, or SINC, envelope. The series of digital data-words presented at the input of the DAC represent an impulse stream. It is the spectrum of this impulse stream, which is the desired output signal. Due to the zero-order hold effect of the DAC, however, the output spectrum is the product of the zero-order hold spectrum (the SINC envelope) and the Fourier transform of the impulse stream. Thus, there is an intrinsic distortion in the output spectrum, which follows the SINC response. The SINC response is deterministic and totally predictable. Thus, it is possible to predistort the input data stream in a manner that compensates for the SINC envelope distortion. This can be accomplished by means of an ISF. The ISF incorporated on the AD9856 is a 17-tap, linear phase FIR filter. Its frequency response characteristic is the inverse of the SINC envelope. Data sent through the ISF is altered to correct for the SINC envelope distortion. Note, however, that the ISF is sampled at the same rate as the DAC. Thus, the effective range of the SINC envelope compensation only extends to the Nyquist frequency (1/2 of the DAC sample rate). |
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