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AD9953PCB Fiches technique(PDF) 10 Page - Analog Devices |
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AD9953PCB Fiches technique(HTML) 10 Page - Analog Devices |
10 / 42 page PRELIMINARY TECHNICAL DATA AD9953 Theory of Operation Component Blocks DDS Core The output frequency (fo) of the DDS is a function of the frequency of system clock (SYSCLK), the value of the frequency tuning word (FTW), and the capacity of the accumulator (232, in this case). The exact relationship is given below with fs defined as the frequency of SYSCLK. fo = (FTW)(fs) / 2 32 { 0 ≤ FTW ≤ 231 fo = fs* ( 1 – ( FTW / 2 32 ) ) { 231 < FTW < 232 -1 The AD9953 frequency tuning word(s) are unsigned numbers, where 80000000(hex) represents the highest output frequency possible, commonly referred to as the Nyquist frequency. Values ranging from than 80000001(hex) to FFFFFFFF (hex) will be expressed as aliased frequencies less than Nyquist. An example using a 3-bit phase accumulator will illustrate this principle. For a tuning word of 001, the phase accumulator output (PAO) increments from all zeros to all ones and repeats when the accumulator overflows after clock cycle number 8. For the tuning word of 111, the phase accumulator output (PAO) decrements from all ones to all zeros and repeats when the accumulator overflows after clock cycle number 8. While the phase accumulator outputs are “reversed” with respect to clock cycles, the outputs provide identical inputs to the phase to amplitude converter, which means the DDS output frequencies are identical. Mathematically, for a 3-bit accumulator, the following equations apply: fo = fs* (FTW / 2 3 ) { 0 ≤ FTW ≤ 22 fo = fs* ( 1 – ( FTW / 2 3 ) ) { 22 < FTW < 23 -1 For the 001 frequency tuning word: Fout = Fs * 1/23 = 1/8*Fs And for the 111 frequency tuning word: Fout = Fs * (1 – 7/8) = 1/8*Fs The value at the output of the Phase Accumulator is translated to an amplitude value via the COS(x) functional block and routed to the DAC. REV. PrB 1/30/03 Page 10 Analog Devices, Inc. |
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