AD9953ASV datasheet(10/42 Pages) AD

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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

{ 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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