AD9954SLASHPCB datasheet(13/36 Pages) AD

13 / 36 page

AD9954

Rev. 0 | Page 13 of 36

THEORY OF OPERATION

COMPONENT BLOCKS

DDS Core

The output frequency (fO) of the DDS is a function of the fre-

quency 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.

()

≤

FTW

with

FTW

(

)

(

)

–

FTW

with

FTW

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.

In certain applications it is desirable to force the output signal to

zero phase. Simply setting the FTW to 0 does not accomplish

this; it only results in the DDS core holding its current phase

value. Thus, a control bit is required to force the phase accumu-

lator output to zero.

At power-up, the clear phase accumulator bit is set to Logic 1,

but the buffer memory for this bit is cleared (Logic 0). There-

fore, upon power-up, the phase accumulator will remain clear

until the first I/O UPDATE is issued.

Phase-Locked Loop (PLL)

The PLL allows multiplication of the REFCLK frequency. Con-

trol of the PLL is accomplished by programming the 5-bit

REFCLK multiplier portion of Control Function Register No. 2,

Bits <7:3>.

When programmed for values ranging from 0x04 to 0x14

(4 decimal to 20 decimal), the PLL multiplies the REFCLK input

frequency by the corresponding decimal value. However, the

maximum output frequency of the PLL is restricted to

400 MHz. Whenever the PLL value is changed, the user should

be aware that time must be allocated to allow the PLL to lock

(approximately 1 ms).

The PLL is bypassed by programming a value outside the range

of 4 to 20 (decimal). When bypassed, the PLL is shut down to

conserve power.

Clock Input

The AD9954 supports various clock methodologies. Support for

differential or single-ended input clocks, and enabling of an on-

chip oscillator, and/or a phase-locked loop (PLL) multiplier are

all controlled via user programmable bits. The AD9954 may be

configured in one of six operating modes to generate the system

clock. The modes are configured using the CLKMODESELECT

pin, CFR1<4> and CFR2<7:3>. Connecting the external pin

CLKMODESELECT to Logic High enables the on-chip crystal

oscillator circuit. With the on-chip oscillator enabled, users of

the AD9954 connect an external crystal to the REFCLK and

REFCLKB inputs to produce a low frequency reference clock in

the range of 20 MHz to 30 MHz. The signal generated by the

oscillator is buffered before it is delivered to the rest of the chip.

This buffered signal is available via the CRYSTAL OUT pin. Bit

CFR1<4> can be used to enable or disable the buffer, turning on

or off the system clock. The oscillator itself is not powered

down in order to avoid long startup times associated with turn-

ing on a crystal oscillator. Writing CFR2<9> to Logic High

enables the crystal oscillator output buffer. Logic Low at

CFR2<9> disables the oscillator output buffer.

Connecting CLKMODESELECT to Logic Low disables the on-

chip oscillator and the oscillator output buffer. With the oscilla-

tor disabled, an external oscillator must provide the REFCLK

and/or REFCLKB signals. For differential operation, these pins

are driven with complementary signals. For single-ended opera-

tion, a 0.1 µF capacitor should be connected between the

unused pin and the analog power supply. With the capacitor in

place, the clock input pin bias voltage is 1.35 V. In addition, the

PLL may be used to multiply the reference frequency by an

integer value in the range of 4 to 20. Table 5 summarizes the

clock modes of operation. Note the PLL multiplier is controlled

via the CFR2<7:3> bits, independent of the CFR1<4> bit.

Table 5.Clock Input Modes of Operation

CFR1<4>

CLKMODESELECT

CFR2<7:3>

Oscillator Enabled?

System Clock

Frequency Range (MHz)

Low

High

3 < M < 21

Yes

FCLK = FOSC × M

80 < FCLK < 400

Low

High

M < 4 or M > 20

Yes

FCLK = FOSC

20 < FCLK < 30

Low

3 < M < 21

FCLK = FOSC × M

80 < FCLK < 400

Low

M < 4 or M > 20

FCLK = FOSC

10 < FCLK < 400

High

FCLK = 0

N/A

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