AD9850 datasheet(8/19 Pages) AD

8 / 19 page

AD9850

–8–

REV. E

+VS

DATA

BUS

LOW-PASS

FILTER

GND

PROCESSOR

XTAL

OSC

CLK

IOUTB

VINN

VINP

QOUT

QOUTB

IOUT

100k

200

100

470pF

200

COMP

TRUE

CMOS

CLOCK

OUTPUTS

RSET

AD9850

5-POLE ELLIPTICAL

42MHz LOW-PASS

200

IMPEDANCE

8-b

5 PARALLEL DATA,

OR 1-b

40 SERIAL DATA,

RESET, AND 2

CLOCK LINES

Figure 13. Basic AD9850 Clock Generator Application

with Low-Pass Filter

VCA

IF IN

ADC ENCODE

I/Q MIXER

AND

LOW-PASS

FILTER

AD9059

DUAL 8-BIT

ADC

DIGITAL

DEMODULATOR

BASEBAND

DIGITAL

DATA

OUT

ADC CLOCK

FREQUENCY

LOCKED TO Tx CHIP/

SYMBOL PN RATE

AD9850

CLOCK

GENERATOR

CHIP/SYMBOL/PN

RATE DATA

125MHz

REFERENCE

CLOCK

AGC

Figure 14. AD9850 Clock Generator Application in a

Spread-Spectrum Receiver

FREQUENCY

TUNING

WORD

AD9850

COMPLETE-DDS

125MHz

REFERENCE

FILTER

FREQUENCY

OUT

FILTER

a. Frequency/Phase–Agile Local Oscillator

TUNING

WORD

AD9850

COMPLETE-

DDS

125MHz

REFERENCE

CLOCK

FILTER

FREQUENCY

OUT

PHASE

COMPARATOR

LOOP

FILTER

VCO

DIVIDE-BY-N

b. Frequency/Phase–Agile Reference for PLL

TUNING WORD

REF

FREQUENCY

OUT

PHASE

COMPARATOR

LOOP

FILTER

VCO

FILTER

PROGRAMMABLE

“DIVIDE-BY-N”

FUNCTION

AD9850

COMPLETE-

DDS

c. Digitally-Programmable ”Divide-by-N“ Function in PLL

Figure 15. AD9850 Complete-DDS Synthesizer in

Frequency Up-Conversion Applications

THEORY OF OPERATION AND APPLICATION

The AD9850 uses direct digital synthesis (DDS) technology, in

the form of a numerically controlled oscillator, to generate a

frequency/phase-agile sine wave. The digital sine wave is con-

verted to analog form via an internal 10-bit high speed D/A

converter, and an onboard high speed comparator is provided to

translate the analog sine wave into a low jitter TTL/CMOS-

compatible output square wave. DDS technology is an innova-

tive circuit architecture that allows fast and precise manipulation

of its output frequency under full digital control. DDS also

enables very high resolution in the incremental selection of

output frequency; the AD9850 allows an output frequency

resolution of 0.0291 Hz with a 125 MHz reference clock ap-

plied. The AD9850’s output waveform is phase-continuous

when changed.

The basic functional block diagram and signal flow of the

AD9850 configured as a clock generator is shown in Figure 16.

The DDS circuitry is basically a digital frequency divider function

whose incremental resolution is determined by the frequency of

the reference clock divided by the 2

N number of bits in the

tuning word. The phase accumulator is a variable-modulus

counter that increments the number stored in it each time it

receives a clock pulse. When the counter overflows it wraps

around, making the phase accumulator’s output contiguous.

The frequency tuning word sets the modulus of the counter that

effectively determines the size of the increment (

∆ Phase) that

gets added to the value in the phase accumulator on the next

clock pulse. The larger the added increment, the faster the ac-

cumulator overflows, which results in a higher output fre-

quency. The AD9850 uses an innovative and proprietary

algorithm that mathematically converts the 14-bit truncated

value of the phase accumulator to the appropriate COS value.

This unique algorithm uses a much reduced ROM look-up table

and DSP techniques to perform this function, which contributes

to the small size and low power dissipation of the AD9850. The

relationship of the output frequency, reference clock, and tuning

word of the AD9850 is determined by the formula:

fOUT = (

∆ Phase × CLKIN)/232

where:

∆ Phase = value of 32-bit tuning word

CLKIN = input reference clock frequency in MHz

fOUT = frequency of the output signal in MHz

The digital sine wave output of the DDS block drives the inter-

nal high speed 10-bit D/A converter that reconstructs the sine

AD9850 Datasheet(PDF) 8 Page - Analog Devices

AD9850 Datasheet(HTML) 8 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Datasheet Download

Link URL