DESCRIPCIÓN GENERAL

The AD9851 is a highly integrated device that uses advanced DDS technology, coupled with an internal high speed, high performance D/A converter, and comparator, to form a dig i tal ly programmable frequency synthesizer and clock generator function. When referenced to an accurate clock source, the AD9851 generates a stable frequency and phase-programmable digitized analog output sine wave. This sine wave can be used directly as a frequency source, or internally converted to a square wave for agile-clock generator applications. The AD9851’s innovative high speed DDS core accepts a 32-bit frequency tuning word, which results in an output tuning res o lu tion of approximately 0.04 Hz with a 180 MHz system clock. The AD9851 con tains a unique 6 REFCLK Multiplier circuit that eliminates the need for a high speed reference oscillator. The 6 REFCLK Multiplier has min i mal impact on SFDR and phase noise charac ter is tics. The AD9851 provides fi ve bits of programmable phase mod u la tion resolution to enable phase shifting of its output in in cre ments of 11.25°.

The AD9851 contains an internal high speed comparator that can be confi gured to accept the (externally) fi ltered output of the DAC to generate a low jitter output pulse. The frequency tuning, control, and phase modulation words are asynchronously loaded into the AD9851 via a parallel or serial loading format. The parallel load format consists of fi ve it er a tive loads of an 8-bit control word (byte). The fi rst 8-bit byte controls output phase, 6 REFCLK Multiplier, power-down enable and loading for mat; the remaining bytes comprise the 32-bit frequency tuning word. Serial loading is accomplished via a 40-bit serial data stream entering through one of the parallel input bus lines. The AD9851 uses advanced CMOS technology to provide this breakthrough level of functionality on just 555 mW of power dissipation (5 V supply), at the maximum clock rate of 180 MHz. The AD9851 is available in a space-saving 28-lead SSOP, surface-mount package that is pin-for-pin compatible with the popular AD9850 125 MHz DDS. It is specifi ed to operate over the extended industrial temperature range of –40°C to +85°C at >3.0 V supply voltage. Below 3.0 V, the specifi cations apply over the commercial temperature range of 0°C to 85°C.

 

CARACTERÍSTICAS

180 MHz Clock Rate with Selectable 6 Reference Clock Multiplier

On-Chip High Performance 10-Bit DAC and High Speed Comparator with Hysteresis

SFDR >43 dB @ 70 MHz AOUT

32-Bit Frequency Tuning Word

Simplifi ed Control Interface: Parallel or Serial Asynchronous Loading Format

5-Bit Phase Modulation and Offset Capability

Comparator Jitter <80 ps p-p @ 20 MHz

2.7 V to 5.25 V Single-Supply Operation

Low Power: 555 mW @ 180 MHz

Power-Down Function, 4 mW @ 2.7 V

Ultrasmall 28-Lead SSOP Packaging

 

APLICACIONES

Frequency/Phase-Agile Sine Wave Synthesis

Clock Recovery and Locking Circuitry for Digital Communications

Digitally Controlled ADC Encode Generator

Agile Local Oscillator Applications in Communications

Quadrature Oscillator

CW, AM, FM, FSK, MSK Mode Transmitter

 

THEORY OF OPERATION AND APPLICATION

The AD9851 uses direct digital synthesis (DDS) technology, in the form of a nu mer i cal ly controlled oscillator (NCO), to gen er ate a frequency/phase-agile sine wave. The digital sine wave is converted to analog form via an internal 10-bit high speed D/A converter. An on-board 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 in no va tive circuit architecture that allows fast and precise ma nip u la tion of its output word, under full digital control. DDS also enables very high resolution in the incremental se lec tion of output fre quen cy. The AD9851 allows an output fre quen cy resolution of approximately 0.04 Hz at an 180 MSPS clock rate with the option of directly using the reference clock or by engaging the 6 REFCLK multiplier. The AD9851’s output waveform is phase-con tin u ous from one output fre quen cy change to another.