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| 型号： | EVAL-AD9835EBZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 6/28页 |
| 文件大小： | 0K |
| 描述： | BOARD EVALUATION FOR AD9835 |
| 产品培训模块： | Direct Digital Synthesis Tutorial Series (1 of 7): Introduction Direct Digital Synthesizer Tutorial Series (7 of 7): DDS in Action Direct Digital Synthesis Tutorial Series (3 of 7): Angle to Amplitude Converter Direct Digital Synthesis Tutorial Series (6 of 7): SINC Envelope Correction Direct Digital Synthesis Tutorial Series (4 of 7): Digital-to-Analog Converter Direct Digital Synthesis Tutorial Series (2 of 7): The Accumulator |
| 标准包装： | 1 |
| 主要目的： | 计时，直接数字合成（DDS） |
| 已用 IC / 零件： | AD9835 |
| 已供物品： | 板 |
| 相关产品： | AD9835BRUZ-REEL7-ND - IC DDS 10BIT 50MHZ 16-TSSOP AD9835BRUZ-REEL-ND - IC DDS 10BIT 50MHZ 16-TSSOP AD9835BRUZ-ND - IC DDS 10BIT 50MHZ 16-TSSOP AD9835BRU-REEL7-ND - IC DDS 10BIT 50MHZ 16-TSSOP AD9835BRU-ND - IC DDS DAC 10BIT 50MHZ 16-TSSOP |
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AD9835
Data Sheet
Rev. A | Page 14 of 28
CIRCUIT DESCRIPTION
The AD9835 provides an exciting level of integration for the
RF communications system designer. The AD9835 combines
the numerical controlled oscillator (NCO), COS lookup table,
frequency and phase modulators, and a digital-to- analog
converter on a single integrated circuit.
The internal circuitry of the AD9835 consists of three main
sections. These are
numerical controlled oscillator (NCO) and phase
modulator
COS lookup table
digital-to-analog converter
The AD9835 is a fully integrated direct digital synthesis (DDS)
chip. The chip requires one reference clock, one low precision
resistor, and eight decoupling capacitors to provide digitally-
created sine waves up to 25 MHz. In addition to the generation
of this RF signal, the chip is fully capable of a broad range of
simple and complex modulation schemes. These modulation
schemes are fully implemented in the digital domain allowing
accurate and simple realization of complex modulation
algorithms using DSP techniques.
NUMERICAL CONTROLLED OSCILLATOR AND
PHASE MODULATOR
This consists of two frequency select registers, a phase accumulator
and four phase offset registers. The main component of the NCO is
a 32-bit phase accumulator, which assembles the phase component
of the output signal. Continuous time signals have a phase range of
0 π to 2 π. Outside this range of numbers, the sinusoid functions
repeat themselves in a periodic manner.
The digital implementation is no different. The accumulator
simply scales the range of phase numbers into a multibit
digital word. The phase accumulator in the AD9835 is
Likewise, the ΔPhase term is scaled into this range of numbers
0 < ΔPhase < 232 1. Making these substitutions into the
equation above
f = ΔPhase × fMCLK/232
where
0 < ΔPhase < 232
The input to the phase accumulator (that is, the phase step)
can be selected either from the FREQ0 register or FREQ1
register and this is controlled by the FSELECT pin or the
FSELECT bit. NCOs inherently generate continuous phase
signals, thus avoiding any output discontinuity when switching
between frequencies.
Following the NCO, a phase offset can be added to perform phase
modulation using the 12-bit PHASE registers. The contents of
this register are added to the most significant bits of the NCO.
The AD9835 has four PHASE registers, the resolution of these
registers being 2 π/4096.
COS LOOKUP TABLE (LUT)
To make the output useful, the signal must be converted from
phase information into a sinusoidal value. Since phase information
maps directly into amplitude, a ROM LUT converts the phase
information into amplitude. To do this, the digital phase infor-
mation is used to address a COS ROM LUT. Although the NCO
contains a 32-bit phase accumulator, the output of the NCO is
truncated to 12 bits. Using the full resolution of the phase accu-
mulator is impractical and unnecessary as this would require a
lookup table of 232 entries.
It is necessary only to have sufficient phase resolution in the LUTs
such that the dc error of the output waveform is dominated by
the quantization error in the DAC. This requires the lookup
table to have two more bits of phase resolution than the
10-bit DAC.
DIGITAL-TO-ANALOG CONVERTER
The AD9835 includes a high impedance current source 10-bit
DAC, capable of driving a wide range of loads at different speeds.
Full-scale output current can be adjusted, for optimum power
and external load requirements, through the use of a single
external resistor (RSET).
The DAC is configured for single-ended operation. The load
resistor can be any value required, as long as the full-scale
voltage developed across it does not exceed the voltage compliance
range. Since full-scale current is controlled by RSET, adjustments
to RSET can balance changes made to the load resistor. However,
if the DAC full-scale output current is significantly less than 4 mA,
the DAC’s linearity may degrade.
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