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EVAL-AD9831EB Datasheet(PDF) 10 Page - Analog Devices |
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EVAL-AD9831EB Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page AD9831 –10– REV. A Numerical Controlled Oscillator + Phase Modulator This consists of two frequency select registers, a phase accumu- lator 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 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 AD9831 is implemented with 32 bits. Therefore, in the AD9831, 2 π = 232. Likewise, the ∆Phase term is scaled into this range of numbers 0 < ∆Phase < 232 – 1. Making these substitutions into the equation above f = ∆Phase × f MCLK/2 32 where 0 < ∆Phase < 232 With a clock signal of 25 MHz and a phase word of 051EB852 hex f = 51EB852 × 25 MHz/232 = 0.500000000465 MHz The input to the phase accumulator (i.e., the phase step) can be selected either from the FREQ0 Register or FREQ1 Register and this is controlled by the FSELECT pin. 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 con- tents of this register are added to the most significant bits of the NCO. The AD9831 has four PHASE registers, the resolution of these registers being 2 π/4096. Sine Look-Up Table (LUT) To make the output useful, the signal must be converted from phase information into a sinusoidal value. Since phase informa- tion maps directly into amplitude, a ROM LUT converts the phase information into amplitude. To do this, the digital phase information is used to address a sine 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 accumulator is impractical and unnecessary as this would require a look-up table of 2 32 entries. It is necessary only to have sufficient phase resolution in the LUTs such that the dc error of the output waveform is domi- nated by the quantization error in the DAC. This requires the look-up table to have two more bits of phase resolution than the 10-bit DAC. Digital-to-Analog Converter The AD9831 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 volt- age developed across it does not exceed the voltage compliance range. Since full-scale current is controlled by RSET, adjust- ments 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. CIRCUIT DESCRIPTION The AD9831 provides an exciting new level of integration for the RF/Communications system designer. The AD9831 com- bines the Numerical Controlled Oscillator (NCO), SINE Look- Up Table, Frequency and Phase Modulators, and a Digital-to- Analog Converter on a single integrated circuit. The internal circuitry of the AD9831 consists of three main sections. These are: Numerical Controlled Oscillator (NCO) + Phase Modulator SINE Look-Up Table Digital-to-Analog Converter The AD9831 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 12.5 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. THEORY OF OPERATION Sine waves are typically thought of in terms of their magnitude form a(t) = sin ( ωt). However, these are nonlinear and not easy to generate except through piece wise construction. On the other hand, the angular information is linear in nature. That is, the phase angle rotates through a fixed angle for each unit of time. The angular rate depends on the frequency of the signal by the traditional rate of ω = 2πf. MAGNITUDE PHASE +1 0 –1 2 π 0 Figure 21. Sine Wave Knowing that the phase of a sine wave is linear and given a reference interval (clock period), the phase rotation for that period can be determined. ∆Phase = ωδt Solving for ω ω = ∆Phase/δt = 2πf Solving for f and substituting the reference clock frequency for the reference period (1/fMCLK = δt) f = ∆Phase × f MCLK/2 π The AD9831 builds the output based on this simple equation. A simple DDS chip can implement this equation with three major subcircuits. |
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