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EVAL-AD9833EB Datasheet(PDF) 8 Page - Analog Devices |
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EVAL-AD9833EB Datasheet(HTML) 8 Page - Analog Devices |
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8 / 18 page  AD9833 –8– REV PrG PRELIMINARY TECHNICAL DATA CIRCUIT DESCRIPTION The AD9833 is a fully integrated Direct Digital Synthesis (DDS) chip. The chip requires one reference clock, one low precision resistor and decoupling capacitors to pro- vide 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 us- ing DSP techniques. The internal circuitry of the AD9833 consists of the fol- lowing main sections: a Numerical Controlled Oscillator (NCO), Frequency and Phase Modulators, SIN ROM, a Digital-to-Analog Converter, and a Regulator. Numerical Controlled Oscillator + Phase Modulator This consists of two frequency select registers, a phase accumulator, two phase offset registers and a phase offset adder. The main component of the NCO is a 28-bit phase accumulator which assembles the phase component of the output signal. Continuous time signals have a phase range of 0 to 2 . 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 AD9833 is implemented with 28 bits. Therefore, in the AD9833, 2 = 2 28. Likewise, the ∆Phase term is scaled into this range of numbers 0 < ∆Phase < 228 – 1. Making these substitu- tions into the equation above f = ∆Phase x f MCLK/2 28 where 0 < ∆Phase < 228 - 1. 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 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 one of these phase registers is added to the most significant bits of the NCO. The AD9833 has two Phase registers, the resolution of these registers being 2 π/4096. SIN ROM To make the output from the NCO useful, it must be converted from phase information into a sinusoidal value. Since phase information maps directly into amplitude, the SIN ROM uses the digital phase information as an ad- dress to a look-up table, and converts the phase information into amplitude. Although the NCO contains a 28-bit phase accumulator, the output of the NCO is trun- cated 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 28 entries. It is necessary only to have sufficient phase resolution such that the errors due to truncation are smaller than the resolution of the 10-bit DAC. This requires the SIN ROM to have two bits of phase resolution more than the 10-bit DAC. The SIN ROM is enabled using the MODE bit (D1) in the control register. This is explained further in Table 11. Digital-to-Analog Converter The AD9833 includes a high impedance current source 10-bit DAC. The DAC receives the digital words from the SIN ROM and converts them into the corresponding analog voltages. The DAC is configured for single-ended operation. An external load resistor is not required as the device has a 200 Ω resistor on board. The DAC generates an output voltage of typically 0.6 Vpp. Regulator VDD provides the power supply required for the analog section and the digital section of the AD9833. This supply can have a value of +2.3V to +5.5V The internal digital section of the AD9833 is operated at 2.5 V. An on-board regulator steps down the voltage ap- plied at VDD to 2.5 V. When the applied voltage at the VDD pin of the AD9833 is equal to or less than 2.7 V, the pins CAP/2.5V and VDD should be tied together, thus by-passing the on-board regulator. |
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