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AD9834 Datasheet(PDF) 9 Page - Analog Devices |
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AD9834 Datasheet(HTML) 9 Page - Analog Devices |
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9 / 20 page  AD9834 – 9– REV PrM PRELIMINARY TECHNICAL DATA CIRCUIT DESCRIPTION The AD9834 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 us- ing DSP techniques. The internal circuitry of the AD9834 consists of the fol- lowing main sections: a Numerical Controlled Oscillator (NCO), Frequency and Phase Modulators, SIN ROM, a Digital-to-Analog Converter, a Comparator 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 AD9834 is implemented with 28 bits. Therefore, in the AD9834, 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 pin or the FSEL 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 AD9834 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 bits MODE and OPBITEN in the control register. This is explained fur- ther in Table 14. Digital-to-Analog Converter The AD9834 includes a high impedance current source 10-bit DAC, capable of driving a wide range of loads. 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 can be configured for either single-ended or differential operation. IOUT and IOUTB can be con- nected through equal external resistors to AGND to develop complementary output voltages. The load resis- tors 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 resistors. Comparator The AD9834 can be used to generate synthesised digital clock signals. This can be done by using the on-board self-biasing comparator, which converts the DAC's sinu- soidal signal to a square wave. The output from the DAC may be filtered externally before being applied to the comparator input. The comparator reference voltage is the time-average of the signal applied to VIN. The comparator can accept a signal of 1 Vpp. As the comparator's input is ac-coupled, to operate correctly as a zero crossing dectector, it requires a minimum input frequency of 3 MHz. The comparator's output will be a square wave with an amplitude from 0 V to DVDD. To enable the comparator, bits SIGNPIB and OPBITEN in the control resister are set to '1'. This is explained fur- ther in Table 13. Regulator The AD9834 has separate power supplies for the analog and digital section. AVDD provides the power supply required for the analog section, while DVDD provides the power supply for the digital section. Both of these supplies can have a value of +2.3V to +5.5V, and are independant of each other e.g. the analog section can be operated at 5V and the digital section can be operated at 3V or vice versa. The internal digital section of the AD9834 is operated at 2.5 V. An on-board regulator steps down the voltage ap- plied at DVDD to 2.5 V. The digital inteface (serial port) of the AD9834 is also operated from DVDD. These digi- tal signals are level shifted within the AD9834 to make them 2.5V compatible. When the applied voltage at the DVDD pin of the AD9834 is equal to or less than 2.5V, the pins CAP/2.5V and DVDD should be tied together, thus by-passing the on-board regulator. |
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