AD9837 datasheet(12/28 Pages) AD

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AD9837

Rev. 0 | Page 12 of 28

CIRCUIT DESCRIPTION

The AD9837 is a fully integrated direct digital synthesis (DDS)

chip. The chip requires a reference clock and decoupling capa-

citors to provide digitally created sine waves up to 8 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.

The internal circuitry of the AD9837 consists of the following

main sections: a numerically controlled oscillator (NCO),

frequency and phase modulators, SIN ROM, a digital-to-analog

converter, and a regulator.

NUMERICALLY CONTROLLED OSCILLATOR PLUS

PHASE MODULATOR

The AD9837 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.

Continuous time signals have a phase range of 0 to 2π. 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 AD9837

is implemented with 28 bits. Therefore, in the AD9837, 2π = 228.

Likewise, the ΔPhase term is scaled into this range of numbers:

0 < ΔPhase < 228 − 1

With these substitutions, Equation 3 becomes

f =

ΔPhase × fMCLK∕228

(4)

where 0 < ΔPhase < 228 − 1.

The input to the phase accumulator can be selected from either

the FREQ0 register or the FREQ1 register and is controlled by

the FSEL bit in the control register. 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 MSBs of the NCO. The

AD9837 has two phase registers; their resolution is 2π/4096.

SIN ROM

To make the output from the NCO useful, it must be converted

from phase information into a sinusoidal value. Because phase

information maps directly to amplitude, the SIN ROM uses the

digital phase information as an address to a lookup table and

converts the phase information into amplitude.

Although the NCO contains a 28-bit phase accumulator, the out-

put of the NCO is truncated to 12 bits. Using the full resolution

of the phase accumulator is impractical and unnecessary because

a lookup table of 228 entries would be required. It is only necessary

to have sufficient phase resolution such that the errors due to

truncation are smaller than the resolution of the 10-bit DAC.

Therefore, the SIN ROM must have two bits of phase resolution

more than the 10-bit DAC.

The SIN ROM is enabled using the MODE bit (Bit D1) in the

control register (see Table 16).

DIGITAL-TO-ANALOG CONVERTER (DAC)

The AD9837 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 because the device has an on-board

200 Ω resistor. The DAC generates an output voltage of 0.6 V p-p

typical.

REGULATOR

VDD provides the power supply required for the analog section

and the digital section of the AD9837. This supply can have a

value of 2.3 V to 5.5 V.

The internal digital section of the AD9837 is operated at 2.5 V.

An on-board regulator steps down the voltage applied at VDD

to 2.5 V. When the applied voltage at the VDD pin of the AD9837

is less than or equal to 2.7 V, the CAP/2.5V and VDD pins should

be tied together to bypass the on-board regulator.

AD9837 Datasheet(PDF) 12 Page - Analog Devices

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