AD9858/FDPCB Analog Devices Inc, AD9858/FDPCB Datasheet - Page 14

• Manufacturer Part Number: AD9858/FDPCB
• Description: BOARD EVAL FOR AD9858/FD
• Manufacturer: Analog Devices Inc

Specifications of AD9858/FDPCB

• Rohs Status: RoHS non-compliant
• Design Resources: Low Jitter Sampling Clock Generator for High Performance ADCs Using AD9958/9858 and AD9515 (CN0109)
• Lead Free Status / Rohs Status: Not Compliant

AD9858
THEORY OF OPERATION
The AD9858 DDS is a flexible device that can address a wide range
of applications. The device consists of a numerically controlled
oscillator (NCO) with a 32-bit phase accumulator, 14-bit phase
offset adjustment, a power efficient DDS core, and a 1 GSPS,
10-bit DAC. The AD9858 incorporates additional capabilities
for automated frequency sweeping. The device also offers an
analog mixer, a PFD, and a programmable CP with advanced fast
lock capability. These RF building blocks can be used for various
frequency synthesis loops or, as needed, in system design.
The AD9858 can directly generate frequencies up to 400 MHz
when driven at a 1 GHz internal clock speed. This clock can be
derived from an external clock source of up to 2 GHz by using
the on-chip, divide-by-2 feature. The on-chip mixer, PFD, and CP
make possible a variety of synthesizer configurations capable of
generating frequencies in the 1 GHz to 2 GHz range or higher.
The AD9858 offers the advantages of a DDS with the additional
flexibility to work in concert with analog frequency synthesis
techniques (PLL, mixing) to generate precision frequency signals
with high resolution, fast frequency hopping, fast settling time,
and automated frequency sweeping capabilities.
Writing data to its on-chip digital registers that control all
operations of the device easily configures the AD9858. The
AD9858 offers a choice of serial or parallel ports for controlling
the device. Four user profiles can be selected by a pair of
external pins. These profiles allow independent setting of the
frequency tuning word and the phase offset adjustment word
for each of the four selectable configurations.
The AD9858 can be programmed to operate in single-tone mode
or in frequency sweeping mode. To save on power consumption,
there is also a programmable full sleep mode, during which most
of the device is powered down to reduce current flow.
The operation of a direct digital synthesizer (DDS) is described
in detail in a tutorial available from Analog Devices at
www.analog.com/dds.
COMPONENT BLOCKS
DDS Core
The DDS core generates the numeric values that represent a
sinusoid in the digital domain. Depending on the operating
mode of the DDS, this sinusoid may be changed in frequency,
phase, or perhaps modulated by an information carrying signal.
The frequency of the output signal is determined by a user-
programmed frequency tuning word (FTW). The relation of
the output frequency of the device to the system clock (SYSCLK)
is determined by
where N = 32.
f
OUT
=
(
FTW
×
2
SYSCLK
N
)
Rev. C | Page 14 of 32
For a more detailed explanation of a DDS core, consult the DDS
tutorial at www.analog.com/dds.
DAC Output
The AD9858 includes a 10-bit current output DAC. Two
complementary outputs provide a combined full-scale output
current (IOUT, IOUT ). Differential outputs reduce the amount
of common-mode noise that may be present at the DAC output,
offering the advantage of an increased signal-to-noise ratio (SNR).
The full-scale current is controlled by means of an external
resistor (R
ground. The full-scale current is proportional to the resistor
value as
The maximum full-scale output current of the combined DAC
outputs is 40 mA, but limiting the output to 20 mA provides the
best spurious-free dynamic range (SFDR) performance. The DAC
output compliance range is (AV
Voltages developed beyond this range cause excessive DAC
distortion and can damage the DAC output circuitry. Proper
attention should be paid to the load termination to keep the
output voltage within this compliance range. When terminating
the differential outputs into a transformer, the center tap should
be attached to AVDD.
PLL Frequency Synthesizer
The PLL frequency synthesizer is a group of independent
synthesis blocks designed to be used with the DDS to expand
the range of synthesis applications. These blocks are a digital
PFD that drives a CP. The charge pump incorporates fast locking
logic, described in the Fast Locking Logic section. Based on
system requirements, the user supplies an external loop filter
and VCO. A high speed analog mixer is included for translation
synthesis loops. Using the different blocks in the PLL frequency
synthesizer, in conjunction with the DDS, the user can create
translation loops (also known as offset loops), fractional divider
loops, and traditional PLL loops to multiply the output of the DDS
in frequency.
Phase Frequency Detector (PFD)
The phase frequency detector has two inputs: PD
Both are analog inputs that can operate in differential mode or
single-ended mode. Both operate at frequencies up to 150 MHz,
although signals of up to 400 MHz can be accommodated on the
inputs when the divide-by-4 functions are used. The expected
input level for both PD
(differential) and 400 mV p-p (single-ended). A programmable
divider that offers division ratios of M, N = {1, 2, 4} immediately
follows the input. The division ratio is controlled by means of
the control function register.
R
SET
= 39.19/ I
SET
) connected between the DACISET pin and analog
OUT
IN
and DIV
DD
− 1.5 V) to (AV
IN
is in the range of 800 mV p-p
DD
IN
+ 0.5 V).
and DIV
IN
.