Analog Devices AN-587 Application Notes

AN-587

a

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APPLICATION NOTE

Synchronizing Multiple AD9850/AD9851 DDS-Based Synthesizers

by David Brandon of Analog Devices, Inc.

INTRODUCTION

Many applications require the generation of two or
more sinusoidal signals with a known phase relation­ship (e.g., quadrature). The AD9850 and AD9851 DDS
ICs from Analog Devices are capable of providing such
signals. This application note offers detailed instruc­tions on how to synchronize two or more of these
devices, and considers possible sources of phase error.

REF CLOCK

The first requirement for successful synchronization of
multiple AD9850/AD9851s is that there must be minimal
phase error between the REF CLK inputs to all DDSs.
Any difference in phase between REF CLK edges will result
in a proportional phase difference at the DDS outputs.
The user must employ careful clock distribution practice
in the layout of the PCB to ensure coincident REF CLK
edges (see Figure 1).

The AD9850/AD9851 REF CLK input circuitry is single­ended so it is necessary that the REF CLK have minimum
input jitter and fast rise/fall times (less than 5 ns is rec­ommended). A slow rise time on REF CLK can introduce
errors because the voltage trip point of the input circuit
varies from device to device. These attributes would
also apply to W_CLK and FQ_UD inputs.

AD9850/AD9851 I/O ACCESS DETAILS

Once a fast-edged and properly routed REF CLK signal is
provided, the next timing requirement is the coincident
transfer of the data into the DDS program registers. The
FQ_UD signal transfers the data to the DDS core. Syn­chronization of multiple DDSs requires that the FQ_UD
rising edge occur simultaneously at all DDSs, just like
the REF CLK. In addition, the FQ_UD must occur at the
proper time with respect to the REF CLK.

REV. 0

A = B = C

REF
CLK

OPTIMUM LAYOUT

DDS1

A

B

DDS2

C

DDS3

Figure 1. REF CLK Distribution

REF
CLK

DDS1

DDS2

DDS3

© Analog Devices, Inc., 2002

AN-587

FACTORY CODES

SHIFT

REGISTER

SLEEP MODE

AD9851 6

MULTIPLIER

40

D

Q

CK

RCK

REGISTER

3

Q

Q

5

32

Q

R

DATA

W_CLK

FQ_UD

RESET

REF
CLK

CONTROL

FUNCTIONS

8

Figure 2. AD9850/AD9851 Functional Block Diagram

Figure 2 is a functional block diagram of the AD9850/AD9851.
There are only small differences between the two devices.
The AD9851 has a 6x clock multiplier (PLL) and MUX, but
the AD9850 does not.

There are two registers in front of the DDS core. The shift
register accepts five bytes in parallel mode or 40 bits in
serial mode. W_CLK latches data into this register. The
second register, after it is triggered by FQ_UD, pre­sents the contents of the shift register to the DDS core
on the next rising edge of the SYSTEM CLOCK.

It is essential that a proper time relationship exist between
the FQ_UD and SYSTEM CLOCK (Figure 2). Improper tim­ing of these signals can result in partial loading of the
tuning word, inhibiting the synchronization of the DDSs.
The FQ_UD must have proper setup times prior to a ris­ing REF CLK edge. The proper timing will be addressed
in the Synchronization Instructions section, and is
shown in figures 4 and 6.

PHASE OFFSET

PHASE

ACCUMULATOR

R

6 REF CLK
MULTIPLIER

10

DAC

SYSTEM CLOCK

COMP

AD9851

14 14

PHASE

AMPLITUDE

MUX

TO

RESET

A RESET must be given after power-up and prior to
transferring any data to the DDS. This places the DAC
output to a known state, which becomes the common
reference point that allows the synchronization of mul­tiple DDSs.

RESET forces the AD9850/AD9851’s phase state to become
COS(0). When new data is sent simultaneously to mul­tiple DDSs, a coherent phase relationship is maintained,
or the relative phase can be shifted between devices by
means of the phase offset adjustment register. The
AD9850 and AD9851 have 5 bits of phase offset adjust­ments, amounting to a phase resolution of 11.25 degrees.
The phase offset adder is located between the phase accu­mulator and the phase-to-amplitude converter.

NOTE: The RESET does not reset the shift register. It
only resets the FQ_UD’s register and the phase accumu­lator to COS(0). The shift register should be treated as
containing “random” data after a RESET and may inad­vertently contain a reserved “factory code” that causes
the DDS to behave in unintended ways. For this reason,
a FQ_UD should not be sent until the shift register has
been programmed with the intended data.

–2–

REV. 0

AN-587

SYNCHRONIZATION INSTRUCTIONS

Figure 3 presents one possible reference design for suc­cessful synchronization of multiple DDSs. This example
shows how to place two DDSs into a quadrature phase
relationship.

In Figure 3, the D flip-flop enables the FQ_UD to be syn­chronous with the REF CLK and provides a setup time
delay. Proper operation may require additional time delay
in the FQ_UD path. This delay depends upon the CK-to-Q
propagation time of the flip-flop. The recommended
timing relationship between the FQ_UD (Pin 8) and the
REF CLK (Pin 9) is depicted in Figure 4.

Here are some general instructions and recommendations
for placing two DDSs into a

quadrature

phase relationship
(refer to Figure 3). There are two sets of instructions, with
and without the 6 REF CLK multiplier enabled.

CK

D FLOP

DQ

FQ_UD

RESET

MICROPROCESSOR

OR FPGA

W_CLK-1

W_CLK-2

8-BIT DATABUS

Instructions for synchronizing two DDSs in quadrature
without the AD9851’s 6 REF CLK multiplier enabled:

1. Power up all devices and apply the common REF CLK.

2. Send a common RESET with a minimum high time of
five REF CLK periods.

3. Program DDS #1 to the desired frequency and a
phase offset of 0 degrees

without

issuing an FQ_UD.

4. Program DDS #2 to the same frequency and a phase
offset of 90 degrees without issuing an FQ_UD.

5. Assert a

common

FQ_UD. This will result in the DAC
outputs becoming active simultaneously at the cor­rect frequency and phase offset as programmed.

6. See Figure 4 for the recommended timing between
REF CLK and FQ_UD.

REF CLK

AD9850/AD9851

#1

DATA

W_CLK

FQ_UD

RESET

RESET

FQ_UD

W_CLK

AD9850/AD9851

#2

DATA

90

PHASE

DIFFERENCE

REV. 0

REF CLK

FQ_UD

REF CLK

Figure 3. Application Circuit

AD9850/AD9851 TIMING RELATIONSHIP

WITHOUT THE 6 REF CLK MULTIPLIER ENABLED

1.5ns
(MIN)

NOT

VALID

THIS TIMING CAN BE TRANSLATED INTO A MINIMUM SETUP
TIME OF 0ns AND A MINIMUM HOLD TIME OF 2 SYSTEM
CLOCK PERIODS.

FQ_UD MUST OCCUR

WITHIN THIS RANGE

VALID

Figure 4. Proper Timing Relationship between REF CLK and FQ_UD

–3–