AD900 Analog Devices, Inc., AD900 Datasheet - Page 9

• Manufacturer Part Number: AD900
• Description: Highspeed 6-bit Aidconverter
• Manufacturer: Analog Devices, Inc.

AD9006/AD9016
Clocking the Converter
The encode command circuit~ of the AD9006/AD9016
(ENCODE and ENCODE) are designed to be driven by a dif-
ferential ECL source.
A differential signal is recommended as the encode command to
The rising edge of the ENCODE signal initiates the conversion
reduce jitter of the encode signal; increased jitter raises the noise
floor of the converter. Full logic levels are preferred for trigger-
ing the dock circuits, but reduced levels can also be used. Cau-
tion should be exercised when using reduced-level encode com-
mands because their slew rates will be decreased, which can
raise the noise floor.
Refer again 10 the timing diagrams for the AD9006 and AD9016.
always required for encode commands, but are especially critical
process in the AD9006 unit. This same signal, delayed, becomes
the DATA READY and complementary DATA READY
pulses. Fast rise and fall limes «0.5ns)
for high frequency analog signals.
trailing edge of the next ENCODE command.
with a 50% duty cycle, the dynamic performance at high encode
Two possible methods of clocking the AD9006/AD9016 are
In the AD90l6, the leadingedges oCthe DATA READY
DATA READY B pulses are triggered by the trailing edge of an
ENCODE command. Their trailing edges are triggered by the
Although the AD9006/AD9016 is designed and tested to operate
rates can be improved by changing the dUty cycle.
shown in Figure 3. Users planning to implement these circuits
need to be aware they may nOt function over the same tempera-
tUre ranges possible with the converters.
Both ECL oscillators and saw filter oscillators are available as
comercial products, with each type operating at some pre-
selected frequency. The type of oscillator which is selected is a
Correct layout of high speed circuits is always critical, but is
ANALOGDEVICES fAX-ON-DEMAND HOTLINE
function of the desired operating frequency for the circuit being
designed.
Layout and Power Supplies
particularly important when both analog and digital signals are
involved.
OSCilLATOR
ECl
Q
Q
B.
and "dean" edges are
ENCODE
ENCODE
AD9016
AD90061
- Page 2~
Figure 3. Clock Circuits
A and
-10-
Analog signal paths should be kept as short as practical, and be
properly terminated to avoid reflections and signal distortions.
The analog inpUt and voltage references should be kept away
from digital signal paths; this reduces the possibility of capac-
itvely coupling digital switching noise into the analog section of
the circuit.
Digital signal paths should also be kept short, and digital run
lengths should be matched because propagation delays through
digital paths become significant at high data rates. Proper ECL
terminations should be used at or near the packages containing
successive gates.
Ideally, analog signal paths and digital signal paths should be
routed as far away from one another as possible and should
never closely parallel one another's paths. If they must cross,
they should do so at right angles to avoid interference.
In any layout of high speed circuits, the layoUt of ground con-
nections is the most important factOr. To reduce noise and inter-
ference on the circuit ground, a double-sided copper-clad
printed circuit board (PCB) is recommended. Every part of the
board not used for components or conducting runs should be
ground plane. Components are mounted on one side; the oppo-
site side is used for power and signa] connections.
It is especially important to retain the continuity of the ground
plane under and around the AD9006/AD9016 converter. If the
system design separates the digital and analog ground retUrns,
both should be connected together and to ground close to the
unit to form a continuous ground plane around the AID section
of the system.
are the preferred choices for high speed circuits. Switching
of nns - and the spikes generated in switchers can produce hard-
Low noise, low ripple temperatUre-stable linear power supplies
power supplies often seem to meet these criteria, including rip-
ple specifications.
to-filter, uncontrollable noise peaks with amplitUdes of several
hundred millivolts. Their high frequency components may be
extremely difficult to keep out of the ground system.
designs, they should be carefully shielded and their oUtputs
If switching power supplies cannot be avoided for high speed
should be well filtered.
OSCILLATOR
SAW FILTER
SINE WAVE
OUT
I
ECL
But ripple specs
HIGH SPEED
=:1
(~~::51TOR
b.
ENCODE
ENCODE
are generally expressedin
AD9016
AD90061
REV.A
tenns