AD9020/PCB AD [Analog Devices], AD9020/PCB Datasheet - Page 10

• Manufacturer Part Number: AD9020/PCB
• Description: 10-Bit 60 MSPS A/D Converter
• Manufacturer: AD [Analog Devices]

AD9020
Timing
In the AD9020, the rising edge of the ENCODE signal triggers
the A/D conversion by latching the comparators. (See the
AD9020 Timing Diagram.)
The ENCODE is TTL/CMOS compatible and should be driven
from a low jitter (phase noise) source. Jitter on the ENCODE
signal will raise the noise floor of the converter. Fast, clean
edges will reduce the jitter in the signal and allow optimum ac
performance. Locking the system clock to a crystal oscillator
also helps reduce jitter. The AD9020 is designed to operate with
a 50% duty cycle; small (10%) variations in duty cycle should
not degrade performance.
Data Format
The format of the output data (D
MSB INVERT and LSBs INVERT pins. These inputs are dc
control inputs, and should be connected to GROUND or +V
The AD9020 Truth Table gives information to choose from
among Binary, Inverted Binary, Twos Complement and In-
verted Twos Complement coding.
The OVERFLOW output is an indication that the analog input
signal has exceeded the voltage at +V
overflow transition voltage and output delay are not tested or in-
cluded in the data sheet limits. Performance of the overflow in-
dicator is dependent on circuit layout and slew rate of the
encode signal. The operation of this function does not affect the
other data bits (D
requiring a critical measure of the analog input voltage.
0
–D
9
). It is not recommended for applications
ANALOG
ENCODE
OUTPUT
INPUT
DATA
0
–D
SENSE
9
) is controlled by the
. The accuracy of the
N
t
OD
AD9020 Timing Diagram
t
a
N
S
.
–10–
DATA FOR N
Layout and Power Supplies
Proper layout of high speed circuits is always critical but is par-
ticularly important when both analog and digital signals are
involved.
Analog signal paths should be kept as short as possible and be
properly terminated to avoid reflections. The analog input volt-
age and the voltage references should be kept away from digital
signal paths; this reduces the amount of digital switching noise
that is capacitively coupled into the analog section of the circuit.
Digital signal paths should also be kept short, and run lengths
should be matched to avoid propagation delay mismatch.
In high speed circuits, layout of the ground circuit is a critical
factor. A single, low impedance ground plane, on the compo-
nent side of the board, will reduce noise on the circuit ground.
Power supplies should be capacitively coupled to the ground
plane to reduce noise in the circuit. Multilayer boards allow
designers to lay out signal traces without interrupting the
ground plane and provide low impedance power planes.
It is especially important to maintain the continuity of the
ground plane under and around the AD9020. In systems with
dedicated digital and analog grounds, all grounds of the
AD9020 should be connected to the analog ground plane.
The power supplies (+V
lated from the supplies used for external devices; this further re-
duces the amount of noise coupled into the A/D converter.
Sockets limit the dynamic performance and should be used only
for prototypes or evaluation—PCK Elastomerics Part # CCS-68-
55 is recommended for the LCC package. (Tel. [phone])
An evaluation board is available to aid designers and provide a
suggested layout.
N + 1
N + 1
t
OD
t
a
– Aperture Delay
– Output Delay
S
DATA FOR N + 1
and –V
S
) of the AD9020 should be iso-
REV. A