Datasheet

Data Sheet AD7294
Rev. H | Page 45 of 48
LAYOUT AND CONFIGURATION
POWER SUPPLY BYPASSING AND GROUNDING
For optimum performance, carefully consider the power supply
and ground return layout on any PCB where the AD7294 is
used. The PCB containing the AD7294 should have separate
analog and digital sections, each having its own area of the
board. The AD7294 should be located in the analog section
on any PCB.
Decouple the power supply to the AD7294 to ground with
10 µF and 0.1 µF capacitors. Place the capacitors as physically
close as possible to the device, with the 0.1 µF capacitor ideally
right up against the device. It is important that the 0.1 µF
capacitor have low effective series resistance (ESR) and low
effective series inductance (ESL); common ceramic types of
capacitors are suitable. The 0.1 µF capacitor provides a low
impedance path to ground for high frequencies caused by
transient currents due to internal logic switching. The 10 µF
capacitors are the tantalum bead type.
The power supply line should have as large a trace as possible to
provide a low impedance path and reduce glitch effects on the
supply line. Shield clocks and other components with fast
switching digital signals from other parts of the board by a
digital ground. Avoid crossover of digital and analog signals,
if possible. When traces cross on opposite sides of the board,
ensure that they run at right angles to each other to reduce
feedthrough effects on the board. The best board layout
technique is the microstrip technique where the component
side of the board is dedicated to the ground plane only and the
signal traces are placed on the solder side; however, this is not
always possible with a 2-layer board.
Layout Considerations for External Temperature Sensors
Power amplifier boards can be electrically noisy environments,
and care must be taken to protect the analog inputs from noise,
particularly when measuring the very small voltages from a
remote diode sensor.
Take the following precautions:
• Place the remote sensing diode as close as possible to
the AD7294. If the worst noise sources are avoided, this
distance can be 4 inches to 8 inches.
• Route the D+ and D− tracks close together, in parallel,
with grounded guard tracks on each side. Provide a ground
plane under the tracks, if possible.
• Use wide tracks to minimize inductance and reduce noise
pickup. A 10 mil track minimum width and spacing is
recommended, as shown in Figure 61.
AGND
D1+
D1–
AGND
0.25mm
0.25mm
0.25mm
0.25mm
0.25mm
0.25mm
0.25mm
05747-049
Figure 61. Arrangement of Signal Tracks
• Try to minimize the number of copper/solder joints
because they can cause thermocouple effects. Where
copper/solder joints are used, make sure that they are
in both the Dx+ and Dx− path and are at the same
temperature.
• Place a 10 pF capacitor between the base and emitter of the
discrete diode, as close as possible to the diode.
• If the distance to the remote sensor is more than 20 cm, the
use of twisted-pair cable is recommended.
Because the measurement technique uses switched current
sources, excessive cable and/or filter capacitance can affect the
measurement. When using long cables, the filter capacitor can
be reduced or removed.