AD7298 Analog Devices, AD7298 Datasheet - Page 23

• Manufacturer Part Number: AD7298
• Description: 8-Channel, 1MSPS, 12-Bit SAR ADC with Temperature Sensor
• Manufacturer: Analog Devices

Specifications of AD7298

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 1MSPS
• Interface: SPI
• Analog Input Type: SE-Uni
• Ain Range: Uni (Vref),Uni 1.0V,Uni 1.25,Uni 2.0V,Uni 2.5V
• Adc Architecture: SAR
• Pkg Type: CSP

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 AD7298 is
used. The PCB containing the AD7298 should have separate
analog and digital sections, each having its own area of the
board. The AD7298 should be located in the analog section
on any PCB.
Decouple the power supply to the AD7298 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.
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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.
TEMPERATURE MONITORING
The AD7298 is ideal for monitoring the thermal environment.
The die accurately reflects the exact thermal conditions that
affect nearby integrated circuits. The AD7298 measures and
converts the temperature at the surface of its own semicon-
ductor chip.
When it is used to measure the temperature of a nearby heat
source, the thermal impedance between the heat source and the
AD7298 must be considered. When the thermal impedance is
determined, the temperature of the heat source can be inferred
from the AD7298 output.
As much as 60% of the heat transferred from the heat source to
the thermal sensor on the AD7298 die is discharged via the
copper tracks and the bond pads. Of the pads on the AD7298,
the GND pad transfers most of the heat. Therefore, to measure
the temperature of a heat source, it is recommended that the
thermal resistance between the AD7298 GND pad and the
GND of the heat source be reduced as much as possible.
AD7298