AD7927 Analog Devices, AD7927 Datasheet - Page 26

• Manufacturer Part Number: AD7927
• Description: 8-Channel, 200 kSPS, 12-Bit ADC with Sequencer in 20-Lead TSSOP
• Manufacturer: Analog Devices

Specifications of AD7927

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 200kSPS
• Interface: Ser,SPI
• Analog Input Type: SE-Uni
• Ain Range: Uni (Vref),Uni (Vref) x 2
• Adc Architecture: SAR
• Pkg Type: SOP

AD7927
APPLICATION HINTS
GROUNDING AND LAYOUT
The AD7927 has very good immunity to noise on the power
supplies as can be seen in Figure 6. However, care should still
be taken with regard to grounding and layout.
The printed circuit board that houses the AD7927 should be
designed such that the analog and digital sections are separated
and confined to certain areas of the board. This facilitates the
use of ground planes that can be separated easily. A minimum
etch technique is generally best for ground planes as it gives the
best shielding. All three AGND pins of the AD7927 should be
sunk in the AGND plane. Digital and analog ground planes
should be joined at only one place. If the AD7927 is in a system
where multiple devices require an AGND to DGND connec-
tion, the connection should still be made at one point only, a
star ground point that should be established as close as possible
to the AD7927.
Avoid running digital lines under the device as these couple
noise onto the die. The analog ground plane should be allowed
to run under the AD7927 to avoid noise coupling. The power
supply lines to the AD7927 should use as large a trace as possi-
ble to provide low impedance paths and reduce the effects of
glitches on the power supply line. Fast switching signals, like
clocks, should be shielded with digital ground to avoid radiating
noise to other sections of the board, and clock signals should
never be run near the analog inputs. Avoid crossover of digital
and analog signals. Traces on opposite sides of the board should
run at right angles to each other. This reduces the effects of
feedthrough through the board. A microstrip technique is by
far the best, but is not always possible with a double-sided
board. In this technique, the component side of the board is
dedicated to ground planes while signals are placed on the
solder side.
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Good decoupling is also important. All analog supplies should
be decoupled with 10 μF tantalum in parallel with 0.1 μF capaci-
tors to AGND. To achieve the best from these decoupling
components, they must be placed as close as possible to the
device, ideally right up against the device. The 0.1 μF capacitors
should have low effective series resistance (ESR) and effective
series inductance (ESI), such as the common ceramic types or
surface mount types, which provide a low impedance path to
ground at high frequencies to handle transient currents due to
internal logic switching.
EVALUATING THE AD7927 PERFORMANCE
The recommended layout for the AD7927 is outlined in the
evaluation board for the AD7927. The evaluation board pack-
age includes a fully assembled and tested evaluation board,
documentation, and software for controlling the board from
the PC via the evaluation-board controller. The evaluation-
board controller can be used in conjunction with the AD7927
Evaluation Board as well as many other Analog Devices, Inc.
evaluation boards ending in the CB designator to demonstrate/
evaluate the ac and dc performance of the AD7927.
The software allows the user to perform ac (fast Fourier
transform) and dc (histogram of codes) tests on the AD7927.
The software and documentation are on a CD shipped with
the evaluation board.
Data Sheet