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REV. B
AD7851
Evaluating the AD7851 Performance
The recommended layout for the AD7851 is outlined in the
evaluation board for the AD7851. The evaluation board package
includes a fully assembled and tested evaluation board, documen-
tation, and software for controlling the board from the PC via the
EVAL-CONTROL BOARD. The EVAL-CONTROL BOARD
can be used in conjunction with the AD7851 Evaluation Board,
as well as many other Analog Devices evaluation boards ending in
the CB designator, to demonstrate/evaluate the ac and dc perfor-
mance of the AD7851.
The software allows the user to perform ac (Fast Fourier Trans-
form) and dc (histogram of codes) tests on the AD7851. It also
gives full access to all the AD7851 on-chip registers allowing for
various calibration and power-down options to be programmed.
AD785x Family
All parts are 12 bit, 200 kSPS, 3.0 V to 5.5 V, except the
AD7856 which is 14 bit, 285 kSPS, 5 V supply.
AD7853 – Single-Channel Serial
AD7854 – Single-Channel Parallel
AD7856 – Eight-Channel Serial
AD7858 – Eight-Channel Serial
AD7859 – Eight-Channel Parallel
APPLICATION HINTS
Grounding and Layout
The analog and digital supplies to the AD7851 are independent
and separately pinned out to minimize coupling between the
analog and digital sections of the device. The part has very good
immunity to noise on the power supplies as can be seen by the
PSRR versus frequency graph. However, care should still be
taken with regard to grounding and layout.
The printed circuit board that houses the AD7851 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. Digital and analog ground planes should only be
joined in one place. If the AD7851 is the only device requiring
an AGND to DGND connection, then the ground planes should
be connected at the AGND and DGND pins of the AD7851. If
the AD7851 is in a system where multiple devices require AGND
to DGND connections, the connection should still be made at
one point only, a star ground point which should be established as
close as possible to the AD7851.
Avoid running digital lines under the device as these will couple
noise onto the die. The analog ground plane should be allowed
to run under the AD7851 to avoid noise coupling. The power
supply lines to the AD7851 should use as large a trace as pos-
sible 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 will reduce 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.
Good decoupling is also important. All analog supplies should
be decoupled with 10 µF tantalum in parallel with 0.1 µF ca-
pacitors to AGND. All digital supplies should have a 0.1 µF disc
ceramic capacitor to DGND. To achieve the best from these de-
coupling components, they must be placed as close as possible
to the device, ideally right up against the device. In systems
where a common supply voltage is used to drive both the AV
DD
and DV
DD
of the AD7851, it is recommended that the system’s
AV
DD
supply is used. In this case, there should be a 10 Ω resistor
between the AV
DD
pin and DV
DD
pin. This supply should have the
recommended analog supply decoupling capacitors between the
AV
DD
pin of the AD7851 and AGND and the recommended
digital supply decoupling capacitor between the DV
DD
pin of the
AD7851 and DGND.
