
AD7854/AD7854L
–26–
REV. 0
APPLICAT ION HINT S
Grounding and Layout
T he analog and digital supplies of the AD7854/AD7854L are
independent and separately pinned out to minimize coupling
between the analog and digital sections of the device. T he 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.
T he printed circuit board on which the AD7854/AD7854L is
mounted should be designed such that the analog and digital
sections are separated and confined to certain areas of the
board. T his facilitates the use of ground planes that can be
easily separated. 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 AD7854/AD7854L 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 AD7854/
AD7854L . If the AD7854/AD7854L 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
AD7854/AD7854L.
Avoid running digital lines under the device as these couple
noise onto the die. T he analog ground plane should be allowed
to run under the AD7854/AD7854L to avoid noise coupling.
T he power supply lines to the AD7854/AD7854L should use as
large a trace as possible to provide low impedance paths and
reduce the effects of glitches on the power supply line. Fast
switching signals like clocks and the data inputs 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.
T races on opposite sides of the board should run at right angles
to each other. T his 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.
Good decoupling is also important. All analog supplies should
be decoupled with a 10
μ
F tantalum capacitor in parallel with
0.1
μ
F disc ceramic capacitor to AGND. All digital supplies
should have a 0.1
μ
F disc ceramic capacitor to DGND. T o
achieve the best performance from these decoupling compo-
nents, 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
AD7854/AD7854L, it is recommended that the system’s AV
DD
supply is used. In this case an optional 10
resistor between
the AV
DD
pin and DV
DD
pin can help to filter noise from digital
circuitry. T his supply should have the recommended analog
supply decoupling capacitors between the AV
DD
pin of the
AD7854/AD7854L and AGND and the recommended digital
supply decoupling capacitor between the DV
DD
pin of the
AD7854/AD7854L and DGND.
E valuating the AD7854/AD7854L Performance
T he recommended layout for the AD7854/AD7854L is outlined
in the evaluation board for the AD7854/AD7854L. T he evalua-
tion board package includes a fully assembled and tested evalua-
tion board, documentation, and software for controlling the
board from the PC via the EVAL-CONT ROL BOARD. T he
EVAL-CONT ROL BOARD can be used in conjunction with
the AD7854/AD7854L Evaluation board, as well as many other
Analog Devices evaluation boards ending in the CB designator,
to demonstrate/evaluate the ac and dc performance of the
AD7854/AD7854L.
T he software allows the user to perform ac (fast Fourier trans-
form) and dc (histogram of codes) tests on the AD7854/
AD7854L. It also gives full access to all the AD7854/AD7854L
on-chip registers allowing for various calibration and power-
down options to be programmed.
AD785x Family
All parts are 12 bits, 200 kSPS, 3.0 V to 5.5 V.
AD7853 – Single Channel Serial
AD7854 – Single Channel Parallel
AD7858 – E ight Channel Serial
AD7859 – E ight Channel Parallel