AD7858/AD7858L
APPLICATION HINTS
Grounding and Layout
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 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. In
systems where a common supply voltage is used to drive both
the AVDD and DVDD of the AD7858/AD7858L, it is recom-
mended that the system’s AVDD supply be used. In this case
there should be a 10 Ω resistor between the AVDD pin and
DVDD pin. This supply should have the recommended analog
supply decoupling capacitors between the AVDD pin of the
AD7858/AD7858L and AGND and the recommended digital
supply decoupling capacitor between the DVDD pin of the
AD7858/AD7858L and DGND.
The analog and digital supplies to the AD7858/AD7858L 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 vs. Frequency graph. However, care
should still be taken with regard to grounding and layout.
The printed circuit board that houses the AD7858/AD7858L
should be designed such that the analog and digital sections are
separated and confined to certain areas of the board. This facili-
tates 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 AD7858/AD7858L 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 AD7858/AD7858L. If the AD7858/
AD7858L 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 AD7858/AD7858L.
Evaluating the AD7858/AD7858L Performance
The recommended layout for the AD7858/AD7858L is out-
lined in the evaluation board for the AD7858/AD7858L. The
evaluation board package includes a fully assembled and tested
evaluation board, documentation, 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 AD7858/AD7858L Evaluation board, as well as many
other Analog Devices evaluation boards ending in the CB desig-
nator, to demonstrate/evaluate the ac and dc performance of the
AD7858/AD7858L.
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 AD7858/AD7858L to avoid noise coupling.
The power supply lines to the AD7858/AD7858L 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 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 oppo-
site 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.
The software allows the user to perform ac (fast Fourier trans-
form) and dc (histogram of codes) tests on the AD7858/
AD7858L. It also gives full access to all the AD7858/AD7858L
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 – Eight Channel Serial
AD7859 – Eight Channel Parallel
–30–
REV. B
ADI [ ADI ]
