AD7859/AD7859L
REV. A
–27–
DGND connection, then the ground planes should be con-
nected at the AGND and DGND pins of the AD7859/
AD7859L. If the AD7859/AD7859L is in a system where mul-
tiple devices require AGND to DGND connections, the con-
nection should still be made at one point only, a star ground
point which should be established as close as possible to the
AD7859/AD7859L.
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 AD7859/AD7859L to avoid noise coupling.
The power supply lines to the AD7859/AD7859L 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.
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 al-
ways 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 µF tantalum capacitor in parallel with
0.1 µF disc ceramic capacitor to AGND. All digital supplies
should have a 0.1 µF disc ceramic capacitor to DGND. To
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
AD7859/AD7859L, it is recommended that the system’s AV
DD
supply is used. In this case an optional 10 Ω resistor between
the AV
DD
pin and DV
DD
pin can help to filter noise from digital
circuitry. This supply should have the recommended analog
supply decoupling capacitors between the AV
DD
pin of the
AD7859/AD7859L and AGND and the recommended digital
supply decoupling capacitor between the DV
DD
pin of the
AD7859/AD7859L and DGND.
Evaluating the AD7859/AD7859L Performance
The recommended layout for the AD7859/AD7859L is outlined
in the evaluation board for the AD7859/AD7859L. The 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-CONTROL BOARD. The
EVAL-CONTROL BOARD can be used in conjunction with
the AD7859/AD7859L 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
AD7859/AD7859L.
The software allows the user to perform ac (fast Fourier trans-
form) and dc (histogram of codes) tests on the AD7859/
AD7859L. It also gives full access to all the AD7859/AD7859L
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
