AD7856_15 Datasheet, PDF (30/32 Pages) Analog Devices – 5 V Single Supply, 8-Channel 14-Bit 285 kSPS Sampling ADC

AD7856
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 AD7856 to avoid noise coupling. The power
supply lines to the AD7856 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 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 AV_DD and DV_DD of the AD7856, it is recommended that
the system’s AV_DD supply be 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 capaci-
tors between the AV_DD pin of the AD7856 and AGND and the
recommended digital supply decoupling capacitor between the
DV_DD pin of the AD7856 and DGND.
Evaluating the AD7856 Performance
The recommended layout for the AD7856 is outlined in the
evaluation board for the AD7856. 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 EVAL-CONTROL BOARD. The EVAL-CON-
TROL BOARD can be used in conjunction with the AD7856
Evaluation board, as well as many other Analog Devices evalua-
tion boards ending in the CB designator, to demonstrate/evalu-
ate the ac and dc performance of the AD7856.
The software allows the user to perform ac (fast Fourier trans-
form) and dc (histogram of codes) tests on the AD7856. It also
gives full access to all the AD7856 on-chip registers allowing for
various calibration and power-down options to be programmed.
AD785x Family
12 bits, 200␣ kSPS, 3.0␣ V to 5.5␣ V:
AD7853 – Single-Channel Serial
AD7854 – Single-Channel Parallel
AD7858 – 8-Channel Serial
AD7859 – 8-Channel Parallel
14 bits, 333 kSPS, 4.75 V to 5.25 V:
AD7851 – Single-Channel Serial
–30–
REV. A