AD7854 Analog Devices, AD7854 Datasheet - Page 26

• Manufacturer Part Number: AD7854
• Description: 3 V to 5 V Single Supply, 200 kSPS, 12-Bit, Parallel Sampling ADC
• Manufacturer: Analog Devices

Specifications of AD7854

• Resolution (bits): 12bit
• # Chan: 1
• Sample Rate: 200kSPS
• Interface: Byte,Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (Vref) p-p
• Adc Architecture: SAR
• Pkg Type: DIP,SOIC,SOP

AD7854/AD7854L
APPLICATION HINTS
Grounding and Layout
The 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. 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 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. This 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. The analog ground plane should be allowed
to run under the AD7854/AD7854L to avoid noise coupling.
The 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.
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
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.
–26–
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
AD7854/AD7854L, it is recommended that the system’s AV
supply is used. In this case an optional 10 Ω resistor between
the AV
circuitry. This supply should have the recommended analog
supply decoupling capacitors between the AV
AD7854/AD7854L and AGND and the recommended digital
supply decoupling capacitor between the DV
AD7854/AD7854L and DGND.
Evaluating the AD7854/AD7854L Performance
The recommended layout for the AD7854/AD7854L is outlined
in the evaluation board for the AD7854/AD7854L. The evalua-
tion 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 AD7854/AD7854L 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
AD7854/AD7854L.
The 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 – Eight Channel Serial
AD7859 – Eight Channel Parallel
DD
pin and DV
DD
pin can help to filter noise from digital
DD
and DV
DD
DD
pin of the
pin of the
DD
REV. B
of the
DD