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AD7674 Datasheet(PDF) 21 Page - Analog Devices

Part No. AD7674
Description  16-Bit, 250 kSPS CMOS ADC
Download  24 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
Logo AD - Analog Devices

AD7674 Datasheet(HTML) 21 Page - Analog Devices

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Because the Serial Port within the ADSP-21065L will be seeing a
discontinuous clock, an initial word reading has to be done after
the ADSP-21065L has been reset to ensure that the Serial Port
is properly synchronized to this clock during each following data
read operation.
Figure 23. Interfacing to the ADSP-21065L Using the
Serial Master Mode
The AD7663 has very good immunity to noise on the power
supplies as can be seen in Figure 9. However, care should still be
taken with regard to grounding layout.
The printed circuit board that houses the AD7663 should be
designed so the analog and digital sections are separated and con-
fined to certain areas of the board. This facilitates the use of
ground planes that can be easily separated. Digital and analog
ground planes should be joined in only one place, preferably
underneath the AD7663 or at least as close as possible to the
AD7663. If the AD7663 is in a system where multiple devices
require analog-to-digital ground connections, the connection
should still be made at one point only, a star ground point that
should be established as close as possible to the AD7663.
It is recommended to 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 AD7663 to avoid noise cou-
pling. Fast switching signals like
CNVST or clocks should be
shielded with digital ground to avoid radiating noise to other
sections of the board and should never run near analog signal paths.
Crossover of digital and analog signals should be avoided. Traces
on different but close layers of the board should run at right
angles to each other. This will reduce the effect of feedthrough
through the board.
The power supply lines to the AD7663 should use as large a trace
as possible to provide low impedance paths and reduce the effect of
glitches on the power supply lines. Good decoupling is also impor-
tant to lower the supplies’ impedance presented to the AD7663
and to reduce the magnitude of the supply spikes. Decoupling
ceramic capacitors, typically 100 nF, should be placed on all of
the power supply pins AVDD, DVDD, and OVDD close to, and
ideally right up against these pins and their corresponding ground
pins. Additionally, low ESR 10 µF capacitors should be located
in the vicinity of the ADC to further reduce low frequency ripple.
The DVDD supply of the AD7663 can be either a separate
supply or come from the analog supply, AVDD, or from the
digital interface supply, OVDD. When the system digital supply
is noisy or fast switching digital signals are present, it is recom-
mended, if no separate supply is available, to connect the DVDD
digital supply to the analog supply, AVDD, through an RC filter
as shown in Figure 5, and to connect the system supply to the
interface digital supply OVDD and the remaining digital circuitry.
When DVDD is powered from the system supply, it is useful to
insert a bead to further reduce high frequency spikes.
The AD7663 has five different ground pins: INGND, REFGND,
AGND, DGND, and OGND. INGND is used to sense the
analog input signal. REFGND senses the reference voltage and
should be a low impedance return to the reference because it
carries pulsed currents. AGND is the ground to which most internal
ADC analog signals are referenced. This ground must be con-
nected with the least resistance to the analog ground plane. DGND
must be tied to the analog or digital ground plane depending on
the configuration. OGND is connected to the digital system ground.
The layout of the decoupling of the reference voltage is important.
The decoupling capacitor should be close to the ADC and con-
nected with short and large traces to minimize parasitic inductances.
Evaluating the AD7663 Performance
A recommended layout for the AD7663 is outlined in the evalu-
ation board for the AD7663. The evaluation board package includes
a fully assembled and tested evaluation board, documentation,
and software for controlling the board from a PC via the Eval-
Control Board.

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