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AD7674 Datasheet(PDF) 25 Page - Analog Devices

Part No. AD7674
Description  18-Bit, 2.5 LSB INL, 800 kSPS SAR ADC
Download  28 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
Logo AD - Analog Devices

AD7674 Datasheet(HTML) 25 Page - Analog Devices

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Rev. A | Page 25 of 28
The AD7674 has very good immunity to noise on the power
supplies. However, care should still be taken with regard to
grounding layout.
The printed circuit board that houses the AD7674 should be
designed so that the analog and digital sections are separated
and confined to certain areas of the board. This calls for the use
of ground planes, which can be easily separated. Digital and
analog ground planes should be joined in only one place,
preferably underneath the AD7674, or at least as close to the
AD7674 as possible. If the AD7674 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 to the AD7674
as possible.
The user should 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 AD7674 to avoid noise
coupling. 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
AD7674 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 important to lower the
supply’s impedance presented to the AD7674 and to reduce the
magnitude of the supply spikes. Decoupling ceramic capacitors,
typically 100 nF, should be placed close to and ideally right up
against each power supply pin (AVDD, DVDD, and OVDD)
and their corresponding ground pins. Additionally, low ESR
10 μF capacitors should be located near the ADC to further
reduce low frequency ripple.
The DVDD supply of the AD7674 can be a separate supply or
can come from the analog supply, AVDD, or the digital
interface supply, OVDD. When the system digital supply is
noisy or when fast switching digital signals are present, and if
no separate supply is available, the user should connect the
DVDD digital supply to the analog supply AVDD through an
RC filter, (see Figure 27), and 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 AD7674 has four different ground pins: REFGND, AGND,
DGND, and OGND. 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 connected 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 should be connected with short and large traces to
minimize parasitic inductances.
An evaluation board for the AD7674 allows a quick means to
measure both DC (histograms and time domain) and AC (time
and frequency domain) performances of the converter. The
EVAL-AD7674CBZ is an evaluation board package that
includes a fully assembled and tested evaluation board,
documentation, and software. The accompanying software
requires the use of a capture board which must be ordered
seperately from the evaluation board (see the Ordering Guide
for information). The evaluation board can also be used in a
stand alone configuration and does not use the software when
in this mode. Refer to the EVAL-AD76XXEDZ and EVAL-
AD76XXCBZ documents available from www.analog.com for
evaluation board details.
Two types of data capture boards can be used with the EVAL-
USB based (EVAL-CED1Z recommended)
Parallel port based (EVAL-CONTROL BRD3Z not
recommended as many newer PC’s do not include parallel
ports any longer)
The recommended board layout for the AD7674 is outlined in
the evaluation board data sheet.

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