REV. A
AD7675
–18–
AD7675*
MC68HC11*
SER/PAR
IRQ
MISO/SDI
SCK
I/O PORT
BUSY
SDOUT
SCLK
CNVST
EXT/INT
CS
RD
INVSCLK
DVDD
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 22. Interfacing the AD7675 to SPI Interface
ADSP-21065L in Master Serial Interface
As shown in Figure 23, the AD7675 can be interfaced to the
ADSP-21065L using the serial interface in master mode without any
glue logic required. This mode combines the advantages of reducing
the wire connections and the ability to read the data during or after
conversion maximum speed transfer (DIVSCLK[0:1] both low).
The AD7675 is configured for the internal clock mode (EXT/
INT low) and acts, therefore, as the master device. The convert
command can be generated by either an external low jitter oscil-
lator or, as shown, by a FLAG output of the ADSP-21065L or
by a frame output TFS of one serial port of the ADSP-21065L
that can be used like a timer. The serial port on the ADSP-
21065L is configured for external clock (IRFS = 0), rising edge
active (CKRE = 1), external late framed sync signals (IRFS = 0,
LAFS = 1, RFSR = 1) and active high (LRFS = 0). The serial
port of the ADSP-21065L is configured by writing to its receive
control register (SRCTL)—see ADSP-2106x SHARC User’s
Manual. 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.
AD7675*
ADSP-21065L*
SHARC
SER/PAR
RFS
DR
RCLK
FLAG OR TFS
SYNC
SDOUT
SCLK
CNVST
RDC/SDIN
RD
EXT/INT
CS
DVDD
*ADDITIONAL PINS OMITTED FOR CLARITY
INVSYNC
INVSCLK
Figure 23. Interfacing to the ADSP-21065L Using
the Serial Master Mode
APPLICATION HINTS
Layout
The AD7675 has very good immunity to noise on the power
supplies as can be seen in Figure 21. However, care should still
be taken with regard to grounding layout.
The printed circuit board that houses the AD7675 should be
designed so 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. Digital and analog
ground planes should be joined in only one place, preferably
underneath the AD7675, or, at least, as close as possible to the
AD7675. If the AD7675 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 AD7675.
It is recommended that running digital lines under the device
should be avoided as these will couple noise onto the die. The
analog ground plane should be allowed to run under the AD7675
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 AD7675 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 AD7675 and reduce the magnitude of the supply spikes.
Decoupling ceramic capacitors, typically 100 nF, should be
placed on each power supply’s pins, AVDD, DVDD, and OVDD,
close to and ideally right up against these pins and their corre-
sponding 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 AD7675 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 connect the system supply to the inter-
face 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 AD7675 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 connected
with short and large traces to minimize parasitic inductances.
Evaluating the AD7675 Performance
A recommended layout for the AD7675 is outlined in the evalu-
ation board for the AD7675. The evaluation board package
includes a fully assembled and tested evaluation board, docu-
mentation, and software for controlling the board from a PC
via the Eval-Control BRD2.
