AD6620
decimation rates for the CIC filters must then be selected such
that their performance is near that of the desired channel require-
ments. Finally, an algorithm such as the Parks-McClellan or
Remez exchange is used to compute the final spectral require-
ments, including droop correction for passband loss of the CIC
filters. If the designed filter meets the requirements, then the
filter is acceptable. If not, another combination of CIC filter
decimation must be examined. Tables III and IV greatly simplify
distribution and selection of CIC requirements. The filter
software available from Analog Devices helps to automate
this procedure.
APPLICATIONS
EVALUATION BOARD
An evaluation board is available for the AD6620. This evalua-
tion board comes complete with an AD6620 and interfaces to a
PC through the printer port. The evaluation board comes com-
plete with software to drive the evaluation board and to design
optimized filters for use with the AD6620. The evaluation board
includes a high speed data interface that mates directly with
evaluation boards for high performance converters such as the
AD6600 and AD6640, allowing digital receivers to be bread-
boarded with only an external RF/IF converter and an interface
to the DSP.
NO
SELECT FILTER
REJECTION
REQUIREMENTS
DOES CIC2 FILTER
PROTECT ENOUGH
BANDWIDTH?
The control software allows access to all of the internal registers
to provide complete programming of the device in a lab setting.
The software can process high speed data as well as digitally
filtered data from the AD6620 allowing analysis of both pre and
post filter channel characteristics. The controlling software can
also be used to verify the filter performance by sweeping the NCO,
greatly simplifying verification of any given filter design.
SELECT
DECIMATION
RATE FOR CIC2
YES
SELECT
DECIMATION
RATE FOR CIC5
DOES CIC5 FILTER
PROTECT ENOUGH
BANDWIDTH?
NO
LATCH
YES
DESIGN RCF WITH
REMEZ EXCHANGE
HI SPEED
DATA
LATCH
HEADER
LATCH
FIFO
AD6620
DOES COMPOSITE
FILTER PROVIDE
THE DESIRED
RESULTS?
LATCH
NO
YES
TRANSCEIVER
Figure 48. Diagram of Filter Design Software
PC PRINTER PORT
SERIAL BUFFERING
Figure 47. Evaluation Board Block Diagram
The AD6620 serial outputs are designed to operate at very high
speed. As such, care must be taken when driving the serial output
lines. These high speed lines must be treated as transmission
lines. Critical lines include the SCLK, SDFS, SDFE, SDI and
SDO. It is recommended that these lines be series source termi-
nated with the characteristic impedance of the driven line. If the
lines are longer than a few inches, digital line buffers should be
used as shown below. Buffering in this manner will prevent
reflections on the serial lines from disrupting operation of the
AD6620. A good reference on transmission lines is found in the
“MECL System Design Handbook” by Motorola Inc., Stock
code HB205R1/D.
As shown in the block diagram below, the high speed data into
the evaluation board is sent to both the AD6620 and the by-pass
latches. On the output of the AD6620, data is available in either
serial or parallel mode. In serial mode, data may be sent directly
to a DSP for system bread-boarding. In parallel mode, the data
may be sent to the on-board FIFO for spectral analysis by the
included software. For additional information, refer to the evalua-
tion board manual.
FILTER DESIGN
The AD6620 implements a pair of cascaded CIC filters with a
sum of products FIR filter. The frequency characteristics of the
CIC filters have already been documented. Additional reading
on this class of filters can be found in “An Economical Class of
Digital Filters for Decimation and Interpolation,” by Eugene B.
Hogenauer, IEEE Transactions on Acoustics, Speech, and Signal
Processing, Volume ASSP-29, Number 2, April 1981.
SCLK
SDO
SCLK
SDO
AD6620
SDFS
SDFS
The characteristics of the FIR filter are fully programmable.
The coefficients of this filter may be generated in any number
of ways, using standard procedures such as Parks-McClellan.
Available software from Analog Devices that assists in the design
of filters for this product. This software allows comparison
between different distributions of decimation. The software
works independently of the evaluation board, but easily allows
transfer of design data directly to the evaluation board for
immediate verification of the designed filter.
Figure 49. Serial Line Buffering and Series Source
Termination
DSP/SHARC INTERFACING
With little effort, the AD6620 will interface to nearly all indus-
try standard DSPs, as shown in the figure below. The figures
below show operation in TDM applications as well as in serial
slave mode.
In TDM mode the first AD6620 is configured to be the master.
This chip is the first to access the serial data bus. When the
master has data available in its output shifters, it generates an
SDFS telling the DSP that serial data will follow. At this point,
The normal procedure for designing a filter for the chip is as
shown in the flow chart. First, the desired characteristics must
be determined based on the receive channel requirements. The
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