LF43168
DEVICES INCORPORATED
Dual 8-Tap FIR Filter
F
IGURE
7.
I
NTERLEAVED
C
OEFFICIENT
F
ILTER
C
ONFIGURATION
N = Delay Length (Decimation Factor)
Interleaved Coefficient Filters
Figure 7 shows the filter configuration
when the device is programmed for
interleaved coefficients. An inter-
leaved coefficient set contains two
separate odd-tap, even-symmetric
coefficient sets which have been
interleaved together (see Figure 8). If
two data sets are interleaved into the
same serial data stream, they can both
be filtered by different coefficient sets
if the two coefficient sets are also
interleaved. The LF43168 is config-
ured as an interleaved coefficient filter
by programming the device for
interleaved coefficient sets, even-
symmetric coefficients, odd number of
filter taps, and data reversal disabled.
Note that coefficient 3, in Figure 7,
must be divided by two to get the
correct result.
Asymmetric Coefficient Filters
It is possible to have asymmetric
coefficient filters. Asymmetric coeffi-
cient sets do not exhibit even or odd
symmetric properties. A 4-tap asym-
metric filter is possible by putting the
device in even-tap, pass A mode and
then feeding the asymmetric coeffi-
cient set to the multipliers. An 8-tap
asymmetric filter is possible if the
device is clocked twice as fast as the
input data rate. It will take two CLK
cycles to calculate the output. On the
first CLK cycle, the reverse decimation
path is selected to feed data to the
filter multipliers. On the second CLK
cycle, the coefficients sent to the
multipliers are changed (if necessary)
and the forward decimation path is
selected to feed data to the filter
multipliers.
N
N
N
N
DATA IN
N
N
N
A
B+A
B
A
B+A
B
A
B+A
B
A
B+A
B
COEF 0
COEF 1
COEF 2
COEF 3
2
ODD-TAP INTERLEAVED FILTER
F
IGURE
8.
I
NTERLEAVED
C
OEFFICIENT
S
ET
E
XAMPLE
7 6 5 4 3 2 1
Odd-Tap, Even-Symmetric
Coefficient Set A
14 13 12 11 10 9 8 7 6 5 4 3 2 1
Interleaved Coefficient Set
Consisting of Sets A and B
7 6 5 4 3 2 1
Odd-Tap, Even-Symmetric
Coefficient Set B
Video Imaging Products
9
03/28/2000–LDS.43168-H
LOGIC [ LOGIC DEVICES INCORPORATED ]
